2144 lines
87 KiB
C++
2144 lines
87 KiB
C++
//===-- MachineBlockPlacement.cpp - Basic Block Code Layout optimization --===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements basic block placement transformations using the CFG
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// structure and branch probability estimates.
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//
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// The pass strives to preserve the structure of the CFG (that is, retain
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// a topological ordering of basic blocks) in the absence of a *strong* signal
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// to the contrary from probabilities. However, within the CFG structure, it
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// attempts to choose an ordering which favors placing more likely sequences of
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// blocks adjacent to each other.
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//
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// The algorithm works from the inner-most loop within a function outward, and
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// at each stage walks through the basic blocks, trying to coalesce them into
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// sequential chains where allowed by the CFG (or demanded by heavy
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// probabilities). Finally, it walks the blocks in topological order, and the
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// first time it reaches a chain of basic blocks, it schedules them in the
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// function in-order.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/CodeGen/Passes.h"
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#include "llvm/CodeGen/TargetPassConfig.h"
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#include "BranchFolding.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/CodeGen/MachineBasicBlock.h"
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#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
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#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
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#include "llvm/CodeGen/MachineDominators.h"
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#include "llvm/CodeGen/MachineFunction.h"
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#include "llvm/CodeGen/MachineFunctionPass.h"
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#include "llvm/CodeGen/MachineLoopInfo.h"
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#include "llvm/CodeGen/MachineModuleInfo.h"
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#include "llvm/CodeGen/TailDuplicator.h"
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#include "llvm/Support/Allocator.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Target/TargetInstrInfo.h"
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#include "llvm/Target/TargetLowering.h"
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#include "llvm/Target/TargetSubtargetInfo.h"
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#include <algorithm>
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using namespace llvm;
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#define DEBUG_TYPE "block-placement"
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STATISTIC(NumCondBranches, "Number of conditional branches");
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STATISTIC(NumUncondBranches, "Number of unconditional branches");
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STATISTIC(CondBranchTakenFreq,
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"Potential frequency of taking conditional branches");
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STATISTIC(UncondBranchTakenFreq,
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"Potential frequency of taking unconditional branches");
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static cl::opt<unsigned> AlignAllBlock("align-all-blocks",
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cl::desc("Force the alignment of all "
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"blocks in the function."),
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cl::init(0), cl::Hidden);
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static cl::opt<unsigned> AlignAllNonFallThruBlocks(
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"align-all-nofallthru-blocks",
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cl::desc("Force the alignment of all "
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"blocks that have no fall-through predecessors (i.e. don't add "
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"nops that are executed)."),
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cl::init(0), cl::Hidden);
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// FIXME: Find a good default for this flag and remove the flag.
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static cl::opt<unsigned> ExitBlockBias(
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"block-placement-exit-block-bias",
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cl::desc("Block frequency percentage a loop exit block needs "
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"over the original exit to be considered the new exit."),
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cl::init(0), cl::Hidden);
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// Definition:
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// - Outlining: placement of a basic block outside the chain or hot path.
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static cl::opt<bool> OutlineOptionalBranches(
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"outline-optional-branches",
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cl::desc("Outlining optional branches will place blocks that are optional "
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"branches, i.e. branches with a common post dominator, outside "
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"the hot path or chain"),
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cl::init(false), cl::Hidden);
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static cl::opt<unsigned> OutlineOptionalThreshold(
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"outline-optional-threshold",
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cl::desc("Don't outline optional branches that are a single block with an "
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"instruction count below this threshold"),
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cl::init(4), cl::Hidden);
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static cl::opt<unsigned> LoopToColdBlockRatio(
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"loop-to-cold-block-ratio",
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cl::desc("Outline loop blocks from loop chain if (frequency of loop) / "
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"(frequency of block) is greater than this ratio"),
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cl::init(5), cl::Hidden);
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static cl::opt<bool>
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PreciseRotationCost("precise-rotation-cost",
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cl::desc("Model the cost of loop rotation more "
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"precisely by using profile data."),
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cl::init(false), cl::Hidden);
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static cl::opt<bool>
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ForcePreciseRotationCost("force-precise-rotation-cost",
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cl::desc("Force the use of precise cost "
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"loop rotation strategy."),
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cl::init(false), cl::Hidden);
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static cl::opt<unsigned> MisfetchCost(
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"misfetch-cost",
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cl::desc("Cost that models the probabilistic risk of an instruction "
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"misfetch due to a jump comparing to falling through, whose cost "
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"is zero."),
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cl::init(1), cl::Hidden);
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static cl::opt<unsigned> JumpInstCost("jump-inst-cost",
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cl::desc("Cost of jump instructions."),
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cl::init(1), cl::Hidden);
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static cl::opt<bool>
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TailDupPlacement("tail-dup-placement",
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cl::desc("Perform tail duplication during placement. "
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"Creates more fallthrough opportunites in "
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"outline branches."),
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cl::init(true), cl::Hidden);
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static cl::opt<bool>
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BranchFoldPlacement("branch-fold-placement",
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cl::desc("Perform branch folding during placement. "
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"Reduces code size."),
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cl::init(true), cl::Hidden);
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// Heuristic for tail duplication.
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static cl::opt<unsigned> TailDuplicatePlacementThreshold(
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"tail-dup-placement-threshold",
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cl::desc("Instruction cutoff for tail duplication during layout. "
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"Tail merging during layout is forced to have a threshold "
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"that won't conflict."), cl::init(2),
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cl::Hidden);
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extern cl::opt<unsigned> StaticLikelyProb;
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extern cl::opt<unsigned> ProfileLikelyProb;
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namespace {
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class BlockChain;
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/// \brief Type for our function-wide basic block -> block chain mapping.
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typedef DenseMap<MachineBasicBlock *, BlockChain *> BlockToChainMapType;
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}
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namespace {
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/// \brief A chain of blocks which will be laid out contiguously.
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///
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/// This is the datastructure representing a chain of consecutive blocks that
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/// are profitable to layout together in order to maximize fallthrough
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/// probabilities and code locality. We also can use a block chain to represent
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/// a sequence of basic blocks which have some external (correctness)
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/// requirement for sequential layout.
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///
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/// Chains can be built around a single basic block and can be merged to grow
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/// them. They participate in a block-to-chain mapping, which is updated
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/// automatically as chains are merged together.
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class BlockChain {
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/// \brief The sequence of blocks belonging to this chain.
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///
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/// This is the sequence of blocks for a particular chain. These will be laid
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/// out in-order within the function.
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SmallVector<MachineBasicBlock *, 4> Blocks;
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/// \brief A handle to the function-wide basic block to block chain mapping.
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///
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/// This is retained in each block chain to simplify the computation of child
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/// block chains for SCC-formation and iteration. We store the edges to child
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/// basic blocks, and map them back to their associated chains using this
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/// structure.
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BlockToChainMapType &BlockToChain;
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public:
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/// \brief Construct a new BlockChain.
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///
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/// This builds a new block chain representing a single basic block in the
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/// function. It also registers itself as the chain that block participates
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/// in with the BlockToChain mapping.
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BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB)
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: Blocks(1, BB), BlockToChain(BlockToChain), UnscheduledPredecessors(0) {
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assert(BB && "Cannot create a chain with a null basic block");
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BlockToChain[BB] = this;
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}
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/// \brief Iterator over blocks within the chain.
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typedef SmallVectorImpl<MachineBasicBlock *>::iterator iterator;
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/// \brief Beginning of blocks within the chain.
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iterator begin() { return Blocks.begin(); }
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/// \brief End of blocks within the chain.
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iterator end() { return Blocks.end(); }
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bool remove(MachineBasicBlock* BB) {
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for(iterator i = begin(); i != end(); ++i) {
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if (*i == BB) {
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Blocks.erase(i);
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return true;
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}
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}
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return false;
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}
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/// \brief Merge a block chain into this one.
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///
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/// This routine merges a block chain into this one. It takes care of forming
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/// a contiguous sequence of basic blocks, updating the edge list, and
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/// updating the block -> chain mapping. It does not free or tear down the
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/// old chain, but the old chain's block list is no longer valid.
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void merge(MachineBasicBlock *BB, BlockChain *Chain) {
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assert(BB);
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assert(!Blocks.empty());
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// Fast path in case we don't have a chain already.
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if (!Chain) {
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assert(!BlockToChain[BB]);
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Blocks.push_back(BB);
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BlockToChain[BB] = this;
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return;
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}
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assert(BB == *Chain->begin());
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assert(Chain->begin() != Chain->end());
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// Update the incoming blocks to point to this chain, and add them to the
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// chain structure.
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for (MachineBasicBlock *ChainBB : *Chain) {
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Blocks.push_back(ChainBB);
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assert(BlockToChain[ChainBB] == Chain && "Incoming blocks not in chain");
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BlockToChain[ChainBB] = this;
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}
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}
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#ifndef NDEBUG
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/// \brief Dump the blocks in this chain.
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LLVM_DUMP_METHOD void dump() {
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for (MachineBasicBlock *MBB : *this)
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MBB->dump();
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}
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#endif // NDEBUG
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/// \brief Count of predecessors of any block within the chain which have not
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/// yet been scheduled. In general, we will delay scheduling this chain
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/// until those predecessors are scheduled (or we find a sufficiently good
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/// reason to override this heuristic.) Note that when forming loop chains,
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/// blocks outside the loop are ignored and treated as if they were already
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/// scheduled.
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///
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/// Note: This field is reinitialized multiple times - once for each loop,
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/// and then once for the function as a whole.
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unsigned UnscheduledPredecessors;
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};
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}
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namespace {
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class MachineBlockPlacement : public MachineFunctionPass {
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/// \brief A typedef for a block filter set.
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typedef SmallSetVector<MachineBasicBlock *, 16> BlockFilterSet;
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/// \brief work lists of blocks that are ready to be laid out
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SmallVector<MachineBasicBlock *, 16> BlockWorkList;
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SmallVector<MachineBasicBlock *, 16> EHPadWorkList;
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/// \brief Machine Function
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MachineFunction *F;
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/// \brief A handle to the branch probability pass.
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const MachineBranchProbabilityInfo *MBPI;
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/// \brief A handle to the function-wide block frequency pass.
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std::unique_ptr<BranchFolder::MBFIWrapper> MBFI;
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/// \brief A handle to the loop info.
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MachineLoopInfo *MLI;
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/// \brief Preferred loop exit.
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/// Member variable for convenience. It may be removed by duplication deep
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/// in the call stack.
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MachineBasicBlock *PreferredLoopExit;
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/// \brief A handle to the target's instruction info.
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const TargetInstrInfo *TII;
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/// \brief A handle to the target's lowering info.
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const TargetLoweringBase *TLI;
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/// \brief A handle to the post dominator tree.
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MachineDominatorTree *MDT;
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/// \brief Duplicator used to duplicate tails during placement.
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///
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/// Placement decisions can open up new tail duplication opportunities, but
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/// since tail duplication affects placement decisions of later blocks, it
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/// must be done inline.
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TailDuplicator TailDup;
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/// \brief A set of blocks that are unavoidably execute, i.e. they dominate
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/// all terminators of the MachineFunction.
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SmallPtrSet<MachineBasicBlock *, 4> UnavoidableBlocks;
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/// \brief Allocator and owner of BlockChain structures.
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///
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/// We build BlockChains lazily while processing the loop structure of
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/// a function. To reduce malloc traffic, we allocate them using this
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/// slab-like allocator, and destroy them after the pass completes. An
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/// important guarantee is that this allocator produces stable pointers to
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/// the chains.
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SpecificBumpPtrAllocator<BlockChain> ChainAllocator;
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/// \brief Function wide BasicBlock to BlockChain mapping.
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///
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/// This mapping allows efficiently moving from any given basic block to the
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/// BlockChain it participates in, if any. We use it to, among other things,
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/// allow implicitly defining edges between chains as the existing edges
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/// between basic blocks.
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DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain;
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#ifndef NDEBUG
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/// The set of basic blocks that have terminators that cannot be fully
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/// analyzed. These basic blocks cannot be re-ordered safely by
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/// MachineBlockPlacement, and we must preserve physical layout of these
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/// blocks and their successors through the pass.
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SmallPtrSet<MachineBasicBlock *, 4> BlocksWithUnanalyzableExits;
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#endif
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/// Decrease the UnscheduledPredecessors count for all blocks in chain, and
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/// if the count goes to 0, add them to the appropriate work list.
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void markChainSuccessors(BlockChain &Chain, MachineBasicBlock *LoopHeaderBB,
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const BlockFilterSet *BlockFilter = nullptr);
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/// Decrease the UnscheduledPredecessors count for a single block, and
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/// if the count goes to 0, add them to the appropriate work list.
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void markBlockSuccessors(
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BlockChain &Chain, MachineBasicBlock *BB, MachineBasicBlock *LoopHeaderBB,
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const BlockFilterSet *BlockFilter = nullptr);
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BranchProbability
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collectViableSuccessors(MachineBasicBlock *BB, BlockChain &Chain,
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const BlockFilterSet *BlockFilter,
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SmallVector<MachineBasicBlock *, 4> &Successors);
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bool shouldPredBlockBeOutlined(MachineBasicBlock *BB, MachineBasicBlock *Succ,
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BlockChain &Chain,
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const BlockFilterSet *BlockFilter,
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BranchProbability SuccProb,
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BranchProbability HotProb);
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bool repeatedlyTailDuplicateBlock(
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MachineBasicBlock *BB, MachineBasicBlock *&LPred,
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MachineBasicBlock *LoopHeaderBB,
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BlockChain &Chain, BlockFilterSet *BlockFilter,
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MachineFunction::iterator &PrevUnplacedBlockIt);
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bool maybeTailDuplicateBlock(MachineBasicBlock *BB, MachineBasicBlock *LPred,
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const BlockChain &Chain,
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BlockFilterSet *BlockFilter,
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MachineFunction::iterator &PrevUnplacedBlockIt,
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bool &DuplicatedToPred);
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bool
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hasBetterLayoutPredecessor(MachineBasicBlock *BB, MachineBasicBlock *Succ,
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BlockChain &SuccChain, BranchProbability SuccProb,
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BranchProbability RealSuccProb, BlockChain &Chain,
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const BlockFilterSet *BlockFilter);
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MachineBasicBlock *selectBestSuccessor(MachineBasicBlock *BB,
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BlockChain &Chain,
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const BlockFilterSet *BlockFilter);
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MachineBasicBlock *
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selectBestCandidateBlock(BlockChain &Chain,
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SmallVectorImpl<MachineBasicBlock *> &WorkList);
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MachineBasicBlock *
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getFirstUnplacedBlock(const BlockChain &PlacedChain,
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MachineFunction::iterator &PrevUnplacedBlockIt,
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const BlockFilterSet *BlockFilter);
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/// \brief Add a basic block to the work list if it is appropriate.
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///
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/// If the optional parameter BlockFilter is provided, only MBB
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/// present in the set will be added to the worklist. If nullptr
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/// is provided, no filtering occurs.
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void fillWorkLists(MachineBasicBlock *MBB,
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SmallPtrSetImpl<BlockChain *> &UpdatedPreds,
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const BlockFilterSet *BlockFilter);
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void buildChain(MachineBasicBlock *BB, BlockChain &Chain,
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BlockFilterSet *BlockFilter = nullptr);
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MachineBasicBlock *findBestLoopTop(MachineLoop &L,
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const BlockFilterSet &LoopBlockSet);
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MachineBasicBlock *findBestLoopExit(MachineLoop &L,
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const BlockFilterSet &LoopBlockSet);
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BlockFilterSet collectLoopBlockSet(MachineLoop &L);
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void buildLoopChains(MachineLoop &L);
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void rotateLoop(BlockChain &LoopChain, MachineBasicBlock *ExitingBB,
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const BlockFilterSet &LoopBlockSet);
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void rotateLoopWithProfile(BlockChain &LoopChain, MachineLoop &L,
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const BlockFilterSet &LoopBlockSet);
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void collectMustExecuteBBs();
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void buildCFGChains();
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void optimizeBranches();
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void alignBlocks();
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public:
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static char ID; // Pass identification, replacement for typeid
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MachineBlockPlacement() : MachineFunctionPass(ID) {
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initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
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}
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bool runOnMachineFunction(MachineFunction &F) override;
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addRequired<MachineBranchProbabilityInfo>();
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AU.addRequired<MachineBlockFrequencyInfo>();
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AU.addRequired<MachineDominatorTree>();
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AU.addRequired<MachineLoopInfo>();
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AU.addRequired<TargetPassConfig>();
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MachineFunctionPass::getAnalysisUsage(AU);
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}
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};
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}
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char MachineBlockPlacement::ID = 0;
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char &llvm::MachineBlockPlacementID = MachineBlockPlacement::ID;
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INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement",
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"Branch Probability Basic Block Placement", false, false)
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INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
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INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
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INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
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INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
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INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement",
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"Branch Probability Basic Block Placement", false, false)
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#ifndef NDEBUG
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/// \brief Helper to print the name of a MBB.
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///
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/// Only used by debug logging.
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static std::string getBlockName(MachineBasicBlock *BB) {
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std::string Result;
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raw_string_ostream OS(Result);
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OS << "BB#" << BB->getNumber();
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OS << " ('" << BB->getName() << "')";
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OS.flush();
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return Result;
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}
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#endif
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/// \brief Mark a chain's successors as having one fewer preds.
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///
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/// When a chain is being merged into the "placed" chain, this routine will
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/// quickly walk the successors of each block in the chain and mark them as
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/// having one fewer active predecessor. It also adds any successors of this
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/// chain which reach the zero-predecessor state to the appropriate worklist.
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void MachineBlockPlacement::markChainSuccessors(
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BlockChain &Chain, MachineBasicBlock *LoopHeaderBB,
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const BlockFilterSet *BlockFilter) {
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// Walk all the blocks in this chain, marking their successors as having
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// a predecessor placed.
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for (MachineBasicBlock *MBB : Chain) {
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markBlockSuccessors(Chain, MBB, LoopHeaderBB, BlockFilter);
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}
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}
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/// \brief Mark a single block's successors as having one fewer preds.
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///
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/// Under normal circumstances, this is only called by markChainSuccessors,
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/// but if a block that was to be placed is completely tail-duplicated away,
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/// and was duplicated into the chain end, we need to redo markBlockSuccessors
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/// for just that block.
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void MachineBlockPlacement::markBlockSuccessors(
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BlockChain &Chain, MachineBasicBlock *MBB, MachineBasicBlock *LoopHeaderBB,
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const BlockFilterSet *BlockFilter) {
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// Add any successors for which this is the only un-placed in-loop
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// predecessor to the worklist as a viable candidate for CFG-neutral
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// placement. No subsequent placement of this block will violate the CFG
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// shape, so we get to use heuristics to choose a favorable placement.
|
|
for (MachineBasicBlock *Succ : MBB->successors()) {
|
|
if (BlockFilter && !BlockFilter->count(Succ))
|
|
continue;
|
|
BlockChain &SuccChain = *BlockToChain[Succ];
|
|
// Disregard edges within a fixed chain, or edges to the loop header.
|
|
if (&Chain == &SuccChain || Succ == LoopHeaderBB)
|
|
continue;
|
|
|
|
// This is a cross-chain edge that is within the loop, so decrement the
|
|
// loop predecessor count of the destination chain.
|
|
if (SuccChain.UnscheduledPredecessors == 0 ||
|
|
--SuccChain.UnscheduledPredecessors > 0)
|
|
continue;
|
|
|
|
auto *NewBB = *SuccChain.begin();
|
|
if (NewBB->isEHPad())
|
|
EHPadWorkList.push_back(NewBB);
|
|
else
|
|
BlockWorkList.push_back(NewBB);
|
|
}
|
|
}
|
|
|
|
/// This helper function collects the set of successors of block
|
|
/// \p BB that are allowed to be its layout successors, and return
|
|
/// the total branch probability of edges from \p BB to those
|
|
/// blocks.
|
|
BranchProbability MachineBlockPlacement::collectViableSuccessors(
|
|
MachineBasicBlock *BB, BlockChain &Chain, const BlockFilterSet *BlockFilter,
|
|
SmallVector<MachineBasicBlock *, 4> &Successors) {
|
|
// Adjust edge probabilities by excluding edges pointing to blocks that is
|
|
// either not in BlockFilter or is already in the current chain. Consider the
|
|
// following CFG:
|
|
//
|
|
// --->A
|
|
// | / \
|
|
// | B C
|
|
// | \ / \
|
|
// ----D E
|
|
//
|
|
// Assume A->C is very hot (>90%), and C->D has a 50% probability, then after
|
|
// A->C is chosen as a fall-through, D won't be selected as a successor of C
|
|
// due to CFG constraint (the probability of C->D is not greater than
|
|
// HotProb to break top-order). If we exclude E that is not in BlockFilter
|
|
// when calculating the probability of C->D, D will be selected and we
|
|
// will get A C D B as the layout of this loop.
|
|
auto AdjustedSumProb = BranchProbability::getOne();
|
|
for (MachineBasicBlock *Succ : BB->successors()) {
|
|
bool SkipSucc = false;
|
|
if (Succ->isEHPad() || (BlockFilter && !BlockFilter->count(Succ))) {
|
|
SkipSucc = true;
|
|
} else {
|
|
BlockChain *SuccChain = BlockToChain[Succ];
|
|
if (SuccChain == &Chain) {
|
|
SkipSucc = true;
|
|
} else if (Succ != *SuccChain->begin()) {
|
|
DEBUG(dbgs() << " " << getBlockName(Succ) << " -> Mid chain!\n");
|
|
continue;
|
|
}
|
|
}
|
|
if (SkipSucc)
|
|
AdjustedSumProb -= MBPI->getEdgeProbability(BB, Succ);
|
|
else
|
|
Successors.push_back(Succ);
|
|
}
|
|
|
|
return AdjustedSumProb;
|
|
}
|
|
|
|
/// The helper function returns the branch probability that is adjusted
|
|
/// or normalized over the new total \p AdjustedSumProb.
|
|
static BranchProbability
|
|
getAdjustedProbability(BranchProbability OrigProb,
|
|
BranchProbability AdjustedSumProb) {
|
|
BranchProbability SuccProb;
|
|
uint32_t SuccProbN = OrigProb.getNumerator();
|
|
uint32_t SuccProbD = AdjustedSumProb.getNumerator();
|
|
if (SuccProbN >= SuccProbD)
|
|
SuccProb = BranchProbability::getOne();
|
|
else
|
|
SuccProb = BranchProbability(SuccProbN, SuccProbD);
|
|
|
|
return SuccProb;
|
|
}
|
|
|
|
/// When the option OutlineOptionalBranches is on, this method
|
|
/// checks if the fallthrough candidate block \p Succ (of block
|
|
/// \p BB) also has other unscheduled predecessor blocks which
|
|
/// are also successors of \p BB (forming triangular shape CFG).
|
|
/// If none of such predecessors are small, it returns true.
|
|
/// The caller can choose to select \p Succ as the layout successors
|
|
/// so that \p Succ's predecessors (optional branches) can be
|
|
/// outlined.
|
|
/// FIXME: fold this with more general layout cost analysis.
|
|
bool MachineBlockPlacement::shouldPredBlockBeOutlined(
|
|
MachineBasicBlock *BB, MachineBasicBlock *Succ, BlockChain &Chain,
|
|
const BlockFilterSet *BlockFilter, BranchProbability SuccProb,
|
|
BranchProbability HotProb) {
|
|
if (!OutlineOptionalBranches)
|
|
return false;
|
|
// If we outline optional branches, look whether Succ is unavoidable, i.e.
|
|
// dominates all terminators of the MachineFunction. If it does, other
|
|
// successors must be optional. Don't do this for cold branches.
|
|
if (SuccProb > HotProb.getCompl() && UnavoidableBlocks.count(Succ) > 0) {
|
|
for (MachineBasicBlock *Pred : Succ->predecessors()) {
|
|
// Check whether there is an unplaced optional branch.
|
|
if (Pred == Succ || (BlockFilter && !BlockFilter->count(Pred)) ||
|
|
BlockToChain[Pred] == &Chain)
|
|
continue;
|
|
// Check whether the optional branch has exactly one BB.
|
|
if (Pred->pred_size() > 1 || *Pred->pred_begin() != BB)
|
|
continue;
|
|
// Check whether the optional branch is small.
|
|
if (Pred->size() < OutlineOptionalThreshold)
|
|
return false;
|
|
}
|
|
return true;
|
|
} else
|
|
return false;
|
|
}
|
|
|
|
// When profile is not present, return the StaticLikelyProb.
|
|
// When profile is available, we need to handle the triangle-shape CFG.
|
|
static BranchProbability getLayoutSuccessorProbThreshold(
|
|
MachineBasicBlock *BB) {
|
|
if (!BB->getParent()->getFunction()->getEntryCount())
|
|
return BranchProbability(StaticLikelyProb, 100);
|
|
if (BB->succ_size() == 2) {
|
|
const MachineBasicBlock *Succ1 = *BB->succ_begin();
|
|
const MachineBasicBlock *Succ2 = *(BB->succ_begin() + 1);
|
|
if (Succ1->isSuccessor(Succ2) || Succ2->isSuccessor(Succ1)) {
|
|
/* See case 1 below for the cost analysis. For BB->Succ to
|
|
* be taken with smaller cost, the following needs to hold:
|
|
* Prob(BB->Succ) > 2* Prob(BB->Pred)
|
|
* So the threshold T
|
|
* T = 2 * (1-Prob(BB->Pred). Since T + Prob(BB->Pred) == 1,
|
|
* We have T + T/2 = 1, i.e. T = 2/3. Also adding user specified
|
|
* branch bias, we have
|
|
* T = (2/3)*(ProfileLikelyProb/50)
|
|
* = (2*ProfileLikelyProb)/150)
|
|
*/
|
|
return BranchProbability(2 * ProfileLikelyProb, 150);
|
|
}
|
|
}
|
|
return BranchProbability(ProfileLikelyProb, 100);
|
|
}
|
|
|
|
/// Checks to see if the layout candidate block \p Succ has a better layout
|
|
/// predecessor than \c BB. If yes, returns true.
|
|
bool MachineBlockPlacement::hasBetterLayoutPredecessor(
|
|
MachineBasicBlock *BB, MachineBasicBlock *Succ, BlockChain &SuccChain,
|
|
BranchProbability SuccProb, BranchProbability RealSuccProb,
|
|
BlockChain &Chain, const BlockFilterSet *BlockFilter) {
|
|
|
|
// There isn't a better layout when there are no unscheduled predecessors.
|
|
if (SuccChain.UnscheduledPredecessors == 0)
|
|
return false;
|
|
|
|
// There are two basic scenarios here:
|
|
// -------------------------------------
|
|
// Case 1: triangular shape CFG (if-then):
|
|
// BB
|
|
// | \
|
|
// | \
|
|
// | Pred
|
|
// | /
|
|
// Succ
|
|
// In this case, we are evaluating whether to select edge -> Succ, e.g.
|
|
// set Succ as the layout successor of BB. Picking Succ as BB's
|
|
// successor breaks the CFG constraints (FIXME: define these constraints).
|
|
// With this layout, Pred BB
|
|
// is forced to be outlined, so the overall cost will be cost of the
|
|
// branch taken from BB to Pred, plus the cost of back taken branch
|
|
// from Pred to Succ, as well as the additional cost associated
|
|
// with the needed unconditional jump instruction from Pred To Succ.
|
|
|
|
// The cost of the topological order layout is the taken branch cost
|
|
// from BB to Succ, so to make BB->Succ a viable candidate, the following
|
|
// must hold:
|
|
// 2 * freq(BB->Pred) * taken_branch_cost + unconditional_jump_cost
|
|
// < freq(BB->Succ) * taken_branch_cost.
|
|
// Ignoring unconditional jump cost, we get
|
|
// freq(BB->Succ) > 2 * freq(BB->Pred), i.e.,
|
|
// prob(BB->Succ) > 2 * prob(BB->Pred)
|
|
//
|
|
// When real profile data is available, we can precisely compute the
|
|
// probability threshold that is needed for edge BB->Succ to be considered.
|
|
// Without profile data, the heuristic requires the branch bias to be
|
|
// a lot larger to make sure the signal is very strong (e.g. 80% default).
|
|
// -----------------------------------------------------------------
|
|
// Case 2: diamond like CFG (if-then-else):
|
|
// S
|
|
// / \
|
|
// | \
|
|
// BB Pred
|
|
// \ /
|
|
// Succ
|
|
// ..
|
|
//
|
|
// The current block is BB and edge BB->Succ is now being evaluated.
|
|
// Note that edge S->BB was previously already selected because
|
|
// prob(S->BB) > prob(S->Pred).
|
|
// At this point, 2 blocks can be placed after BB: Pred or Succ. If we
|
|
// choose Pred, we will have a topological ordering as shown on the left
|
|
// in the picture below. If we choose Succ, we have the solution as shown
|
|
// on the right:
|
|
//
|
|
// topo-order:
|
|
//
|
|
// S----- ---S
|
|
// | | | |
|
|
// ---BB | | BB
|
|
// | | | |
|
|
// | pred-- | Succ--
|
|
// | | | |
|
|
// ---succ ---pred--
|
|
//
|
|
// cost = freq(S->Pred) + freq(BB->Succ) cost = 2 * freq (S->Pred)
|
|
// = freq(S->Pred) + freq(S->BB)
|
|
//
|
|
// If we have profile data (i.e, branch probabilities can be trusted), the
|
|
// cost (number of taken branches) with layout S->BB->Succ->Pred is 2 *
|
|
// freq(S->Pred) while the cost of topo order is freq(S->Pred) + freq(S->BB).
|
|
// We know Prob(S->BB) > Prob(S->Pred), so freq(S->BB) > freq(S->Pred), which
|
|
// means the cost of topological order is greater.
|
|
// When profile data is not available, however, we need to be more
|
|
// conservative. If the branch prediction is wrong, breaking the topo-order
|
|
// will actually yield a layout with large cost. For this reason, we need
|
|
// strong biased branch at block S with Prob(S->BB) in order to select
|
|
// BB->Succ. This is equivalent to looking the CFG backward with backward
|
|
// edge: Prob(Succ->BB) needs to >= HotProb in order to be selected (without
|
|
// profile data).
|
|
// --------------------------------------------------------------------------
|
|
// Case 3: forked diamond
|
|
// S
|
|
// / \
|
|
// / \
|
|
// BB Pred
|
|
// | \ / |
|
|
// | \ / |
|
|
// | X |
|
|
// | / \ |
|
|
// | / \ |
|
|
// S1 S2
|
|
//
|
|
// The current block is BB and edge BB->S1 is now being evaluated.
|
|
// As above S->BB was already selected because
|
|
// prob(S->BB) > prob(S->Pred). Assume that prob(BB->S1) >= prob(BB->S2).
|
|
//
|
|
// topo-order:
|
|
//
|
|
// S-------| ---S
|
|
// | | | |
|
|
// ---BB | | BB
|
|
// | | | |
|
|
// | Pred----| | S1----
|
|
// | | | |
|
|
// --(S1 or S2) ---Pred--
|
|
//
|
|
// topo-cost = freq(S->Pred) + freq(BB->S1) + freq(BB->S2)
|
|
// + min(freq(Pred->S1), freq(Pred->S2))
|
|
// Non-topo-order cost:
|
|
// In the worst case, S2 will not get laid out after Pred.
|
|
// non-topo-cost = 2 * freq(S->Pred) + freq(BB->S2).
|
|
// To be conservative, we can assume that min(freq(Pred->S1), freq(Pred->S2))
|
|
// is 0. Then the non topo layout is better when
|
|
// freq(S->Pred) < freq(BB->S1).
|
|
// This is exactly what is checked below.
|
|
// Note there are other shapes that apply (Pred may not be a single block,
|
|
// but they all fit this general pattern.)
|
|
BranchProbability HotProb = getLayoutSuccessorProbThreshold(BB);
|
|
|
|
// Make sure that a hot successor doesn't have a globally more
|
|
// important predecessor.
|
|
BlockFrequency CandidateEdgeFreq = MBFI->getBlockFreq(BB) * RealSuccProb;
|
|
bool BadCFGConflict = false;
|
|
|
|
for (MachineBasicBlock *Pred : Succ->predecessors()) {
|
|
if (Pred == Succ || BlockToChain[Pred] == &SuccChain ||
|
|
(BlockFilter && !BlockFilter->count(Pred)) ||
|
|
BlockToChain[Pred] == &Chain)
|
|
continue;
|
|
// Do backward checking.
|
|
// For all cases above, we need a backward checking to filter out edges that
|
|
// are not 'strongly' biased. With profile data available, the check is
|
|
// mostly redundant for case 2 (when threshold prob is set at 50%) unless S
|
|
// has more than two successors.
|
|
// BB Pred
|
|
// \ /
|
|
// Succ
|
|
// We select edge BB->Succ if
|
|
// freq(BB->Succ) > freq(Succ) * HotProb
|
|
// i.e. freq(BB->Succ) > freq(BB->Succ) * HotProb + freq(Pred->Succ) *
|
|
// HotProb
|
|
// i.e. freq((BB->Succ) * (1 - HotProb) > freq(Pred->Succ) * HotProb
|
|
// Case 1 is covered too, because the first equation reduces to:
|
|
// prob(BB->Succ) > HotProb. (freq(Succ) = freq(BB) for a triangle)
|
|
BlockFrequency PredEdgeFreq =
|
|
MBFI->getBlockFreq(Pred) * MBPI->getEdgeProbability(Pred, Succ);
|
|
if (PredEdgeFreq * HotProb >= CandidateEdgeFreq * HotProb.getCompl()) {
|
|
BadCFGConflict = true;
|
|
break;
|
|
}
|
|
}
|
|
|
|
if (BadCFGConflict) {
|
|
DEBUG(dbgs() << " Not a candidate: " << getBlockName(Succ) << " -> " << SuccProb
|
|
<< " (prob) (non-cold CFG conflict)\n");
|
|
return true;
|
|
}
|
|
|
|
return false;
|
|
}
|
|
|
|
/// \brief Select the best successor for a block.
|
|
///
|
|
/// This looks across all successors of a particular block and attempts to
|
|
/// select the "best" one to be the layout successor. It only considers direct
|
|
/// successors which also pass the block filter. It will attempt to avoid
|
|
/// breaking CFG structure, but cave and break such structures in the case of
|
|
/// very hot successor edges.
|
|
///
|
|
/// \returns The best successor block found, or null if none are viable.
|
|
MachineBasicBlock *
|
|
MachineBlockPlacement::selectBestSuccessor(MachineBasicBlock *BB,
|
|
BlockChain &Chain,
|
|
const BlockFilterSet *BlockFilter) {
|
|
const BranchProbability HotProb(StaticLikelyProb, 100);
|
|
|
|
MachineBasicBlock *BestSucc = nullptr;
|
|
auto BestProb = BranchProbability::getZero();
|
|
|
|
SmallVector<MachineBasicBlock *, 4> Successors;
|
|
auto AdjustedSumProb =
|
|
collectViableSuccessors(BB, Chain, BlockFilter, Successors);
|
|
|
|
DEBUG(dbgs() << "Selecting best successor for: " << getBlockName(BB) << "\n");
|
|
for (MachineBasicBlock *Succ : Successors) {
|
|
auto RealSuccProb = MBPI->getEdgeProbability(BB, Succ);
|
|
BranchProbability SuccProb =
|
|
getAdjustedProbability(RealSuccProb, AdjustedSumProb);
|
|
|
|
// This heuristic is off by default.
|
|
if (shouldPredBlockBeOutlined(BB, Succ, Chain, BlockFilter, SuccProb,
|
|
HotProb))
|
|
return Succ;
|
|
|
|
BlockChain &SuccChain = *BlockToChain[Succ];
|
|
// Skip the edge \c BB->Succ if block \c Succ has a better layout
|
|
// predecessor that yields lower global cost.
|
|
if (hasBetterLayoutPredecessor(BB, Succ, SuccChain, SuccProb, RealSuccProb,
|
|
Chain, BlockFilter))
|
|
continue;
|
|
|
|
DEBUG(
|
|
dbgs() << " Candidate: " << getBlockName(Succ) << ", probability: "
|
|
<< SuccProb
|
|
<< (SuccChain.UnscheduledPredecessors != 0 ? " (CFG break)" : "")
|
|
<< "\n");
|
|
|
|
if (BestSucc && BestProb >= SuccProb) {
|
|
DEBUG(dbgs() << " Not the best candidate, continuing\n");
|
|
continue;
|
|
}
|
|
|
|
DEBUG(dbgs() << " Setting it as best candidate\n");
|
|
BestSucc = Succ;
|
|
BestProb = SuccProb;
|
|
}
|
|
if (BestSucc)
|
|
DEBUG(dbgs() << " Selected: " << getBlockName(BestSucc) << "\n");
|
|
|
|
return BestSucc;
|
|
}
|
|
|
|
/// \brief Select the best block from a worklist.
|
|
///
|
|
/// This looks through the provided worklist as a list of candidate basic
|
|
/// blocks and select the most profitable one to place. The definition of
|
|
/// profitable only really makes sense in the context of a loop. This returns
|
|
/// the most frequently visited block in the worklist, which in the case of
|
|
/// a loop, is the one most desirable to be physically close to the rest of the
|
|
/// loop body in order to improve i-cache behavior.
|
|
///
|
|
/// \returns The best block found, or null if none are viable.
|
|
MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
|
|
BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList) {
|
|
// Once we need to walk the worklist looking for a candidate, cleanup the
|
|
// worklist of already placed entries.
|
|
// FIXME: If this shows up on profiles, it could be folded (at the cost of
|
|
// some code complexity) into the loop below.
|
|
WorkList.erase(remove_if(WorkList,
|
|
[&](MachineBasicBlock *BB) {
|
|
return BlockToChain.lookup(BB) == &Chain;
|
|
}),
|
|
WorkList.end());
|
|
|
|
if (WorkList.empty())
|
|
return nullptr;
|
|
|
|
bool IsEHPad = WorkList[0]->isEHPad();
|
|
|
|
MachineBasicBlock *BestBlock = nullptr;
|
|
BlockFrequency BestFreq;
|
|
for (MachineBasicBlock *MBB : WorkList) {
|
|
assert(MBB->isEHPad() == IsEHPad);
|
|
|
|
BlockChain &SuccChain = *BlockToChain[MBB];
|
|
if (&SuccChain == &Chain)
|
|
continue;
|
|
|
|
assert(SuccChain.UnscheduledPredecessors == 0 && "Found CFG-violating block");
|
|
|
|
BlockFrequency CandidateFreq = MBFI->getBlockFreq(MBB);
|
|
DEBUG(dbgs() << " " << getBlockName(MBB) << " -> ";
|
|
MBFI->printBlockFreq(dbgs(), CandidateFreq) << " (freq)\n");
|
|
|
|
// For ehpad, we layout the least probable first as to avoid jumping back
|
|
// from least probable landingpads to more probable ones.
|
|
//
|
|
// FIXME: Using probability is probably (!) not the best way to achieve
|
|
// this. We should probably have a more principled approach to layout
|
|
// cleanup code.
|
|
//
|
|
// The goal is to get:
|
|
//
|
|
// +--------------------------+
|
|
// | V
|
|
// InnerLp -> InnerCleanup OuterLp -> OuterCleanup -> Resume
|
|
//
|
|
// Rather than:
|
|
//
|
|
// +-------------------------------------+
|
|
// V |
|
|
// OuterLp -> OuterCleanup -> Resume InnerLp -> InnerCleanup
|
|
if (BestBlock && (IsEHPad ^ (BestFreq >= CandidateFreq)))
|
|
continue;
|
|
|
|
BestBlock = MBB;
|
|
BestFreq = CandidateFreq;
|
|
}
|
|
|
|
return BestBlock;
|
|
}
|
|
|
|
/// \brief Retrieve the first unplaced basic block.
|
|
///
|
|
/// This routine is called when we are unable to use the CFG to walk through
|
|
/// all of the basic blocks and form a chain due to unnatural loops in the CFG.
|
|
/// We walk through the function's blocks in order, starting from the
|
|
/// LastUnplacedBlockIt. We update this iterator on each call to avoid
|
|
/// re-scanning the entire sequence on repeated calls to this routine.
|
|
MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
|
|
const BlockChain &PlacedChain,
|
|
MachineFunction::iterator &PrevUnplacedBlockIt,
|
|
const BlockFilterSet *BlockFilter) {
|
|
for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F->end(); I != E;
|
|
++I) {
|
|
if (BlockFilter && !BlockFilter->count(&*I))
|
|
continue;
|
|
if (BlockToChain[&*I] != &PlacedChain) {
|
|
PrevUnplacedBlockIt = I;
|
|
// Now select the head of the chain to which the unplaced block belongs
|
|
// as the block to place. This will force the entire chain to be placed,
|
|
// and satisfies the requirements of merging chains.
|
|
return *BlockToChain[&*I]->begin();
|
|
}
|
|
}
|
|
return nullptr;
|
|
}
|
|
|
|
void MachineBlockPlacement::fillWorkLists(
|
|
MachineBasicBlock *MBB,
|
|
SmallPtrSetImpl<BlockChain *> &UpdatedPreds,
|
|
const BlockFilterSet *BlockFilter = nullptr) {
|
|
BlockChain &Chain = *BlockToChain[MBB];
|
|
if (!UpdatedPreds.insert(&Chain).second)
|
|
return;
|
|
|
|
assert(Chain.UnscheduledPredecessors == 0);
|
|
for (MachineBasicBlock *ChainBB : Chain) {
|
|
assert(BlockToChain[ChainBB] == &Chain);
|
|
for (MachineBasicBlock *Pred : ChainBB->predecessors()) {
|
|
if (BlockFilter && !BlockFilter->count(Pred))
|
|
continue;
|
|
if (BlockToChain[Pred] == &Chain)
|
|
continue;
|
|
++Chain.UnscheduledPredecessors;
|
|
}
|
|
}
|
|
|
|
if (Chain.UnscheduledPredecessors != 0)
|
|
return;
|
|
|
|
MBB = *Chain.begin();
|
|
if (MBB->isEHPad())
|
|
EHPadWorkList.push_back(MBB);
|
|
else
|
|
BlockWorkList.push_back(MBB);
|
|
}
|
|
|
|
void MachineBlockPlacement::buildChain(
|
|
MachineBasicBlock *BB, BlockChain &Chain,
|
|
BlockFilterSet *BlockFilter) {
|
|
assert(BB && "BB must not be null.\n");
|
|
assert(BlockToChain[BB] == &Chain && "BlockToChainMap mis-match.\n");
|
|
MachineFunction::iterator PrevUnplacedBlockIt = F->begin();
|
|
|
|
MachineBasicBlock *LoopHeaderBB = BB;
|
|
markChainSuccessors(Chain, LoopHeaderBB, BlockFilter);
|
|
BB = *std::prev(Chain.end());
|
|
for (;;) {
|
|
assert(BB && "null block found at end of chain in loop.");
|
|
assert(BlockToChain[BB] == &Chain && "BlockToChainMap mis-match in loop.");
|
|
assert(*std::prev(Chain.end()) == BB && "BB Not found at end of chain.");
|
|
|
|
|
|
// Look for the best viable successor if there is one to place immediately
|
|
// after this block.
|
|
MachineBasicBlock *BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
|
|
|
|
// If an immediate successor isn't available, look for the best viable
|
|
// block among those we've identified as not violating the loop's CFG at
|
|
// this point. This won't be a fallthrough, but it will increase locality.
|
|
if (!BestSucc)
|
|
BestSucc = selectBestCandidateBlock(Chain, BlockWorkList);
|
|
if (!BestSucc)
|
|
BestSucc = selectBestCandidateBlock(Chain, EHPadWorkList);
|
|
|
|
if (!BestSucc) {
|
|
BestSucc = getFirstUnplacedBlock(Chain, PrevUnplacedBlockIt, BlockFilter);
|
|
if (!BestSucc)
|
|
break;
|
|
|
|
DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
|
|
"layout successor until the CFG reduces\n");
|
|
}
|
|
|
|
// Placement may have changed tail duplication opportunities.
|
|
// Check for that now.
|
|
if (TailDupPlacement && BestSucc) {
|
|
// If the chosen successor was duplicated into all its predecessors,
|
|
// don't bother laying it out, just go round the loop again with BB as
|
|
// the chain end.
|
|
if (repeatedlyTailDuplicateBlock(BestSucc, BB, LoopHeaderBB, Chain,
|
|
BlockFilter, PrevUnplacedBlockIt))
|
|
continue;
|
|
}
|
|
|
|
// Place this block, updating the datastructures to reflect its placement.
|
|
BlockChain &SuccChain = *BlockToChain[BestSucc];
|
|
// Zero out UnscheduledPredecessors for the successor we're about to merge in case
|
|
// we selected a successor that didn't fit naturally into the CFG.
|
|
SuccChain.UnscheduledPredecessors = 0;
|
|
DEBUG(dbgs() << "Merging from " << getBlockName(BB) << " to "
|
|
<< getBlockName(BestSucc) << "\n");
|
|
markChainSuccessors(SuccChain, LoopHeaderBB, BlockFilter);
|
|
Chain.merge(BestSucc, &SuccChain);
|
|
BB = *std::prev(Chain.end());
|
|
}
|
|
|
|
DEBUG(dbgs() << "Finished forming chain for header block "
|
|
<< getBlockName(*Chain.begin()) << "\n");
|
|
}
|
|
|
|
/// \brief Find the best loop top block for layout.
|
|
///
|
|
/// Look for a block which is strictly better than the loop header for laying
|
|
/// out at the top of the loop. This looks for one and only one pattern:
|
|
/// a latch block with no conditional exit. This block will cause a conditional
|
|
/// jump around it or will be the bottom of the loop if we lay it out in place,
|
|
/// but if it it doesn't end up at the bottom of the loop for any reason,
|
|
/// rotation alone won't fix it. Because such a block will always result in an
|
|
/// unconditional jump (for the backedge) rotating it in front of the loop
|
|
/// header is always profitable.
|
|
MachineBasicBlock *
|
|
MachineBlockPlacement::findBestLoopTop(MachineLoop &L,
|
|
const BlockFilterSet &LoopBlockSet) {
|
|
// Placing the latch block before the header may introduce an extra branch
|
|
// that skips this block the first time the loop is executed, which we want
|
|
// to avoid when optimising for size.
|
|
// FIXME: in theory there is a case that does not introduce a new branch,
|
|
// i.e. when the layout predecessor does not fallthrough to the loop header.
|
|
// In practice this never happens though: there always seems to be a preheader
|
|
// that can fallthrough and that is also placed before the header.
|
|
if (F->getFunction()->optForSize())
|
|
return L.getHeader();
|
|
|
|
// Check that the header hasn't been fused with a preheader block due to
|
|
// crazy branches. If it has, we need to start with the header at the top to
|
|
// prevent pulling the preheader into the loop body.
|
|
BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
|
|
if (!LoopBlockSet.count(*HeaderChain.begin()))
|
|
return L.getHeader();
|
|
|
|
DEBUG(dbgs() << "Finding best loop top for: " << getBlockName(L.getHeader())
|
|
<< "\n");
|
|
|
|
BlockFrequency BestPredFreq;
|
|
MachineBasicBlock *BestPred = nullptr;
|
|
for (MachineBasicBlock *Pred : L.getHeader()->predecessors()) {
|
|
if (!LoopBlockSet.count(Pred))
|
|
continue;
|
|
DEBUG(dbgs() << " header pred: " << getBlockName(Pred) << ", has "
|
|
<< Pred->succ_size() << " successors, ";
|
|
MBFI->printBlockFreq(dbgs(), Pred) << " freq\n");
|
|
if (Pred->succ_size() > 1)
|
|
continue;
|
|
|
|
BlockFrequency PredFreq = MBFI->getBlockFreq(Pred);
|
|
if (!BestPred || PredFreq > BestPredFreq ||
|
|
(!(PredFreq < BestPredFreq) &&
|
|
Pred->isLayoutSuccessor(L.getHeader()))) {
|
|
BestPred = Pred;
|
|
BestPredFreq = PredFreq;
|
|
}
|
|
}
|
|
|
|
// If no direct predecessor is fine, just use the loop header.
|
|
if (!BestPred) {
|
|
DEBUG(dbgs() << " final top unchanged\n");
|
|
return L.getHeader();
|
|
}
|
|
|
|
// Walk backwards through any straight line of predecessors.
|
|
while (BestPred->pred_size() == 1 &&
|
|
(*BestPred->pred_begin())->succ_size() == 1 &&
|
|
*BestPred->pred_begin() != L.getHeader())
|
|
BestPred = *BestPred->pred_begin();
|
|
|
|
DEBUG(dbgs() << " final top: " << getBlockName(BestPred) << "\n");
|
|
return BestPred;
|
|
}
|
|
|
|
/// \brief Find the best loop exiting block for layout.
|
|
///
|
|
/// This routine implements the logic to analyze the loop looking for the best
|
|
/// block to layout at the top of the loop. Typically this is done to maximize
|
|
/// fallthrough opportunities.
|
|
MachineBasicBlock *
|
|
MachineBlockPlacement::findBestLoopExit(MachineLoop &L,
|
|
const BlockFilterSet &LoopBlockSet) {
|
|
// We don't want to layout the loop linearly in all cases. If the loop header
|
|
// is just a normal basic block in the loop, we want to look for what block
|
|
// within the loop is the best one to layout at the top. However, if the loop
|
|
// header has be pre-merged into a chain due to predecessors not having
|
|
// analyzable branches, *and* the predecessor it is merged with is *not* part
|
|
// of the loop, rotating the header into the middle of the loop will create
|
|
// a non-contiguous range of blocks which is Very Bad. So start with the
|
|
// header and only rotate if safe.
|
|
BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
|
|
if (!LoopBlockSet.count(*HeaderChain.begin()))
|
|
return nullptr;
|
|
|
|
BlockFrequency BestExitEdgeFreq;
|
|
unsigned BestExitLoopDepth = 0;
|
|
MachineBasicBlock *ExitingBB = nullptr;
|
|
// If there are exits to outer loops, loop rotation can severely limit
|
|
// fallthrough opportunities unless it selects such an exit. Keep a set of
|
|
// blocks where rotating to exit with that block will reach an outer loop.
|
|
SmallPtrSet<MachineBasicBlock *, 4> BlocksExitingToOuterLoop;
|
|
|
|
DEBUG(dbgs() << "Finding best loop exit for: " << getBlockName(L.getHeader())
|
|
<< "\n");
|
|
for (MachineBasicBlock *MBB : L.getBlocks()) {
|
|
BlockChain &Chain = *BlockToChain[MBB];
|
|
// Ensure that this block is at the end of a chain; otherwise it could be
|
|
// mid-way through an inner loop or a successor of an unanalyzable branch.
|
|
if (MBB != *std::prev(Chain.end()))
|
|
continue;
|
|
|
|
// Now walk the successors. We need to establish whether this has a viable
|
|
// exiting successor and whether it has a viable non-exiting successor.
|
|
// We store the old exiting state and restore it if a viable looping
|
|
// successor isn't found.
|
|
MachineBasicBlock *OldExitingBB = ExitingBB;
|
|
BlockFrequency OldBestExitEdgeFreq = BestExitEdgeFreq;
|
|
bool HasLoopingSucc = false;
|
|
for (MachineBasicBlock *Succ : MBB->successors()) {
|
|
if (Succ->isEHPad())
|
|
continue;
|
|
if (Succ == MBB)
|
|
continue;
|
|
BlockChain &SuccChain = *BlockToChain[Succ];
|
|
// Don't split chains, either this chain or the successor's chain.
|
|
if (&Chain == &SuccChain) {
|
|
DEBUG(dbgs() << " exiting: " << getBlockName(MBB) << " -> "
|
|
<< getBlockName(Succ) << " (chain conflict)\n");
|
|
continue;
|
|
}
|
|
|
|
auto SuccProb = MBPI->getEdgeProbability(MBB, Succ);
|
|
if (LoopBlockSet.count(Succ)) {
|
|
DEBUG(dbgs() << " looping: " << getBlockName(MBB) << " -> "
|
|
<< getBlockName(Succ) << " (" << SuccProb << ")\n");
|
|
HasLoopingSucc = true;
|
|
continue;
|
|
}
|
|
|
|
unsigned SuccLoopDepth = 0;
|
|
if (MachineLoop *ExitLoop = MLI->getLoopFor(Succ)) {
|
|
SuccLoopDepth = ExitLoop->getLoopDepth();
|
|
if (ExitLoop->contains(&L))
|
|
BlocksExitingToOuterLoop.insert(MBB);
|
|
}
|
|
|
|
BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(MBB) * SuccProb;
|
|
DEBUG(dbgs() << " exiting: " << getBlockName(MBB) << " -> "
|
|
<< getBlockName(Succ) << " [L:" << SuccLoopDepth << "] (";
|
|
MBFI->printBlockFreq(dbgs(), ExitEdgeFreq) << ")\n");
|
|
// Note that we bias this toward an existing layout successor to retain
|
|
// incoming order in the absence of better information. The exit must have
|
|
// a frequency higher than the current exit before we consider breaking
|
|
// the layout.
|
|
BranchProbability Bias(100 - ExitBlockBias, 100);
|
|
if (!ExitingBB || SuccLoopDepth > BestExitLoopDepth ||
|
|
ExitEdgeFreq > BestExitEdgeFreq ||
|
|
(MBB->isLayoutSuccessor(Succ) &&
|
|
!(ExitEdgeFreq < BestExitEdgeFreq * Bias))) {
|
|
BestExitEdgeFreq = ExitEdgeFreq;
|
|
ExitingBB = MBB;
|
|
}
|
|
}
|
|
|
|
if (!HasLoopingSucc) {
|
|
// Restore the old exiting state, no viable looping successor was found.
|
|
ExitingBB = OldExitingBB;
|
|
BestExitEdgeFreq = OldBestExitEdgeFreq;
|
|
}
|
|
}
|
|
// Without a candidate exiting block or with only a single block in the
|
|
// loop, just use the loop header to layout the loop.
|
|
if (!ExitingBB) {
|
|
DEBUG(dbgs() << " No other candidate exit blocks, using loop header\n");
|
|
return nullptr;
|
|
}
|
|
if (L.getNumBlocks() == 1) {
|
|
DEBUG(dbgs() << " Loop has 1 block, using loop header as exit\n");
|
|
return nullptr;
|
|
}
|
|
|
|
// Also, if we have exit blocks which lead to outer loops but didn't select
|
|
// one of them as the exiting block we are rotating toward, disable loop
|
|
// rotation altogether.
|
|
if (!BlocksExitingToOuterLoop.empty() &&
|
|
!BlocksExitingToOuterLoop.count(ExitingBB))
|
|
return nullptr;
|
|
|
|
DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB) << "\n");
|
|
return ExitingBB;
|
|
}
|
|
|
|
/// \brief Attempt to rotate an exiting block to the bottom of the loop.
|
|
///
|
|
/// Once we have built a chain, try to rotate it to line up the hot exit block
|
|
/// with fallthrough out of the loop if doing so doesn't introduce unnecessary
|
|
/// branches. For example, if the loop has fallthrough into its header and out
|
|
/// of its bottom already, don't rotate it.
|
|
void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain,
|
|
MachineBasicBlock *ExitingBB,
|
|
const BlockFilterSet &LoopBlockSet) {
|
|
if (!ExitingBB)
|
|
return;
|
|
|
|
MachineBasicBlock *Top = *LoopChain.begin();
|
|
bool ViableTopFallthrough = false;
|
|
for (MachineBasicBlock *Pred : Top->predecessors()) {
|
|
BlockChain *PredChain = BlockToChain[Pred];
|
|
if (!LoopBlockSet.count(Pred) &&
|
|
(!PredChain || Pred == *std::prev(PredChain->end()))) {
|
|
ViableTopFallthrough = true;
|
|
break;
|
|
}
|
|
}
|
|
|
|
// If the header has viable fallthrough, check whether the current loop
|
|
// bottom is a viable exiting block. If so, bail out as rotating will
|
|
// introduce an unnecessary branch.
|
|
if (ViableTopFallthrough) {
|
|
MachineBasicBlock *Bottom = *std::prev(LoopChain.end());
|
|
for (MachineBasicBlock *Succ : Bottom->successors()) {
|
|
BlockChain *SuccChain = BlockToChain[Succ];
|
|
if (!LoopBlockSet.count(Succ) &&
|
|
(!SuccChain || Succ == *SuccChain->begin()))
|
|
return;
|
|
}
|
|
}
|
|
|
|
BlockChain::iterator ExitIt = find(LoopChain, ExitingBB);
|
|
if (ExitIt == LoopChain.end())
|
|
return;
|
|
|
|
std::rotate(LoopChain.begin(), std::next(ExitIt), LoopChain.end());
|
|
}
|
|
|
|
/// \brief Attempt to rotate a loop based on profile data to reduce branch cost.
|
|
///
|
|
/// With profile data, we can determine the cost in terms of missed fall through
|
|
/// opportunities when rotating a loop chain and select the best rotation.
|
|
/// Basically, there are three kinds of cost to consider for each rotation:
|
|
/// 1. The possibly missed fall through edge (if it exists) from BB out of
|
|
/// the loop to the loop header.
|
|
/// 2. The possibly missed fall through edges (if they exist) from the loop
|
|
/// exits to BB out of the loop.
|
|
/// 3. The missed fall through edge (if it exists) from the last BB to the
|
|
/// first BB in the loop chain.
|
|
/// Therefore, the cost for a given rotation is the sum of costs listed above.
|
|
/// We select the best rotation with the smallest cost.
|
|
void MachineBlockPlacement::rotateLoopWithProfile(
|
|
BlockChain &LoopChain, MachineLoop &L, const BlockFilterSet &LoopBlockSet) {
|
|
auto HeaderBB = L.getHeader();
|
|
auto HeaderIter = find(LoopChain, HeaderBB);
|
|
auto RotationPos = LoopChain.end();
|
|
|
|
BlockFrequency SmallestRotationCost = BlockFrequency::getMaxFrequency();
|
|
|
|
// A utility lambda that scales up a block frequency by dividing it by a
|
|
// branch probability which is the reciprocal of the scale.
|
|
auto ScaleBlockFrequency = [](BlockFrequency Freq,
|
|
unsigned Scale) -> BlockFrequency {
|
|
if (Scale == 0)
|
|
return 0;
|
|
// Use operator / between BlockFrequency and BranchProbability to implement
|
|
// saturating multiplication.
|
|
return Freq / BranchProbability(1, Scale);
|
|
};
|
|
|
|
// Compute the cost of the missed fall-through edge to the loop header if the
|
|
// chain head is not the loop header. As we only consider natural loops with
|
|
// single header, this computation can be done only once.
|
|
BlockFrequency HeaderFallThroughCost(0);
|
|
for (auto *Pred : HeaderBB->predecessors()) {
|
|
BlockChain *PredChain = BlockToChain[Pred];
|
|
if (!LoopBlockSet.count(Pred) &&
|
|
(!PredChain || Pred == *std::prev(PredChain->end()))) {
|
|
auto EdgeFreq =
|
|
MBFI->getBlockFreq(Pred) * MBPI->getEdgeProbability(Pred, HeaderBB);
|
|
auto FallThruCost = ScaleBlockFrequency(EdgeFreq, MisfetchCost);
|
|
// If the predecessor has only an unconditional jump to the header, we
|
|
// need to consider the cost of this jump.
|
|
if (Pred->succ_size() == 1)
|
|
FallThruCost += ScaleBlockFrequency(EdgeFreq, JumpInstCost);
|
|
HeaderFallThroughCost = std::max(HeaderFallThroughCost, FallThruCost);
|
|
}
|
|
}
|
|
|
|
// Here we collect all exit blocks in the loop, and for each exit we find out
|
|
// its hottest exit edge. For each loop rotation, we define the loop exit cost
|
|
// as the sum of frequencies of exit edges we collect here, excluding the exit
|
|
// edge from the tail of the loop chain.
|
|
SmallVector<std::pair<MachineBasicBlock *, BlockFrequency>, 4> ExitsWithFreq;
|
|
for (auto BB : LoopChain) {
|
|
auto LargestExitEdgeProb = BranchProbability::getZero();
|
|
for (auto *Succ : BB->successors()) {
|
|
BlockChain *SuccChain = BlockToChain[Succ];
|
|
if (!LoopBlockSet.count(Succ) &&
|
|
(!SuccChain || Succ == *SuccChain->begin())) {
|
|
auto SuccProb = MBPI->getEdgeProbability(BB, Succ);
|
|
LargestExitEdgeProb = std::max(LargestExitEdgeProb, SuccProb);
|
|
}
|
|
}
|
|
if (LargestExitEdgeProb > BranchProbability::getZero()) {
|
|
auto ExitFreq = MBFI->getBlockFreq(BB) * LargestExitEdgeProb;
|
|
ExitsWithFreq.emplace_back(BB, ExitFreq);
|
|
}
|
|
}
|
|
|
|
// In this loop we iterate every block in the loop chain and calculate the
|
|
// cost assuming the block is the head of the loop chain. When the loop ends,
|
|
// we should have found the best candidate as the loop chain's head.
|
|
for (auto Iter = LoopChain.begin(), TailIter = std::prev(LoopChain.end()),
|
|
EndIter = LoopChain.end();
|
|
Iter != EndIter; Iter++, TailIter++) {
|
|
// TailIter is used to track the tail of the loop chain if the block we are
|
|
// checking (pointed by Iter) is the head of the chain.
|
|
if (TailIter == LoopChain.end())
|
|
TailIter = LoopChain.begin();
|
|
|
|
auto TailBB = *TailIter;
|
|
|
|
// Calculate the cost by putting this BB to the top.
|
|
BlockFrequency Cost = 0;
|
|
|
|
// If the current BB is the loop header, we need to take into account the
|
|
// cost of the missed fall through edge from outside of the loop to the
|
|
// header.
|
|
if (Iter != HeaderIter)
|
|
Cost += HeaderFallThroughCost;
|
|
|
|
// Collect the loop exit cost by summing up frequencies of all exit edges
|
|
// except the one from the chain tail.
|
|
for (auto &ExitWithFreq : ExitsWithFreq)
|
|
if (TailBB != ExitWithFreq.first)
|
|
Cost += ExitWithFreq.second;
|
|
|
|
// The cost of breaking the once fall-through edge from the tail to the top
|
|
// of the loop chain. Here we need to consider three cases:
|
|
// 1. If the tail node has only one successor, then we will get an
|
|
// additional jmp instruction. So the cost here is (MisfetchCost +
|
|
// JumpInstCost) * tail node frequency.
|
|
// 2. If the tail node has two successors, then we may still get an
|
|
// additional jmp instruction if the layout successor after the loop
|
|
// chain is not its CFG successor. Note that the more frequently executed
|
|
// jmp instruction will be put ahead of the other one. Assume the
|
|
// frequency of those two branches are x and y, where x is the frequency
|
|
// of the edge to the chain head, then the cost will be
|
|
// (x * MisfetechCost + min(x, y) * JumpInstCost) * tail node frequency.
|
|
// 3. If the tail node has more than two successors (this rarely happens),
|
|
// we won't consider any additional cost.
|
|
if (TailBB->isSuccessor(*Iter)) {
|
|
auto TailBBFreq = MBFI->getBlockFreq(TailBB);
|
|
if (TailBB->succ_size() == 1)
|
|
Cost += ScaleBlockFrequency(TailBBFreq.getFrequency(),
|
|
MisfetchCost + JumpInstCost);
|
|
else if (TailBB->succ_size() == 2) {
|
|
auto TailToHeadProb = MBPI->getEdgeProbability(TailBB, *Iter);
|
|
auto TailToHeadFreq = TailBBFreq * TailToHeadProb;
|
|
auto ColderEdgeFreq = TailToHeadProb > BranchProbability(1, 2)
|
|
? TailBBFreq * TailToHeadProb.getCompl()
|
|
: TailToHeadFreq;
|
|
Cost += ScaleBlockFrequency(TailToHeadFreq, MisfetchCost) +
|
|
ScaleBlockFrequency(ColderEdgeFreq, JumpInstCost);
|
|
}
|
|
}
|
|
|
|
DEBUG(dbgs() << "The cost of loop rotation by making " << getBlockName(*Iter)
|
|
<< " to the top: " << Cost.getFrequency() << "\n");
|
|
|
|
if (Cost < SmallestRotationCost) {
|
|
SmallestRotationCost = Cost;
|
|
RotationPos = Iter;
|
|
}
|
|
}
|
|
|
|
if (RotationPos != LoopChain.end()) {
|
|
DEBUG(dbgs() << "Rotate loop by making " << getBlockName(*RotationPos)
|
|
<< " to the top\n");
|
|
std::rotate(LoopChain.begin(), RotationPos, LoopChain.end());
|
|
}
|
|
}
|
|
|
|
/// \brief Collect blocks in the given loop that are to be placed.
|
|
///
|
|
/// When profile data is available, exclude cold blocks from the returned set;
|
|
/// otherwise, collect all blocks in the loop.
|
|
MachineBlockPlacement::BlockFilterSet
|
|
MachineBlockPlacement::collectLoopBlockSet(MachineLoop &L) {
|
|
BlockFilterSet LoopBlockSet;
|
|
|
|
// Filter cold blocks off from LoopBlockSet when profile data is available.
|
|
// Collect the sum of frequencies of incoming edges to the loop header from
|
|
// outside. If we treat the loop as a super block, this is the frequency of
|
|
// the loop. Then for each block in the loop, we calculate the ratio between
|
|
// its frequency and the frequency of the loop block. When it is too small,
|
|
// don't add it to the loop chain. If there are outer loops, then this block
|
|
// will be merged into the first outer loop chain for which this block is not
|
|
// cold anymore. This needs precise profile data and we only do this when
|
|
// profile data is available.
|
|
if (F->getFunction()->getEntryCount()) {
|
|
BlockFrequency LoopFreq(0);
|
|
for (auto LoopPred : L.getHeader()->predecessors())
|
|
if (!L.contains(LoopPred))
|
|
LoopFreq += MBFI->getBlockFreq(LoopPred) *
|
|
MBPI->getEdgeProbability(LoopPred, L.getHeader());
|
|
|
|
for (MachineBasicBlock *LoopBB : L.getBlocks()) {
|
|
auto Freq = MBFI->getBlockFreq(LoopBB).getFrequency();
|
|
if (Freq == 0 || LoopFreq.getFrequency() / Freq > LoopToColdBlockRatio)
|
|
continue;
|
|
LoopBlockSet.insert(LoopBB);
|
|
}
|
|
} else
|
|
LoopBlockSet.insert(L.block_begin(), L.block_end());
|
|
|
|
return LoopBlockSet;
|
|
}
|
|
|
|
/// \brief Forms basic block chains from the natural loop structures.
|
|
///
|
|
/// These chains are designed to preserve the existing *structure* of the code
|
|
/// as much as possible. We can then stitch the chains together in a way which
|
|
/// both preserves the topological structure and minimizes taken conditional
|
|
/// branches.
|
|
void MachineBlockPlacement::buildLoopChains(MachineLoop &L) {
|
|
// First recurse through any nested loops, building chains for those inner
|
|
// loops.
|
|
for (MachineLoop *InnerLoop : L)
|
|
buildLoopChains(*InnerLoop);
|
|
|
|
assert(BlockWorkList.empty());
|
|
assert(EHPadWorkList.empty());
|
|
BlockFilterSet LoopBlockSet = collectLoopBlockSet(L);
|
|
|
|
// Check if we have profile data for this function. If yes, we will rotate
|
|
// this loop by modeling costs more precisely which requires the profile data
|
|
// for better layout.
|
|
bool RotateLoopWithProfile =
|
|
ForcePreciseRotationCost ||
|
|
(PreciseRotationCost && F->getFunction()->getEntryCount());
|
|
|
|
// First check to see if there is an obviously preferable top block for the
|
|
// loop. This will default to the header, but may end up as one of the
|
|
// predecessors to the header if there is one which will result in strictly
|
|
// fewer branches in the loop body.
|
|
// When we use profile data to rotate the loop, this is unnecessary.
|
|
MachineBasicBlock *LoopTop =
|
|
RotateLoopWithProfile ? L.getHeader() : findBestLoopTop(L, LoopBlockSet);
|
|
|
|
// If we selected just the header for the loop top, look for a potentially
|
|
// profitable exit block in the event that rotating the loop can eliminate
|
|
// branches by placing an exit edge at the bottom.
|
|
if (!RotateLoopWithProfile && LoopTop == L.getHeader())
|
|
PreferredLoopExit = findBestLoopExit(L, LoopBlockSet);
|
|
|
|
BlockChain &LoopChain = *BlockToChain[LoopTop];
|
|
|
|
// FIXME: This is a really lame way of walking the chains in the loop: we
|
|
// walk the blocks, and use a set to prevent visiting a particular chain
|
|
// twice.
|
|
SmallPtrSet<BlockChain *, 4> UpdatedPreds;
|
|
assert(LoopChain.UnscheduledPredecessors == 0);
|
|
UpdatedPreds.insert(&LoopChain);
|
|
|
|
for (MachineBasicBlock *LoopBB : LoopBlockSet)
|
|
fillWorkLists(LoopBB, UpdatedPreds, &LoopBlockSet);
|
|
|
|
buildChain(LoopTop, LoopChain, &LoopBlockSet);
|
|
|
|
if (RotateLoopWithProfile)
|
|
rotateLoopWithProfile(LoopChain, L, LoopBlockSet);
|
|
else
|
|
rotateLoop(LoopChain, PreferredLoopExit, LoopBlockSet);
|
|
|
|
DEBUG({
|
|
// Crash at the end so we get all of the debugging output first.
|
|
bool BadLoop = false;
|
|
if (LoopChain.UnscheduledPredecessors) {
|
|
BadLoop = true;
|
|
dbgs() << "Loop chain contains a block without its preds placed!\n"
|
|
<< " Loop header: " << getBlockName(*L.block_begin()) << "\n"
|
|
<< " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
|
|
}
|
|
for (MachineBasicBlock *ChainBB : LoopChain) {
|
|
dbgs() << " ... " << getBlockName(ChainBB) << "\n";
|
|
if (!LoopBlockSet.remove(ChainBB)) {
|
|
// We don't mark the loop as bad here because there are real situations
|
|
// where this can occur. For example, with an unanalyzable fallthrough
|
|
// from a loop block to a non-loop block or vice versa.
|
|
dbgs() << "Loop chain contains a block not contained by the loop!\n"
|
|
<< " Loop header: " << getBlockName(*L.block_begin()) << "\n"
|
|
<< " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
|
|
<< " Bad block: " << getBlockName(ChainBB) << "\n";
|
|
}
|
|
}
|
|
|
|
if (!LoopBlockSet.empty()) {
|
|
BadLoop = true;
|
|
for (MachineBasicBlock *LoopBB : LoopBlockSet)
|
|
dbgs() << "Loop contains blocks never placed into a chain!\n"
|
|
<< " Loop header: " << getBlockName(*L.block_begin()) << "\n"
|
|
<< " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
|
|
<< " Bad block: " << getBlockName(LoopBB) << "\n";
|
|
}
|
|
assert(!BadLoop && "Detected problems with the placement of this loop.");
|
|
});
|
|
|
|
BlockWorkList.clear();
|
|
EHPadWorkList.clear();
|
|
}
|
|
|
|
/// When OutlineOpitonalBranches is on, this method collects BBs that
|
|
/// dominates all terminator blocks of the function \p F.
|
|
void MachineBlockPlacement::collectMustExecuteBBs() {
|
|
if (OutlineOptionalBranches) {
|
|
// Find the nearest common dominator of all of F's terminators.
|
|
MachineBasicBlock *Terminator = nullptr;
|
|
for (MachineBasicBlock &MBB : *F) {
|
|
if (MBB.succ_size() == 0) {
|
|
if (Terminator == nullptr)
|
|
Terminator = &MBB;
|
|
else
|
|
Terminator = MDT->findNearestCommonDominator(Terminator, &MBB);
|
|
}
|
|
}
|
|
|
|
// MBBs dominating this common dominator are unavoidable.
|
|
UnavoidableBlocks.clear();
|
|
for (MachineBasicBlock &MBB : *F) {
|
|
if (MDT->dominates(&MBB, Terminator)) {
|
|
UnavoidableBlocks.insert(&MBB);
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
void MachineBlockPlacement::buildCFGChains() {
|
|
// Ensure that every BB in the function has an associated chain to simplify
|
|
// the assumptions of the remaining algorithm.
|
|
SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
|
|
for (MachineFunction::iterator FI = F->begin(), FE = F->end(); FI != FE;
|
|
++FI) {
|
|
MachineBasicBlock *BB = &*FI;
|
|
BlockChain *Chain =
|
|
new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
|
|
// Also, merge any blocks which we cannot reason about and must preserve
|
|
// the exact fallthrough behavior for.
|
|
for (;;) {
|
|
Cond.clear();
|
|
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
|
|
if (!TII->analyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
|
|
break;
|
|
|
|
MachineFunction::iterator NextFI = std::next(FI);
|
|
MachineBasicBlock *NextBB = &*NextFI;
|
|
// Ensure that the layout successor is a viable block, as we know that
|
|
// fallthrough is a possibility.
|
|
assert(NextFI != FE && "Can't fallthrough past the last block.");
|
|
DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
|
|
<< getBlockName(BB) << " -> " << getBlockName(NextBB)
|
|
<< "\n");
|
|
Chain->merge(NextBB, nullptr);
|
|
#ifndef NDEBUG
|
|
BlocksWithUnanalyzableExits.insert(&*BB);
|
|
#endif
|
|
FI = NextFI;
|
|
BB = NextBB;
|
|
}
|
|
}
|
|
|
|
// Turned on with OutlineOptionalBranches option
|
|
collectMustExecuteBBs();
|
|
|
|
// Build any loop-based chains.
|
|
PreferredLoopExit = nullptr;
|
|
for (MachineLoop *L : *MLI)
|
|
buildLoopChains(*L);
|
|
|
|
assert(BlockWorkList.empty());
|
|
assert(EHPadWorkList.empty());
|
|
|
|
SmallPtrSet<BlockChain *, 4> UpdatedPreds;
|
|
for (MachineBasicBlock &MBB : *F)
|
|
fillWorkLists(&MBB, UpdatedPreds);
|
|
|
|
BlockChain &FunctionChain = *BlockToChain[&F->front()];
|
|
buildChain(&F->front(), FunctionChain);
|
|
|
|
#ifndef NDEBUG
|
|
typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
|
|
#endif
|
|
DEBUG({
|
|
// Crash at the end so we get all of the debugging output first.
|
|
bool BadFunc = false;
|
|
FunctionBlockSetType FunctionBlockSet;
|
|
for (MachineBasicBlock &MBB : *F)
|
|
FunctionBlockSet.insert(&MBB);
|
|
|
|
for (MachineBasicBlock *ChainBB : FunctionChain)
|
|
if (!FunctionBlockSet.erase(ChainBB)) {
|
|
BadFunc = true;
|
|
dbgs() << "Function chain contains a block not in the function!\n"
|
|
<< " Bad block: " << getBlockName(ChainBB) << "\n";
|
|
}
|
|
|
|
if (!FunctionBlockSet.empty()) {
|
|
BadFunc = true;
|
|
for (MachineBasicBlock *RemainingBB : FunctionBlockSet)
|
|
dbgs() << "Function contains blocks never placed into a chain!\n"
|
|
<< " Bad block: " << getBlockName(RemainingBB) << "\n";
|
|
}
|
|
assert(!BadFunc && "Detected problems with the block placement.");
|
|
});
|
|
|
|
// Splice the blocks into place.
|
|
MachineFunction::iterator InsertPos = F->begin();
|
|
DEBUG(dbgs() << "[MBP] Function: "<< F->getName() << "\n");
|
|
for (MachineBasicBlock *ChainBB : FunctionChain) {
|
|
DEBUG(dbgs() << (ChainBB == *FunctionChain.begin() ? "Placing chain "
|
|
: " ... ")
|
|
<< getBlockName(ChainBB) << "\n");
|
|
if (InsertPos != MachineFunction::iterator(ChainBB))
|
|
F->splice(InsertPos, ChainBB);
|
|
else
|
|
++InsertPos;
|
|
|
|
// Update the terminator of the previous block.
|
|
if (ChainBB == *FunctionChain.begin())
|
|
continue;
|
|
MachineBasicBlock *PrevBB = &*std::prev(MachineFunction::iterator(ChainBB));
|
|
|
|
// FIXME: It would be awesome of updateTerminator would just return rather
|
|
// than assert when the branch cannot be analyzed in order to remove this
|
|
// boiler plate.
|
|
Cond.clear();
|
|
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
|
|
|
|
#ifndef NDEBUG
|
|
if (!BlocksWithUnanalyzableExits.count(PrevBB)) {
|
|
// Given the exact block placement we chose, we may actually not _need_ to
|
|
// be able to edit PrevBB's terminator sequence, but not being _able_ to
|
|
// do that at this point is a bug.
|
|
assert((!TII->analyzeBranch(*PrevBB, TBB, FBB, Cond) ||
|
|
!PrevBB->canFallThrough()) &&
|
|
"Unexpected block with un-analyzable fallthrough!");
|
|
Cond.clear();
|
|
TBB = FBB = nullptr;
|
|
}
|
|
#endif
|
|
|
|
// The "PrevBB" is not yet updated to reflect current code layout, so,
|
|
// o. it may fall-through to a block without explicit "goto" instruction
|
|
// before layout, and no longer fall-through it after layout; or
|
|
// o. just opposite.
|
|
//
|
|
// analyzeBranch() may return erroneous value for FBB when these two
|
|
// situations take place. For the first scenario FBB is mistakenly set NULL;
|
|
// for the 2nd scenario, the FBB, which is expected to be NULL, is
|
|
// mistakenly pointing to "*BI".
|
|
// Thus, if the future change needs to use FBB before the layout is set, it
|
|
// has to correct FBB first by using the code similar to the following:
|
|
//
|
|
// if (!Cond.empty() && (!FBB || FBB == ChainBB)) {
|
|
// PrevBB->updateTerminator();
|
|
// Cond.clear();
|
|
// TBB = FBB = nullptr;
|
|
// if (TII->analyzeBranch(*PrevBB, TBB, FBB, Cond)) {
|
|
// // FIXME: This should never take place.
|
|
// TBB = FBB = nullptr;
|
|
// }
|
|
// }
|
|
if (!TII->analyzeBranch(*PrevBB, TBB, FBB, Cond))
|
|
PrevBB->updateTerminator();
|
|
}
|
|
|
|
// Fixup the last block.
|
|
Cond.clear();
|
|
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
|
|
if (!TII->analyzeBranch(F->back(), TBB, FBB, Cond))
|
|
F->back().updateTerminator();
|
|
|
|
BlockWorkList.clear();
|
|
EHPadWorkList.clear();
|
|
}
|
|
|
|
void MachineBlockPlacement::optimizeBranches() {
|
|
BlockChain &FunctionChain = *BlockToChain[&F->front()];
|
|
SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
|
|
|
|
// Now that all the basic blocks in the chain have the proper layout,
|
|
// make a final call to AnalyzeBranch with AllowModify set.
|
|
// Indeed, the target may be able to optimize the branches in a way we
|
|
// cannot because all branches may not be analyzable.
|
|
// E.g., the target may be able to remove an unconditional branch to
|
|
// a fallthrough when it occurs after predicated terminators.
|
|
for (MachineBasicBlock *ChainBB : FunctionChain) {
|
|
Cond.clear();
|
|
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
|
|
if (!TII->analyzeBranch(*ChainBB, TBB, FBB, Cond, /*AllowModify*/ true)) {
|
|
// If PrevBB has a two-way branch, try to re-order the branches
|
|
// such that we branch to the successor with higher probability first.
|
|
if (TBB && !Cond.empty() && FBB &&
|
|
MBPI->getEdgeProbability(ChainBB, FBB) >
|
|
MBPI->getEdgeProbability(ChainBB, TBB) &&
|
|
!TII->reverseBranchCondition(Cond)) {
|
|
DEBUG(dbgs() << "Reverse order of the two branches: "
|
|
<< getBlockName(ChainBB) << "\n");
|
|
DEBUG(dbgs() << " Edge probability: "
|
|
<< MBPI->getEdgeProbability(ChainBB, FBB) << " vs "
|
|
<< MBPI->getEdgeProbability(ChainBB, TBB) << "\n");
|
|
DebugLoc dl; // FIXME: this is nowhere
|
|
TII->removeBranch(*ChainBB);
|
|
TII->insertBranch(*ChainBB, FBB, TBB, Cond, dl);
|
|
ChainBB->updateTerminator();
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
void MachineBlockPlacement::alignBlocks() {
|
|
// Walk through the backedges of the function now that we have fully laid out
|
|
// the basic blocks and align the destination of each backedge. We don't rely
|
|
// exclusively on the loop info here so that we can align backedges in
|
|
// unnatural CFGs and backedges that were introduced purely because of the
|
|
// loop rotations done during this layout pass.
|
|
if (F->getFunction()->optForSize())
|
|
return;
|
|
BlockChain &FunctionChain = *BlockToChain[&F->front()];
|
|
if (FunctionChain.begin() == FunctionChain.end())
|
|
return; // Empty chain.
|
|
|
|
const BranchProbability ColdProb(1, 5); // 20%
|
|
BlockFrequency EntryFreq = MBFI->getBlockFreq(&F->front());
|
|
BlockFrequency WeightedEntryFreq = EntryFreq * ColdProb;
|
|
for (MachineBasicBlock *ChainBB : FunctionChain) {
|
|
if (ChainBB == *FunctionChain.begin())
|
|
continue;
|
|
|
|
// Don't align non-looping basic blocks. These are unlikely to execute
|
|
// enough times to matter in practice. Note that we'll still handle
|
|
// unnatural CFGs inside of a natural outer loop (the common case) and
|
|
// rotated loops.
|
|
MachineLoop *L = MLI->getLoopFor(ChainBB);
|
|
if (!L)
|
|
continue;
|
|
|
|
unsigned Align = TLI->getPrefLoopAlignment(L);
|
|
if (!Align)
|
|
continue; // Don't care about loop alignment.
|
|
|
|
// If the block is cold relative to the function entry don't waste space
|
|
// aligning it.
|
|
BlockFrequency Freq = MBFI->getBlockFreq(ChainBB);
|
|
if (Freq < WeightedEntryFreq)
|
|
continue;
|
|
|
|
// If the block is cold relative to its loop header, don't align it
|
|
// regardless of what edges into the block exist.
|
|
MachineBasicBlock *LoopHeader = L->getHeader();
|
|
BlockFrequency LoopHeaderFreq = MBFI->getBlockFreq(LoopHeader);
|
|
if (Freq < (LoopHeaderFreq * ColdProb))
|
|
continue;
|
|
|
|
// Check for the existence of a non-layout predecessor which would benefit
|
|
// from aligning this block.
|
|
MachineBasicBlock *LayoutPred =
|
|
&*std::prev(MachineFunction::iterator(ChainBB));
|
|
|
|
// Force alignment if all the predecessors are jumps. We already checked
|
|
// that the block isn't cold above.
|
|
if (!LayoutPred->isSuccessor(ChainBB)) {
|
|
ChainBB->setAlignment(Align);
|
|
continue;
|
|
}
|
|
|
|
// Align this block if the layout predecessor's edge into this block is
|
|
// cold relative to the block. When this is true, other predecessors make up
|
|
// all of the hot entries into the block and thus alignment is likely to be
|
|
// important.
|
|
BranchProbability LayoutProb =
|
|
MBPI->getEdgeProbability(LayoutPred, ChainBB);
|
|
BlockFrequency LayoutEdgeFreq = MBFI->getBlockFreq(LayoutPred) * LayoutProb;
|
|
if (LayoutEdgeFreq <= (Freq * ColdProb))
|
|
ChainBB->setAlignment(Align);
|
|
}
|
|
}
|
|
|
|
/// Tail duplicate \p BB into (some) predecessors if profitable, repeating if
|
|
/// it was duplicated into its chain predecessor and removed.
|
|
/// \p BB - Basic block that may be duplicated.
|
|
///
|
|
/// \p LPred - Chosen layout predecessor of \p BB.
|
|
/// Updated to be the chain end if LPred is removed.
|
|
/// \p Chain - Chain to which \p LPred belongs, and \p BB will belong.
|
|
/// \p BlockFilter - Set of blocks that belong to the loop being laid out.
|
|
/// Used to identify which blocks to update predecessor
|
|
/// counts.
|
|
/// \p PrevUnplacedBlockIt - Iterator pointing to the last block that was
|
|
/// chosen in the given order due to unnatural CFG
|
|
/// only needed if \p BB is removed and
|
|
/// \p PrevUnplacedBlockIt pointed to \p BB.
|
|
/// @return true if \p BB was removed.
|
|
bool MachineBlockPlacement::repeatedlyTailDuplicateBlock(
|
|
MachineBasicBlock *BB, MachineBasicBlock *&LPred,
|
|
MachineBasicBlock *LoopHeaderBB,
|
|
BlockChain &Chain, BlockFilterSet *BlockFilter,
|
|
MachineFunction::iterator &PrevUnplacedBlockIt) {
|
|
bool Removed, DuplicatedToLPred;
|
|
bool DuplicatedToOriginalLPred;
|
|
Removed = maybeTailDuplicateBlock(BB, LPred, Chain, BlockFilter,
|
|
PrevUnplacedBlockIt,
|
|
DuplicatedToLPred);
|
|
if (!Removed)
|
|
return false;
|
|
DuplicatedToOriginalLPred = DuplicatedToLPred;
|
|
// Iteratively try to duplicate again. It can happen that a block that is
|
|
// duplicated into is still small enough to be duplicated again.
|
|
// No need to call markBlockSuccessors in this case, as the blocks being
|
|
// duplicated from here on are already scheduled.
|
|
// Note that DuplicatedToLPred always implies Removed.
|
|
while (DuplicatedToLPred) {
|
|
assert (Removed && "Block must have been removed to be duplicated into its "
|
|
"layout predecessor.");
|
|
MachineBasicBlock *DupBB, *DupPred;
|
|
// The removal callback causes Chain.end() to be updated when a block is
|
|
// removed. On the first pass through the loop, the chain end should be the
|
|
// same as it was on function entry. On subsequent passes, because we are
|
|
// duplicating the block at the end of the chain, if it is removed the
|
|
// chain will have shrunk by one block.
|
|
BlockChain::iterator ChainEnd = Chain.end();
|
|
DupBB = *(--ChainEnd);
|
|
// Now try to duplicate again.
|
|
if (ChainEnd == Chain.begin())
|
|
break;
|
|
DupPred = *std::prev(ChainEnd);
|
|
Removed = maybeTailDuplicateBlock(DupBB, DupPred, Chain, BlockFilter,
|
|
PrevUnplacedBlockIt,
|
|
DuplicatedToLPred);
|
|
}
|
|
// If BB was duplicated into LPred, it is now scheduled. But because it was
|
|
// removed, markChainSuccessors won't be called for its chain. Instead we
|
|
// call markBlockSuccessors for LPred to achieve the same effect. This must go
|
|
// at the end because repeating the tail duplication can increase the number
|
|
// of unscheduled predecessors.
|
|
LPred = *std::prev(Chain.end());
|
|
if (DuplicatedToOriginalLPred)
|
|
markBlockSuccessors(Chain, LPred, LoopHeaderBB, BlockFilter);
|
|
return true;
|
|
}
|
|
|
|
/// Tail duplicate \p BB into (some) predecessors if profitable.
|
|
/// \p BB - Basic block that may be duplicated
|
|
/// \p LPred - Chosen layout predecessor of \p BB
|
|
/// \p Chain - Chain to which \p LPred belongs, and \p BB will belong.
|
|
/// \p BlockFilter - Set of blocks that belong to the loop being laid out.
|
|
/// Used to identify which blocks to update predecessor
|
|
/// counts.
|
|
/// \p PrevUnplacedBlockIt - Iterator pointing to the last block that was
|
|
/// chosen in the given order due to unnatural CFG
|
|
/// only needed if \p BB is removed and
|
|
/// \p PrevUnplacedBlockIt pointed to \p BB.
|
|
/// \p DuplicatedToLPred - True if the block was duplicated into LPred. Will
|
|
/// only be true if the block was removed.
|
|
/// \return - True if the block was duplicated into all preds and removed.
|
|
bool MachineBlockPlacement::maybeTailDuplicateBlock(
|
|
MachineBasicBlock *BB, MachineBasicBlock *LPred,
|
|
const BlockChain &Chain, BlockFilterSet *BlockFilter,
|
|
MachineFunction::iterator &PrevUnplacedBlockIt,
|
|
bool &DuplicatedToLPred) {
|
|
|
|
DuplicatedToLPred = false;
|
|
DEBUG(dbgs() << "Redoing tail duplication for Succ#"
|
|
<< BB->getNumber() << "\n");
|
|
bool IsSimple = TailDup.isSimpleBB(BB);
|
|
// Blocks with single successors don't create additional fallthrough
|
|
// opportunities. Don't duplicate them. TODO: When conditional exits are
|
|
// analyzable, allow them to be duplicated.
|
|
if (!IsSimple && BB->succ_size() == 1)
|
|
return false;
|
|
if (!TailDup.shouldTailDuplicate(IsSimple, *BB))
|
|
return false;
|
|
// This has to be a callback because none of it can be done after
|
|
// BB is deleted.
|
|
bool Removed = false;
|
|
auto RemovalCallback =
|
|
[&](MachineBasicBlock *RemBB) {
|
|
// Signal to outer function
|
|
Removed = true;
|
|
|
|
// Conservative default.
|
|
bool InWorkList = true;
|
|
// Remove from the Chain and Chain Map
|
|
if (BlockToChain.count(RemBB)) {
|
|
BlockChain *Chain = BlockToChain[RemBB];
|
|
InWorkList = Chain->UnscheduledPredecessors == 0;
|
|
Chain->remove(RemBB);
|
|
BlockToChain.erase(RemBB);
|
|
}
|
|
|
|
// Handle the unplaced block iterator
|
|
if (&(*PrevUnplacedBlockIt) == RemBB) {
|
|
PrevUnplacedBlockIt++;
|
|
}
|
|
|
|
// Handle the Work Lists
|
|
if (InWorkList) {
|
|
SmallVectorImpl<MachineBasicBlock *> &RemoveList = BlockWorkList;
|
|
if (RemBB->isEHPad())
|
|
RemoveList = EHPadWorkList;
|
|
RemoveList.erase(
|
|
remove_if(RemoveList,
|
|
[RemBB](MachineBasicBlock *BB) {return BB == RemBB;}),
|
|
RemoveList.end());
|
|
}
|
|
|
|
// Handle the filter set
|
|
if (BlockFilter) {
|
|
BlockFilter->remove(RemBB);
|
|
}
|
|
|
|
// Remove the block from loop info.
|
|
MLI->removeBlock(RemBB);
|
|
if (RemBB == PreferredLoopExit)
|
|
PreferredLoopExit = nullptr;
|
|
|
|
DEBUG(dbgs() << "TailDuplicator deleted block: "
|
|
<< getBlockName(RemBB) << "\n");
|
|
};
|
|
auto RemovalCallbackRef =
|
|
llvm::function_ref<void(MachineBasicBlock*)>(RemovalCallback);
|
|
|
|
SmallVector<MachineBasicBlock *, 8> DuplicatedPreds;
|
|
TailDup.tailDuplicateAndUpdate(IsSimple, BB, LPred,
|
|
&DuplicatedPreds, &RemovalCallbackRef);
|
|
|
|
// Update UnscheduledPredecessors to reflect tail-duplication.
|
|
DuplicatedToLPred = false;
|
|
for (MachineBasicBlock *Pred : DuplicatedPreds) {
|
|
// We're only looking for unscheduled predecessors that match the filter.
|
|
BlockChain* PredChain = BlockToChain[Pred];
|
|
if (Pred == LPred)
|
|
DuplicatedToLPred = true;
|
|
if (Pred == LPred || (BlockFilter && !BlockFilter->count(Pred))
|
|
|| PredChain == &Chain)
|
|
continue;
|
|
for (MachineBasicBlock *NewSucc : Pred->successors()) {
|
|
if (BlockFilter && !BlockFilter->count(NewSucc))
|
|
continue;
|
|
BlockChain *NewChain = BlockToChain[NewSucc];
|
|
if (NewChain != &Chain && NewChain != PredChain)
|
|
NewChain->UnscheduledPredecessors++;
|
|
}
|
|
}
|
|
return Removed;
|
|
}
|
|
|
|
bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &MF) {
|
|
if (skipFunction(*MF.getFunction()))
|
|
return false;
|
|
|
|
// Check for single-block functions and skip them.
|
|
if (std::next(MF.begin()) == MF.end())
|
|
return false;
|
|
|
|
F = &MF;
|
|
MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
|
|
MBFI = llvm::make_unique<BranchFolder::MBFIWrapper>(
|
|
getAnalysis<MachineBlockFrequencyInfo>());
|
|
MLI = &getAnalysis<MachineLoopInfo>();
|
|
TII = MF.getSubtarget().getInstrInfo();
|
|
TLI = MF.getSubtarget().getTargetLowering();
|
|
MDT = &getAnalysis<MachineDominatorTree>();
|
|
|
|
// Initialize PreferredLoopExit to nullptr here since it may never be set if
|
|
// there are no MachineLoops.
|
|
PreferredLoopExit = nullptr;
|
|
|
|
if (TailDupPlacement) {
|
|
unsigned TailDupSize = TailDuplicatePlacementThreshold;
|
|
if (MF.getFunction()->optForSize())
|
|
TailDupSize = 1;
|
|
TailDup.initMF(MF, MBPI, /* LayoutMode */ true, TailDupSize);
|
|
}
|
|
|
|
assert(BlockToChain.empty());
|
|
|
|
buildCFGChains();
|
|
|
|
// Changing the layout can create new tail merging opportunities.
|
|
TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
|
|
// TailMerge can create jump into if branches that make CFG irreducible for
|
|
// HW that requires structured CFG.
|
|
bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
|
|
PassConfig->getEnableTailMerge() &&
|
|
BranchFoldPlacement;
|
|
// No tail merging opportunities if the block number is less than four.
|
|
if (MF.size() > 3 && EnableTailMerge) {
|
|
unsigned TailMergeSize = TailDuplicatePlacementThreshold + 1;
|
|
BranchFolder BF(/*EnableTailMerge=*/true, /*CommonHoist=*/false, *MBFI,
|
|
*MBPI, TailMergeSize);
|
|
|
|
if (BF.OptimizeFunction(MF, TII, MF.getSubtarget().getRegisterInfo(),
|
|
getAnalysisIfAvailable<MachineModuleInfo>(), MLI,
|
|
/*AfterBlockPlacement=*/true)) {
|
|
// Redo the layout if tail merging creates/removes/moves blocks.
|
|
BlockToChain.clear();
|
|
// Must redo the dominator tree if blocks were changed.
|
|
MDT->runOnMachineFunction(MF);
|
|
ChainAllocator.DestroyAll();
|
|
buildCFGChains();
|
|
}
|
|
}
|
|
|
|
optimizeBranches();
|
|
alignBlocks();
|
|
|
|
BlockToChain.clear();
|
|
ChainAllocator.DestroyAll();
|
|
|
|
if (AlignAllBlock)
|
|
// Align all of the blocks in the function to a specific alignment.
|
|
for (MachineBasicBlock &MBB : MF)
|
|
MBB.setAlignment(AlignAllBlock);
|
|
else if (AlignAllNonFallThruBlocks) {
|
|
// Align all of the blocks that have no fall-through predecessors to a
|
|
// specific alignment.
|
|
for (auto MBI = std::next(MF.begin()), MBE = MF.end(); MBI != MBE; ++MBI) {
|
|
auto LayoutPred = std::prev(MBI);
|
|
if (!LayoutPred->isSuccessor(&*MBI))
|
|
MBI->setAlignment(AlignAllNonFallThruBlocks);
|
|
}
|
|
}
|
|
|
|
// We always return true as we have no way to track whether the final order
|
|
// differs from the original order.
|
|
return true;
|
|
}
|
|
|
|
namespace {
|
|
/// \brief A pass to compute block placement statistics.
|
|
///
|
|
/// A separate pass to compute interesting statistics for evaluating block
|
|
/// placement. This is separate from the actual placement pass so that they can
|
|
/// be computed in the absence of any placement transformations or when using
|
|
/// alternative placement strategies.
|
|
class MachineBlockPlacementStats : public MachineFunctionPass {
|
|
/// \brief A handle to the branch probability pass.
|
|
const MachineBranchProbabilityInfo *MBPI;
|
|
|
|
/// \brief A handle to the function-wide block frequency pass.
|
|
const MachineBlockFrequencyInfo *MBFI;
|
|
|
|
public:
|
|
static char ID; // Pass identification, replacement for typeid
|
|
MachineBlockPlacementStats() : MachineFunctionPass(ID) {
|
|
initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
|
|
}
|
|
|
|
bool runOnMachineFunction(MachineFunction &F) override;
|
|
|
|
void getAnalysisUsage(AnalysisUsage &AU) const override {
|
|
AU.addRequired<MachineBranchProbabilityInfo>();
|
|
AU.addRequired<MachineBlockFrequencyInfo>();
|
|
AU.setPreservesAll();
|
|
MachineFunctionPass::getAnalysisUsage(AU);
|
|
}
|
|
};
|
|
}
|
|
|
|
char MachineBlockPlacementStats::ID = 0;
|
|
char &llvm::MachineBlockPlacementStatsID = MachineBlockPlacementStats::ID;
|
|
INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
|
|
"Basic Block Placement Stats", false, false)
|
|
INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
|
|
INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
|
|
INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
|
|
"Basic Block Placement Stats", false, false)
|
|
|
|
bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
|
|
// Check for single-block functions and skip them.
|
|
if (std::next(F.begin()) == F.end())
|
|
return false;
|
|
|
|
MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
|
|
MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
|
|
|
|
for (MachineBasicBlock &MBB : F) {
|
|
BlockFrequency BlockFreq = MBFI->getBlockFreq(&MBB);
|
|
Statistic &NumBranches =
|
|
(MBB.succ_size() > 1) ? NumCondBranches : NumUncondBranches;
|
|
Statistic &BranchTakenFreq =
|
|
(MBB.succ_size() > 1) ? CondBranchTakenFreq : UncondBranchTakenFreq;
|
|
for (MachineBasicBlock *Succ : MBB.successors()) {
|
|
// Skip if this successor is a fallthrough.
|
|
if (MBB.isLayoutSuccessor(Succ))
|
|
continue;
|
|
|
|
BlockFrequency EdgeFreq =
|
|
BlockFreq * MBPI->getEdgeProbability(&MBB, Succ);
|
|
++NumBranches;
|
|
BranchTakenFreq += EdgeFreq.getFrequency();
|
|
}
|
|
}
|
|
|
|
return false;
|
|
}
|