413 lines
16 KiB
C++
413 lines
16 KiB
C++
//===-- PhiElimination.cpp - Eliminate PHI nodes by inserting copies ------===//
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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 pass eliminates machine instruction PHI nodes by inserting copy
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// instructions. This destroys SSA information, but is the desired input for
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// some register allocators.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "phielim"
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#include "PHIElimination.h"
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#include "llvm/CodeGen/LiveVariables.h"
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#include "llvm/CodeGen/Passes.h"
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#include "llvm/CodeGen/MachineDominators.h"
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#include "llvm/CodeGen/MachineInstr.h"
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#include "llvm/CodeGen/MachineInstrBuilder.h"
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#include "llvm/CodeGen/MachineLoopInfo.h"
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#include "llvm/CodeGen/MachineRegisterInfo.h"
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#include "llvm/Target/TargetInstrInfo.h"
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#include "llvm/Function.h"
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#include "llvm/Target/TargetMachine.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Support/Compiler.h"
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#include "llvm/Support/Debug.h"
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#include <algorithm>
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#include <map>
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using namespace llvm;
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STATISTIC(NumAtomic, "Number of atomic phis lowered");
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STATISTIC(NumReused, "Number of reused lowered phis");
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char PHIElimination::ID = 0;
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INITIALIZE_PASS(PHIElimination, "phi-node-elimination",
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"Eliminate PHI nodes for register allocation", false, false);
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char &llvm::PHIEliminationID = PHIElimination::ID;
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void llvm::PHIElimination::getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addPreserved<LiveVariables>();
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AU.addPreserved<MachineDominatorTree>();
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AU.addPreserved<MachineLoopInfo>();
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MachineFunctionPass::getAnalysisUsage(AU);
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}
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bool llvm::PHIElimination::runOnMachineFunction(MachineFunction &MF) {
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MRI = &MF.getRegInfo();
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bool Changed = false;
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// Split critical edges to help the coalescer
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if (LiveVariables *LV = getAnalysisIfAvailable<LiveVariables>()) {
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MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
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for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
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Changed |= SplitPHIEdges(MF, *I, *LV, MLI);
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}
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// Populate VRegPHIUseCount
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analyzePHINodes(MF);
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// Eliminate PHI instructions by inserting copies into predecessor blocks.
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for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
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Changed |= EliminatePHINodes(MF, *I);
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// Remove dead IMPLICIT_DEF instructions.
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for (SmallPtrSet<MachineInstr*, 4>::iterator I = ImpDefs.begin(),
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E = ImpDefs.end(); I != E; ++I) {
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MachineInstr *DefMI = *I;
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unsigned DefReg = DefMI->getOperand(0).getReg();
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if (MRI->use_nodbg_empty(DefReg))
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DefMI->eraseFromParent();
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}
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// Clean up the lowered PHI instructions.
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for (LoweredPHIMap::iterator I = LoweredPHIs.begin(), E = LoweredPHIs.end();
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I != E; ++I)
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MF.DeleteMachineInstr(I->first);
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LoweredPHIs.clear();
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ImpDefs.clear();
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VRegPHIUseCount.clear();
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return Changed;
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}
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/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
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/// predecessor basic blocks.
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///
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bool llvm::PHIElimination::EliminatePHINodes(MachineFunction &MF,
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MachineBasicBlock &MBB) {
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if (MBB.empty() || !MBB.front().isPHI())
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return false; // Quick exit for basic blocks without PHIs.
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// Get an iterator to the first instruction after the last PHI node (this may
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// also be the end of the basic block).
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MachineBasicBlock::iterator AfterPHIsIt = SkipPHIsAndLabels(MBB, MBB.begin());
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while (MBB.front().isPHI())
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LowerAtomicPHINode(MBB, AfterPHIsIt);
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return true;
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}
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/// isSourceDefinedByImplicitDef - Return true if all sources of the phi node
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/// are implicit_def's.
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static bool isSourceDefinedByImplicitDef(const MachineInstr *MPhi,
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const MachineRegisterInfo *MRI) {
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for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) {
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unsigned SrcReg = MPhi->getOperand(i).getReg();
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const MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
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if (!DefMI || !DefMI->isImplicitDef())
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return false;
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}
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return true;
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}
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// FindCopyInsertPoint - Find a safe place in MBB to insert a copy from SrcReg
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// when following the CFG edge to SuccMBB. This needs to be after any def of
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// SrcReg, but before any subsequent point where control flow might jump out of
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// the basic block.
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MachineBasicBlock::iterator
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llvm::PHIElimination::FindCopyInsertPoint(MachineBasicBlock &MBB,
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MachineBasicBlock &SuccMBB,
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unsigned SrcReg) {
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// Handle the trivial case trivially.
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if (MBB.empty())
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return MBB.begin();
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// Usually, we just want to insert the copy before the first terminator
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// instruction. However, for the edge going to a landing pad, we must insert
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// the copy before the call/invoke instruction.
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if (!SuccMBB.isLandingPad())
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return MBB.getFirstTerminator();
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// Discover any defs/uses in this basic block.
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SmallPtrSet<MachineInstr*, 8> DefUsesInMBB;
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for (MachineRegisterInfo::reg_iterator RI = MRI->reg_begin(SrcReg),
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RE = MRI->reg_end(); RI != RE; ++RI) {
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MachineInstr *DefUseMI = &*RI;
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if (DefUseMI->getParent() == &MBB)
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DefUsesInMBB.insert(DefUseMI);
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}
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MachineBasicBlock::iterator InsertPoint;
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if (DefUsesInMBB.empty()) {
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// No defs. Insert the copy at the start of the basic block.
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InsertPoint = MBB.begin();
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} else if (DefUsesInMBB.size() == 1) {
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// Insert the copy immediately after the def/use.
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InsertPoint = *DefUsesInMBB.begin();
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++InsertPoint;
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} else {
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// Insert the copy immediately after the last def/use.
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InsertPoint = MBB.end();
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while (!DefUsesInMBB.count(&*--InsertPoint)) {}
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++InsertPoint;
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}
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// Make sure the copy goes after any phi nodes however.
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return SkipPHIsAndLabels(MBB, InsertPoint);
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}
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/// LowerAtomicPHINode - Lower the PHI node at the top of the specified block,
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/// under the assuption that it needs to be lowered in a way that supports
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/// atomic execution of PHIs. This lowering method is always correct all of the
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/// time.
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///
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void llvm::PHIElimination::LowerAtomicPHINode(
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MachineBasicBlock &MBB,
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MachineBasicBlock::iterator AfterPHIsIt) {
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++NumAtomic;
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// Unlink the PHI node from the basic block, but don't delete the PHI yet.
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MachineInstr *MPhi = MBB.remove(MBB.begin());
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unsigned NumSrcs = (MPhi->getNumOperands() - 1) / 2;
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unsigned DestReg = MPhi->getOperand(0).getReg();
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assert(MPhi->getOperand(0).getSubReg() == 0 && "Can't handle sub-reg PHIs");
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bool isDead = MPhi->getOperand(0).isDead();
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// Create a new register for the incoming PHI arguments.
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MachineFunction &MF = *MBB.getParent();
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unsigned IncomingReg = 0;
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bool reusedIncoming = false; // Is IncomingReg reused from an earlier PHI?
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// Insert a register to register copy at the top of the current block (but
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// after any remaining phi nodes) which copies the new incoming register
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// into the phi node destination.
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const TargetInstrInfo *TII = MF.getTarget().getInstrInfo();
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if (isSourceDefinedByImplicitDef(MPhi, MRI))
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// If all sources of a PHI node are implicit_def, just emit an
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// implicit_def instead of a copy.
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BuildMI(MBB, AfterPHIsIt, MPhi->getDebugLoc(),
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TII->get(TargetOpcode::IMPLICIT_DEF), DestReg);
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else {
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// Can we reuse an earlier PHI node? This only happens for critical edges,
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// typically those created by tail duplication.
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unsigned &entry = LoweredPHIs[MPhi];
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if (entry) {
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// An identical PHI node was already lowered. Reuse the incoming register.
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IncomingReg = entry;
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reusedIncoming = true;
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++NumReused;
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DEBUG(dbgs() << "Reusing %reg" << IncomingReg << " for " << *MPhi);
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} else {
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const TargetRegisterClass *RC = MF.getRegInfo().getRegClass(DestReg);
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entry = IncomingReg = MF.getRegInfo().createVirtualRegister(RC);
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}
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BuildMI(MBB, AfterPHIsIt, MPhi->getDebugLoc(),
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TII->get(TargetOpcode::COPY), DestReg)
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.addReg(IncomingReg);
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}
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// Update live variable information if there is any.
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LiveVariables *LV = getAnalysisIfAvailable<LiveVariables>();
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if (LV) {
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MachineInstr *PHICopy = prior(AfterPHIsIt);
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if (IncomingReg) {
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LiveVariables::VarInfo &VI = LV->getVarInfo(IncomingReg);
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// Increment use count of the newly created virtual register.
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VI.NumUses++;
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LV->setPHIJoin(IncomingReg);
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// When we are reusing the incoming register, it may already have been
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// killed in this block. The old kill will also have been inserted at
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// AfterPHIsIt, so it appears before the current PHICopy.
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if (reusedIncoming)
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if (MachineInstr *OldKill = VI.findKill(&MBB)) {
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DEBUG(dbgs() << "Remove old kill from " << *OldKill);
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LV->removeVirtualRegisterKilled(IncomingReg, OldKill);
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DEBUG(MBB.dump());
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}
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// Add information to LiveVariables to know that the incoming value is
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// killed. Note that because the value is defined in several places (once
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// each for each incoming block), the "def" block and instruction fields
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// for the VarInfo is not filled in.
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LV->addVirtualRegisterKilled(IncomingReg, PHICopy);
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}
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// Since we are going to be deleting the PHI node, if it is the last use of
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// any registers, or if the value itself is dead, we need to move this
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// information over to the new copy we just inserted.
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LV->removeVirtualRegistersKilled(MPhi);
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// If the result is dead, update LV.
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if (isDead) {
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LV->addVirtualRegisterDead(DestReg, PHICopy);
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LV->removeVirtualRegisterDead(DestReg, MPhi);
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}
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}
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// Adjust the VRegPHIUseCount map to account for the removal of this PHI node.
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for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2)
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--VRegPHIUseCount[BBVRegPair(MPhi->getOperand(i+1).getMBB()->getNumber(),
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MPhi->getOperand(i).getReg())];
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// Now loop over all of the incoming arguments, changing them to copy into the
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// IncomingReg register in the corresponding predecessor basic block.
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SmallPtrSet<MachineBasicBlock*, 8> MBBsInsertedInto;
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for (int i = NumSrcs - 1; i >= 0; --i) {
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unsigned SrcReg = MPhi->getOperand(i*2+1).getReg();
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unsigned SrcSubReg = MPhi->getOperand(i*2+1).getSubReg();
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assert(TargetRegisterInfo::isVirtualRegister(SrcReg) &&
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"Machine PHI Operands must all be virtual registers!");
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// Get the MachineBasicBlock equivalent of the BasicBlock that is the source
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// path the PHI.
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MachineBasicBlock &opBlock = *MPhi->getOperand(i*2+2).getMBB();
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// If source is defined by an implicit def, there is no need to insert a
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// copy.
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MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
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if (DefMI->isImplicitDef()) {
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ImpDefs.insert(DefMI);
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continue;
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}
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// Check to make sure we haven't already emitted the copy for this block.
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// This can happen because PHI nodes may have multiple entries for the same
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// basic block.
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if (!MBBsInsertedInto.insert(&opBlock))
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continue; // If the copy has already been emitted, we're done.
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// Find a safe location to insert the copy, this may be the first terminator
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// in the block (or end()).
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MachineBasicBlock::iterator InsertPos =
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FindCopyInsertPoint(opBlock, MBB, SrcReg);
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// Insert the copy.
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if (!reusedIncoming && IncomingReg)
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BuildMI(opBlock, InsertPos, MPhi->getDebugLoc(),
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TII->get(TargetOpcode::COPY), IncomingReg).addReg(SrcReg, 0, SrcSubReg);
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// Now update live variable information if we have it. Otherwise we're done
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if (!LV) continue;
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// We want to be able to insert a kill of the register if this PHI (aka, the
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// copy we just inserted) is the last use of the source value. Live
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// variable analysis conservatively handles this by saying that the value is
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// live until the end of the block the PHI entry lives in. If the value
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// really is dead at the PHI copy, there will be no successor blocks which
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// have the value live-in.
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// Also check to see if this register is in use by another PHI node which
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// has not yet been eliminated. If so, it will be killed at an appropriate
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// point later.
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// Is it used by any PHI instructions in this block?
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bool ValueIsUsed = VRegPHIUseCount[BBVRegPair(opBlock.getNumber(), SrcReg)];
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// Okay, if we now know that the value is not live out of the block, we can
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// add a kill marker in this block saying that it kills the incoming value!
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if (!ValueIsUsed && !LV->isLiveOut(SrcReg, opBlock)) {
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// In our final twist, we have to decide which instruction kills the
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// register. In most cases this is the copy, however, the first
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// terminator instruction at the end of the block may also use the value.
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// In this case, we should mark *it* as being the killing block, not the
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// copy.
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MachineBasicBlock::iterator KillInst;
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MachineBasicBlock::iterator Term = opBlock.getFirstTerminator();
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if (Term != opBlock.end() && Term->readsRegister(SrcReg)) {
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KillInst = Term;
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// Check that no other terminators use values.
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#ifndef NDEBUG
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for (MachineBasicBlock::iterator TI = llvm::next(Term);
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TI != opBlock.end(); ++TI) {
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assert(!TI->readsRegister(SrcReg) &&
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"Terminator instructions cannot use virtual registers unless"
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"they are the first terminator in a block!");
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}
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#endif
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} else if (reusedIncoming || !IncomingReg) {
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// We may have to rewind a bit if we didn't insert a copy this time.
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KillInst = Term;
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while (KillInst != opBlock.begin())
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if ((--KillInst)->readsRegister(SrcReg))
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break;
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} else {
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// We just inserted this copy.
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KillInst = prior(InsertPos);
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}
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assert(KillInst->readsRegister(SrcReg) && "Cannot find kill instruction");
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// Finally, mark it killed.
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LV->addVirtualRegisterKilled(SrcReg, KillInst);
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// This vreg no longer lives all of the way through opBlock.
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unsigned opBlockNum = opBlock.getNumber();
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LV->getVarInfo(SrcReg).AliveBlocks.reset(opBlockNum);
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}
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}
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// Really delete the PHI instruction now, if it is not in the LoweredPHIs map.
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if (reusedIncoming || !IncomingReg)
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MF.DeleteMachineInstr(MPhi);
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}
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/// analyzePHINodes - Gather information about the PHI nodes in here. In
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/// particular, we want to map the number of uses of a virtual register which is
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/// used in a PHI node. We map that to the BB the vreg is coming from. This is
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/// used later to determine when the vreg is killed in the BB.
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///
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void llvm::PHIElimination::analyzePHINodes(const MachineFunction& MF) {
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for (MachineFunction::const_iterator I = MF.begin(), E = MF.end();
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I != E; ++I)
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for (MachineBasicBlock::const_iterator BBI = I->begin(), BBE = I->end();
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BBI != BBE && BBI->isPHI(); ++BBI)
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for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
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++VRegPHIUseCount[BBVRegPair(BBI->getOperand(i+1).getMBB()->getNumber(),
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BBI->getOperand(i).getReg())];
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}
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bool llvm::PHIElimination::SplitPHIEdges(MachineFunction &MF,
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MachineBasicBlock &MBB,
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LiveVariables &LV,
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MachineLoopInfo *MLI) {
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if (MBB.empty() || !MBB.front().isPHI() || MBB.isLandingPad())
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return false; // Quick exit for basic blocks without PHIs.
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bool Changed = false;
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for (MachineBasicBlock::const_iterator BBI = MBB.begin(), BBE = MBB.end();
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BBI != BBE && BBI->isPHI(); ++BBI) {
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for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) {
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unsigned Reg = BBI->getOperand(i).getReg();
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MachineBasicBlock *PreMBB = BBI->getOperand(i+1).getMBB();
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// We break edges when registers are live out from the predecessor block
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// (not considering PHI nodes). If the register is live in to this block
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// anyway, we would gain nothing from splitting.
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// Avoid splitting backedges of loops. It would introduce small
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// out-of-line blocks into the loop which is very bad for code placement.
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if (PreMBB != &MBB &&
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!LV.isLiveIn(Reg, MBB) && LV.isLiveOut(Reg, *PreMBB)) {
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if (!MLI ||
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!(MLI->getLoopFor(PreMBB) == MLI->getLoopFor(&MBB) &&
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MLI->isLoopHeader(&MBB)))
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Changed |= PreMBB->SplitCriticalEdge(&MBB, this) != 0;
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}
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}
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}
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return true;
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}
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