629 lines
23 KiB
C++
629 lines
23 KiB
C++
//===-- GlobalMerge.cpp - Internal globals merging -----------------------===//
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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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// This pass merges globals with internal linkage into one. This way all the
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// globals which were merged into a biggest one can be addressed using offsets
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// from the same base pointer (no need for separate base pointer for each of the
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// global). Such a transformation can significantly reduce the register pressure
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// when many globals are involved.
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//
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// For example, consider the code which touches several global variables at
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// once:
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//
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// static int foo[N], bar[N], baz[N];
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//
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// for (i = 0; i < N; ++i) {
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// foo[i] = bar[i] * baz[i];
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// }
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//
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// On ARM the addresses of 3 arrays should be kept in the registers, thus
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// this code has quite large register pressure (loop body):
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//
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// ldr r1, [r5], #4
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// ldr r2, [r6], #4
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// mul r1, r2, r1
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// str r1, [r0], #4
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//
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// Pass converts the code to something like:
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//
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// static struct {
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// int foo[N];
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// int bar[N];
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// int baz[N];
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// } merged;
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//
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// for (i = 0; i < N; ++i) {
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// merged.foo[i] = merged.bar[i] * merged.baz[i];
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// }
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//
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// and in ARM code this becomes:
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//
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// ldr r0, [r5, #40]
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// ldr r1, [r5, #80]
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// mul r0, r1, r0
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// str r0, [r5], #4
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//
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// note that we saved 2 registers here almostly "for free".
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//
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// However, merging globals can have tradeoffs:
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// - it confuses debuggers, tools, and users
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// - it makes linker optimizations less useful (order files, LOHs, ...)
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// - it forces usage of indexed addressing (which isn't necessarily "free")
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// - it can increase register pressure when the uses are disparate enough.
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//
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// We use heuristics to discover the best global grouping we can (cf cl::opts).
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// ===---------------------------------------------------------------------===//
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/SmallBitVector.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/CodeGen/Passes.h"
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#include "llvm/IR/Attributes.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/DataLayout.h"
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#include "llvm/IR/DerivedTypes.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/GlobalVariable.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/Intrinsics.h"
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#include "llvm/IR/Module.h"
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#include "llvm/Pass.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/TargetLowering.h"
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#include "llvm/Target/TargetLoweringObjectFile.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 "global-merge"
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// FIXME: This is only useful as a last-resort way to disable the pass.
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static cl::opt<bool>
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EnableGlobalMerge("enable-global-merge", cl::Hidden,
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cl::desc("Enable the global merge pass"),
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cl::init(true));
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static cl::opt<unsigned>
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GlobalMergeMaxOffset("global-merge-max-offset", cl::Hidden,
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cl::desc("Set maximum offset for global merge pass"),
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cl::init(0));
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static cl::opt<bool> GlobalMergeGroupByUse(
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"global-merge-group-by-use", cl::Hidden,
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cl::desc("Improve global merge pass to look at uses"), cl::init(true));
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static cl::opt<bool> GlobalMergeIgnoreSingleUse(
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"global-merge-ignore-single-use", cl::Hidden,
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cl::desc("Improve global merge pass to ignore globals only used alone"),
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cl::init(true));
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static cl::opt<bool>
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EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
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cl::desc("Enable global merge pass on constants"),
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cl::init(false));
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// FIXME: this could be a transitional option, and we probably need to remove
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// it if only we are sure this optimization could always benefit all targets.
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static cl::opt<cl::boolOrDefault>
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EnableGlobalMergeOnExternal("global-merge-on-external", cl::Hidden,
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cl::desc("Enable global merge pass on external linkage"));
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STATISTIC(NumMerged, "Number of globals merged");
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namespace {
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class GlobalMerge : public FunctionPass {
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const TargetMachine *TM;
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// FIXME: Infer the maximum possible offset depending on the actual users
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// (these max offsets are different for the users inside Thumb or ARM
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// functions), see the code that passes in the offset in the ARM backend
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// for more information.
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unsigned MaxOffset;
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/// Whether we should try to optimize for size only.
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/// Currently, this applies a dead simple heuristic: only consider globals
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/// used in minsize functions for merging.
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/// FIXME: This could learn about optsize, and be used in the cost model.
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bool OnlyOptimizeForSize;
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/// Whether we should merge global variables that have external linkage.
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bool MergeExternalGlobals;
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bool IsMachO;
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bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
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Module &M, bool isConst, unsigned AddrSpace) const;
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/// \brief Merge everything in \p Globals for which the corresponding bit
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/// in \p GlobalSet is set.
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bool doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
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const BitVector &GlobalSet, Module &M, bool isConst,
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unsigned AddrSpace) const;
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/// \brief Check if the given variable has been identified as must keep
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/// \pre setMustKeepGlobalVariables must have been called on the Module that
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/// contains GV
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bool isMustKeepGlobalVariable(const GlobalVariable *GV) const {
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return MustKeepGlobalVariables.count(GV);
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}
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/// Collect every variables marked as "used" or used in a landing pad
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/// instruction for this Module.
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void setMustKeepGlobalVariables(Module &M);
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/// Collect every variables marked as "used"
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void collectUsedGlobalVariables(Module &M);
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/// Keep track of the GlobalVariable that must not be merged away
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SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables;
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public:
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static char ID; // Pass identification, replacement for typeid.
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explicit GlobalMerge()
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: FunctionPass(ID), TM(nullptr), MaxOffset(GlobalMergeMaxOffset),
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OnlyOptimizeForSize(false), MergeExternalGlobals(false) {
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initializeGlobalMergePass(*PassRegistry::getPassRegistry());
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}
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explicit GlobalMerge(const TargetMachine *TM, unsigned MaximalOffset,
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bool OnlyOptimizeForSize, bool MergeExternalGlobals)
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: FunctionPass(ID), TM(TM), MaxOffset(MaximalOffset),
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OnlyOptimizeForSize(OnlyOptimizeForSize),
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MergeExternalGlobals(MergeExternalGlobals) {
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initializeGlobalMergePass(*PassRegistry::getPassRegistry());
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}
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bool doInitialization(Module &M) override;
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bool runOnFunction(Function &F) override;
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bool doFinalization(Module &M) override;
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StringRef getPassName() const override { return "Merge internal globals"; }
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.setPreservesCFG();
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FunctionPass::getAnalysisUsage(AU);
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}
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};
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} // end anonymous namespace
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char GlobalMerge::ID = 0;
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INITIALIZE_PASS_BEGIN(GlobalMerge, "global-merge", "Merge global variables",
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false, false)
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INITIALIZE_PASS_END(GlobalMerge, "global-merge", "Merge global variables",
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false, false)
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bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
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Module &M, bool isConst, unsigned AddrSpace) const {
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auto &DL = M.getDataLayout();
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// FIXME: Find better heuristics
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std::stable_sort(Globals.begin(), Globals.end(),
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[&DL](const GlobalVariable *GV1, const GlobalVariable *GV2) {
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return DL.getTypeAllocSize(GV1->getValueType()) <
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DL.getTypeAllocSize(GV2->getValueType());
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});
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// If we want to just blindly group all globals together, do so.
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if (!GlobalMergeGroupByUse) {
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BitVector AllGlobals(Globals.size());
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AllGlobals.set();
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return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
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}
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// If we want to be smarter, look at all uses of each global, to try to
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// discover all sets of globals used together, and how many times each of
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// these sets occurred.
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//
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// Keep this reasonably efficient, by having an append-only list of all sets
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// discovered so far (UsedGlobalSet), and mapping each "together-ness" unit of
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// code (currently, a Function) to the set of globals seen so far that are
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// used together in that unit (GlobalUsesByFunction).
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//
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// When we look at the Nth global, we now that any new set is either:
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// - the singleton set {N}, containing this global only, or
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// - the union of {N} and a previously-discovered set, containing some
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// combination of the previous N-1 globals.
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// Using that knowledge, when looking at the Nth global, we can keep:
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// - a reference to the singleton set {N} (CurGVOnlySetIdx)
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// - a list mapping each previous set to its union with {N} (EncounteredUGS),
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// if it actually occurs.
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// We keep track of the sets of globals used together "close enough".
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struct UsedGlobalSet {
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UsedGlobalSet(size_t Size) : Globals(Size), UsageCount(1) {}
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BitVector Globals;
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unsigned UsageCount;
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};
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// Each set is unique in UsedGlobalSets.
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std::vector<UsedGlobalSet> UsedGlobalSets;
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// Avoid repeating the create-global-set pattern.
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auto CreateGlobalSet = [&]() -> UsedGlobalSet & {
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UsedGlobalSets.emplace_back(Globals.size());
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return UsedGlobalSets.back();
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};
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// The first set is the empty set.
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CreateGlobalSet().UsageCount = 0;
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// We define "close enough" to be "in the same function".
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// FIXME: Grouping uses by function is way too aggressive, so we should have
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// a better metric for distance between uses.
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// The obvious alternative would be to group by BasicBlock, but that's in
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// turn too conservative..
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// Anything in between wouldn't be trivial to compute, so just stick with
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// per-function grouping.
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// The value type is an index into UsedGlobalSets.
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// The default (0) conveniently points to the empty set.
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DenseMap<Function *, size_t /*UsedGlobalSetIdx*/> GlobalUsesByFunction;
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// Now, look at each merge-eligible global in turn.
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// Keep track of the sets we already encountered to which we added the
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// current global.
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// Each element matches the same-index element in UsedGlobalSets.
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// This lets us efficiently tell whether a set has already been expanded to
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// include the current global.
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std::vector<size_t> EncounteredUGS;
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for (size_t GI = 0, GE = Globals.size(); GI != GE; ++GI) {
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GlobalVariable *GV = Globals[GI];
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// Reset the encountered sets for this global...
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std::fill(EncounteredUGS.begin(), EncounteredUGS.end(), 0);
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// ...and grow it in case we created new sets for the previous global.
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EncounteredUGS.resize(UsedGlobalSets.size());
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// We might need to create a set that only consists of the current global.
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// Keep track of its index into UsedGlobalSets.
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size_t CurGVOnlySetIdx = 0;
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// For each global, look at all its Uses.
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for (auto &U : GV->uses()) {
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// This Use might be a ConstantExpr. We're interested in Instruction
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// users, so look through ConstantExpr...
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Use *UI, *UE;
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if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U.getUser())) {
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if (CE->use_empty())
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continue;
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UI = &*CE->use_begin();
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UE = nullptr;
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} else if (isa<Instruction>(U.getUser())) {
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UI = &U;
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UE = UI->getNext();
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} else {
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continue;
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}
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// ...to iterate on all the instruction users of the global.
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// Note that we iterate on Uses and not on Users to be able to getNext().
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for (; UI != UE; UI = UI->getNext()) {
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Instruction *I = dyn_cast<Instruction>(UI->getUser());
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if (!I)
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continue;
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Function *ParentFn = I->getParent()->getParent();
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// If we're only optimizing for size, ignore non-minsize functions.
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if (OnlyOptimizeForSize && !ParentFn->optForMinSize())
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continue;
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size_t UGSIdx = GlobalUsesByFunction[ParentFn];
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// If this is the first global the basic block uses, map it to the set
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// consisting of this global only.
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if (!UGSIdx) {
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// If that set doesn't exist yet, create it.
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if (!CurGVOnlySetIdx) {
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CurGVOnlySetIdx = UsedGlobalSets.size();
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CreateGlobalSet().Globals.set(GI);
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} else {
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++UsedGlobalSets[CurGVOnlySetIdx].UsageCount;
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}
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GlobalUsesByFunction[ParentFn] = CurGVOnlySetIdx;
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continue;
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}
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// If we already encountered this BB, just increment the counter.
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if (UsedGlobalSets[UGSIdx].Globals.test(GI)) {
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++UsedGlobalSets[UGSIdx].UsageCount;
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continue;
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}
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// If not, the previous set wasn't actually used in this function.
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--UsedGlobalSets[UGSIdx].UsageCount;
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// If we already expanded the previous set to include this global, just
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// reuse that expanded set.
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if (size_t ExpandedIdx = EncounteredUGS[UGSIdx]) {
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++UsedGlobalSets[ExpandedIdx].UsageCount;
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GlobalUsesByFunction[ParentFn] = ExpandedIdx;
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continue;
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}
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// If not, create a new set consisting of the union of the previous set
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// and this global. Mark it as encountered, so we can reuse it later.
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GlobalUsesByFunction[ParentFn] = EncounteredUGS[UGSIdx] =
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UsedGlobalSets.size();
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UsedGlobalSet &NewUGS = CreateGlobalSet();
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NewUGS.Globals.set(GI);
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NewUGS.Globals |= UsedGlobalSets[UGSIdx].Globals;
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}
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}
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}
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// Now we found a bunch of sets of globals used together. We accumulated
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// the number of times we encountered the sets (i.e., the number of blocks
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// that use that exact set of globals).
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//
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// Multiply that by the size of the set to give us a crude profitability
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// metric.
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std::sort(UsedGlobalSets.begin(), UsedGlobalSets.end(),
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[](const UsedGlobalSet &UGS1, const UsedGlobalSet &UGS2) {
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return UGS1.Globals.count() * UGS1.UsageCount <
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UGS2.Globals.count() * UGS2.UsageCount;
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});
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// We can choose to merge all globals together, but ignore globals never used
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// with another global. This catches the obviously non-profitable cases of
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// having a single global, but is aggressive enough for any other case.
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if (GlobalMergeIgnoreSingleUse) {
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BitVector AllGlobals(Globals.size());
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for (size_t i = 0, e = UsedGlobalSets.size(); i != e; ++i) {
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const UsedGlobalSet &UGS = UsedGlobalSets[e - i - 1];
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if (UGS.UsageCount == 0)
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continue;
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if (UGS.Globals.count() > 1)
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AllGlobals |= UGS.Globals;
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}
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return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
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}
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// Starting from the sets with the best (=biggest) profitability, find a
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// good combination.
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// The ideal (and expensive) solution can only be found by trying all
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// combinations, looking for the one with the best profitability.
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// Don't be smart about it, and just pick the first compatible combination,
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// starting with the sets with the best profitability.
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BitVector PickedGlobals(Globals.size());
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bool Changed = false;
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for (size_t i = 0, e = UsedGlobalSets.size(); i != e; ++i) {
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const UsedGlobalSet &UGS = UsedGlobalSets[e - i - 1];
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if (UGS.UsageCount == 0)
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continue;
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if (PickedGlobals.anyCommon(UGS.Globals))
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continue;
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PickedGlobals |= UGS.Globals;
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// If the set only contains one global, there's no point in merging.
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// Ignore the global for inclusion in other sets though, so keep it in
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// PickedGlobals.
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if (UGS.Globals.count() < 2)
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continue;
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Changed |= doMerge(Globals, UGS.Globals, M, isConst, AddrSpace);
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}
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return Changed;
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}
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bool GlobalMerge::doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
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const BitVector &GlobalSet, Module &M, bool isConst,
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unsigned AddrSpace) const {
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assert(Globals.size() > 1);
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Type *Int32Ty = Type::getInt32Ty(M.getContext());
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auto &DL = M.getDataLayout();
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DEBUG(dbgs() << " Trying to merge set, starts with #"
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<< GlobalSet.find_first() << "\n");
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ssize_t i = GlobalSet.find_first();
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while (i != -1) {
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ssize_t j = 0;
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uint64_t MergedSize = 0;
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std::vector<Type*> Tys;
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std::vector<Constant*> Inits;
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bool HasExternal = false;
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StringRef FirstExternalName;
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for (j = i; j != -1; j = GlobalSet.find_next(j)) {
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Type *Ty = Globals[j]->getValueType();
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MergedSize += DL.getTypeAllocSize(Ty);
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if (MergedSize > MaxOffset) {
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break;
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}
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Tys.push_back(Ty);
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Inits.push_back(Globals[j]->getInitializer());
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if (Globals[j]->hasExternalLinkage() && !HasExternal) {
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HasExternal = true;
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FirstExternalName = Globals[j]->getName();
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}
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}
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// If merged variables doesn't have external linkage, we needn't to expose
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// the symbol after merging.
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GlobalValue::LinkageTypes Linkage = HasExternal
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? GlobalValue::ExternalLinkage
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: GlobalValue::InternalLinkage;
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StructType *MergedTy = StructType::get(M.getContext(), Tys);
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Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
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// On Darwin external linkage needs to be preserved, otherwise
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// dsymutil cannot preserve the debug info for the merged
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// variables. If they have external linkage, use the symbol name
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// of the first variable merged as the suffix of global symbol
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// name. This avoids a link-time naming conflict for the
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// _MergedGlobals symbols.
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Twine MergedName =
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(IsMachO && HasExternal)
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? "_MergedGlobals_" + FirstExternalName
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: "_MergedGlobals";
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auto MergedLinkage = IsMachO ? Linkage : GlobalValue::PrivateLinkage;
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auto *MergedGV = new GlobalVariable(
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M, MergedTy, isConst, MergedLinkage, MergedInit, MergedName, nullptr,
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GlobalVariable::NotThreadLocal, AddrSpace);
|
|
|
|
const StructLayout *MergedLayout = DL.getStructLayout(MergedTy);
|
|
|
|
for (ssize_t k = i, idx = 0; k != j; k = GlobalSet.find_next(k), ++idx) {
|
|
GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();
|
|
std::string Name = Globals[k]->getName();
|
|
|
|
// Copy metadata while adjusting any debug info metadata by the original
|
|
// global's offset within the merged global.
|
|
MergedGV->copyMetadata(Globals[k], MergedLayout->getElementOffset(idx));
|
|
|
|
Constant *Idx[2] = {
|
|
ConstantInt::get(Int32Ty, 0),
|
|
ConstantInt::get(Int32Ty, idx),
|
|
};
|
|
Constant *GEP =
|
|
ConstantExpr::getInBoundsGetElementPtr(MergedTy, MergedGV, Idx);
|
|
Globals[k]->replaceAllUsesWith(GEP);
|
|
Globals[k]->eraseFromParent();
|
|
|
|
// When the linkage is not internal we must emit an alias for the original
|
|
// variable name as it may be accessed from another object. On non-Mach-O
|
|
// we can also emit an alias for internal linkage as it's safe to do so.
|
|
// It's not safe on Mach-O as the alias (and thus the portion of the
|
|
// MergedGlobals variable) may be dead stripped at link time.
|
|
if (Linkage != GlobalValue::InternalLinkage || !IsMachO) {
|
|
GlobalAlias::create(Tys[idx], AddrSpace, Linkage, Name, GEP, &M);
|
|
}
|
|
|
|
NumMerged++;
|
|
}
|
|
i = j;
|
|
}
|
|
|
|
return true;
|
|
}
|
|
|
|
void GlobalMerge::collectUsedGlobalVariables(Module &M) {
|
|
// Extract global variables from llvm.used array
|
|
const GlobalVariable *GV = M.getGlobalVariable("llvm.used");
|
|
if (!GV || !GV->hasInitializer()) return;
|
|
|
|
// Should be an array of 'i8*'.
|
|
const ConstantArray *InitList = cast<ConstantArray>(GV->getInitializer());
|
|
|
|
for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
|
|
if (const GlobalVariable *G =
|
|
dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts()))
|
|
MustKeepGlobalVariables.insert(G);
|
|
}
|
|
|
|
void GlobalMerge::setMustKeepGlobalVariables(Module &M) {
|
|
collectUsedGlobalVariables(M);
|
|
|
|
for (Function &F : M) {
|
|
for (BasicBlock &BB : F) {
|
|
Instruction *Pad = BB.getFirstNonPHI();
|
|
if (!Pad->isEHPad())
|
|
continue;
|
|
|
|
// Keep globals used by landingpads and catchpads.
|
|
for (const Use &U : Pad->operands()) {
|
|
if (const GlobalVariable *GV =
|
|
dyn_cast<GlobalVariable>(U->stripPointerCasts()))
|
|
MustKeepGlobalVariables.insert(GV);
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
bool GlobalMerge::doInitialization(Module &M) {
|
|
if (!EnableGlobalMerge)
|
|
return false;
|
|
|
|
IsMachO = Triple(M.getTargetTriple()).isOSBinFormatMachO();
|
|
|
|
auto &DL = M.getDataLayout();
|
|
DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals,
|
|
BSSGlobals;
|
|
bool Changed = false;
|
|
setMustKeepGlobalVariables(M);
|
|
|
|
// Grab all non-const globals.
|
|
for (auto &GV : M.globals()) {
|
|
// Merge is safe for "normal" internal or external globals only
|
|
if (GV.isDeclaration() || GV.isThreadLocal() || GV.hasSection())
|
|
continue;
|
|
|
|
if (!(MergeExternalGlobals && GV.hasExternalLinkage()) &&
|
|
!GV.hasInternalLinkage())
|
|
continue;
|
|
|
|
PointerType *PT = dyn_cast<PointerType>(GV.getType());
|
|
assert(PT && "Global variable is not a pointer!");
|
|
|
|
unsigned AddressSpace = PT->getAddressSpace();
|
|
|
|
// Ignore fancy-aligned globals for now.
|
|
unsigned Alignment = DL.getPreferredAlignment(&GV);
|
|
Type *Ty = GV.getValueType();
|
|
if (Alignment > DL.getABITypeAlignment(Ty))
|
|
continue;
|
|
|
|
// Ignore all 'special' globals.
|
|
if (GV.getName().startswith("llvm.") ||
|
|
GV.getName().startswith(".llvm."))
|
|
continue;
|
|
|
|
// Ignore all "required" globals:
|
|
if (isMustKeepGlobalVariable(&GV))
|
|
continue;
|
|
|
|
if (DL.getTypeAllocSize(Ty) < MaxOffset) {
|
|
if (TM &&
|
|
TargetLoweringObjectFile::getKindForGlobal(&GV, *TM).isBSSLocal())
|
|
BSSGlobals[AddressSpace].push_back(&GV);
|
|
else if (GV.isConstant())
|
|
ConstGlobals[AddressSpace].push_back(&GV);
|
|
else
|
|
Globals[AddressSpace].push_back(&GV);
|
|
}
|
|
}
|
|
|
|
for (auto &P : Globals)
|
|
if (P.second.size() > 1)
|
|
Changed |= doMerge(P.second, M, false, P.first);
|
|
|
|
for (auto &P : BSSGlobals)
|
|
if (P.second.size() > 1)
|
|
Changed |= doMerge(P.second, M, false, P.first);
|
|
|
|
if (EnableGlobalMergeOnConst)
|
|
for (auto &P : ConstGlobals)
|
|
if (P.second.size() > 1)
|
|
Changed |= doMerge(P.second, M, true, P.first);
|
|
|
|
return Changed;
|
|
}
|
|
|
|
bool GlobalMerge::runOnFunction(Function &F) {
|
|
return false;
|
|
}
|
|
|
|
bool GlobalMerge::doFinalization(Module &M) {
|
|
MustKeepGlobalVariables.clear();
|
|
return false;
|
|
}
|
|
|
|
Pass *llvm::createGlobalMergePass(const TargetMachine *TM, unsigned Offset,
|
|
bool OnlyOptimizeForSize,
|
|
bool MergeExternalByDefault) {
|
|
bool MergeExternal = (EnableGlobalMergeOnExternal == cl::BOU_UNSET) ?
|
|
MergeExternalByDefault : (EnableGlobalMergeOnExternal == cl::BOU_TRUE);
|
|
return new GlobalMerge(TM, Offset, OnlyOptimizeForSize, MergeExternal);
|
|
}
|