//===-- PhiElimination.cpp - Eliminate PHI nodes by inserting copies ------===//
//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
// This pass eliminates machine instruction PHI nodes by inserting copy
// instructions. This destroys SSA information, but is the desired input for
// some register allocators.
//
//===----------------------------------------------------------------------===//
-#include "llvm/CodeGen/MachineFunctionPass.h"
-#include "llvm/CodeGen/MachineInstr.h"
-#include "llvm/CodeGen/SSARegMap.h"
+#define DEBUG_TYPE "phielim"
+#include "PHIEliminationUtils.h"
#include "llvm/CodeGen/LiveVariables.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/MachineDominators.h"
+#include "llvm/CodeGen/MachineInstr.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineLoopInfo.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
+#include "llvm/Function.h"
#include "llvm/Target/TargetMachine.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/Debug.h"
+#include <algorithm>
+using namespace llvm;
-namespace {
- struct PNE : public MachineFunctionPass {
- bool runOnMachineFunction(MachineFunction &Fn) {
- bool Changed = false;
+static cl::opt<bool>
+DisableEdgeSplitting("disable-phi-elim-edge-splitting", cl::init(false),
+ cl::Hidden, cl::desc("Disable critical edge splitting "
+ "during PHI elimination"));
- // Eliminate PHI instructions by inserting copies into predecessor blocks.
- //
- for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
- Changed |= EliminatePHINodes(Fn, *I);
+namespace {
+ class PHIElimination : public MachineFunctionPass {
+ MachineRegisterInfo *MRI; // Machine register information
- //std::cerr << "AFTER PHI NODE ELIM:\n";
- //Fn.dump();
- return Changed;
+ public:
+ static char ID; // Pass identification, replacement for typeid
+ PHIElimination() : MachineFunctionPass(ID) {
+ initializePHIEliminationPass(*PassRegistry::getPassRegistry());
}
- virtual void getAnalysisUsage(AnalysisUsage &AU) const {
- AU.addPreserved<LiveVariables>();
- MachineFunctionPass::getAnalysisUsage(AU);
- }
+ virtual bool runOnMachineFunction(MachineFunction &Fn);
+ virtual void getAnalysisUsage(AnalysisUsage &AU) const;
private:
/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions
/// in predecessor basic blocks.
///
bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB);
+ void LowerAtomicPHINode(MachineBasicBlock &MBB,
+ MachineBasicBlock::iterator AfterPHIsIt);
+
+ /// analyzePHINodes - Gather information about the PHI nodes in
+ /// here. In particular, we want to map the number of uses of a virtual
+ /// register which is used in a PHI node. We map that to the BB the
+ /// vreg is coming from. This is used later to determine when the vreg
+ /// is killed in the BB.
+ ///
+ void analyzePHINodes(const MachineFunction& Fn);
+
+ /// Split critical edges where necessary for good coalescer performance.
+ bool SplitPHIEdges(MachineFunction &MF, MachineBasicBlock &MBB,
+ LiveVariables &LV, MachineLoopInfo *MLI);
+
+ typedef std::pair<unsigned, unsigned> BBVRegPair;
+ typedef DenseMap<BBVRegPair, unsigned> VRegPHIUse;
+
+ VRegPHIUse VRegPHIUseCount;
+
+ // Defs of PHI sources which are implicit_def.
+ SmallPtrSet<MachineInstr*, 4> ImpDefs;
+
+ // Map reusable lowered PHI node -> incoming join register.
+ typedef DenseMap<MachineInstr*, unsigned,
+ MachineInstrExpressionTrait> LoweredPHIMap;
+ LoweredPHIMap LoweredPHIs;
};
+}
+
+STATISTIC(NumAtomic, "Number of atomic phis lowered");
+STATISTIC(NumCriticalEdgesSplit, "Number of critical edges split");
+STATISTIC(NumReused, "Number of reused lowered phis");
- RegisterPass<PNE> X("phi-node-elimination",
- "Eliminate PHI nodes for register allocation");
+char PHIElimination::ID = 0;
+char& llvm::PHIEliminationID = PHIElimination::ID;
+
+INITIALIZE_PASS_BEGIN(PHIElimination, "phi-node-elimination",
+ "Eliminate PHI nodes for register allocation",
+ false, false)
+INITIALIZE_PASS_DEPENDENCY(LiveVariables)
+INITIALIZE_PASS_END(PHIElimination, "phi-node-elimination",
+ "Eliminate PHI nodes for register allocation", false, false)
+
+void PHIElimination::getAnalysisUsage(AnalysisUsage &AU) const {
+ AU.addPreserved<LiveVariables>();
+ AU.addPreserved<MachineDominatorTree>();
+ AU.addPreserved<MachineLoopInfo>();
+ MachineFunctionPass::getAnalysisUsage(AU);
}
-const PassInfo *PHIEliminationID = X.getPassInfo();
+bool PHIElimination::runOnMachineFunction(MachineFunction &MF) {
+ MRI = &MF.getRegInfo();
+
+ bool Changed = false;
+
+ // This pass takes the function out of SSA form.
+ MRI->leaveSSA();
+
+ // Split critical edges to help the coalescer
+ if (!DisableEdgeSplitting) {
+ if (LiveVariables *LV = getAnalysisIfAvailable<LiveVariables>()) {
+ MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
+ for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
+ Changed |= SplitPHIEdges(MF, *I, *LV, MLI);
+ }
+ }
+
+ // Populate VRegPHIUseCount
+ analyzePHINodes(MF);
+
+ // Eliminate PHI instructions by inserting copies into predecessor blocks.
+ for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
+ Changed |= EliminatePHINodes(MF, *I);
+
+ // Remove dead IMPLICIT_DEF instructions.
+ for (SmallPtrSet<MachineInstr*, 4>::iterator I = ImpDefs.begin(),
+ E = ImpDefs.end(); I != E; ++I) {
+ MachineInstr *DefMI = *I;
+ unsigned DefReg = DefMI->getOperand(0).getReg();
+ if (MRI->use_nodbg_empty(DefReg))
+ DefMI->eraseFromParent();
+ }
+
+ // Clean up the lowered PHI instructions.
+ for (LoweredPHIMap::iterator I = LoweredPHIs.begin(), E = LoweredPHIs.end();
+ I != E; ++I)
+ MF.DeleteMachineInstr(I->first);
+
+ LoweredPHIs.clear();
+ ImpDefs.clear();
+ VRegPHIUseCount.clear();
+
+ return Changed;
+}
/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
/// predecessor basic blocks.
///
-bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) {
- if (MBB.empty() || MBB.front()->getOpcode() != TargetInstrInfo::PHI)
- return false; // Quick exit for normal case...
-
- LiveVariables *LV = getAnalysisToUpdate<LiveVariables>();
- const TargetInstrInfo &MII = MF.getTarget().getInstrInfo();
- const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo();
-
- while (MBB.front()->getOpcode() == TargetInstrInfo::PHI) {
- MachineInstr *MI = MBB.front();
- // Unlink the PHI node from the basic block... but don't delete the PHI yet
- MBB.erase(MBB.begin());
-
- assert(MI->getOperand(0).isVirtualRegister() &&
- "PHI node doesn't write virt reg?");
-
- unsigned DestReg = MI->getOperand(0).getAllocatedRegNum();
-
- // Create a new register for the incoming PHI arguments
- const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg);
- unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC);
-
- // Insert a register to register copy in the top of the current block (but
- // after any remaining phi nodes) which copies the new incoming register
- // into the phi node destination.
- //
- MachineBasicBlock::iterator AfterPHIsIt = MBB.begin();
- if (AfterPHIsIt != MBB.end())
- while ((*AfterPHIsIt)->getOpcode() == TargetInstrInfo::PHI) ++AfterPHIsIt;
- RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);
-
- // Update live variable information if there is any...
- if (LV) {
- MachineInstr *PHICopy = *(AfterPHIsIt-1);
+bool PHIElimination::EliminatePHINodes(MachineFunction &MF,
+ MachineBasicBlock &MBB) {
+ if (MBB.empty() || !MBB.front().isPHI())
+ return false; // Quick exit for basic blocks without PHIs.
+
+ // Get an iterator to the first instruction after the last PHI node (this may
+ // also be the end of the basic block).
+ MachineBasicBlock::iterator AfterPHIsIt = MBB.SkipPHIsAndLabels(MBB.begin());
+
+ while (MBB.front().isPHI())
+ LowerAtomicPHINode(MBB, AfterPHIsIt);
+
+ return true;
+}
+
+/// isImplicitlyDefined - Return true if all defs of VirtReg are implicit-defs.
+/// This includes registers with no defs.
+static bool isImplicitlyDefined(unsigned VirtReg,
+ const MachineRegisterInfo *MRI) {
+ for (MachineRegisterInfo::def_iterator DI = MRI->def_begin(VirtReg),
+ DE = MRI->def_end(); DI != DE; ++DI)
+ if (!DI->isImplicitDef())
+ return false;
+ return true;
+}
+
+/// isSourceDefinedByImplicitDef - Return true if all sources of the phi node
+/// are implicit_def's.
+static bool isSourceDefinedByImplicitDef(const MachineInstr *MPhi,
+ const MachineRegisterInfo *MRI) {
+ for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2)
+ if (!isImplicitlyDefined(MPhi->getOperand(i).getReg(), MRI))
+ return false;
+ return true;
+}
+
+
+/// LowerAtomicPHINode - Lower the PHI node at the top of the specified block,
+/// under the assumption that it needs to be lowered in a way that supports
+/// atomic execution of PHIs. This lowering method is always correct all of the
+/// time.
+///
+void PHIElimination::LowerAtomicPHINode(
+ MachineBasicBlock &MBB,
+ MachineBasicBlock::iterator AfterPHIsIt) {
+ ++NumAtomic;
+ // Unlink the PHI node from the basic block, but don't delete the PHI yet.
+ MachineInstr *MPhi = MBB.remove(MBB.begin());
+
+ unsigned NumSrcs = (MPhi->getNumOperands() - 1) / 2;
+ unsigned DestReg = MPhi->getOperand(0).getReg();
+ assert(MPhi->getOperand(0).getSubReg() == 0 && "Can't handle sub-reg PHIs");
+ bool isDead = MPhi->getOperand(0).isDead();
+
+ // Create a new register for the incoming PHI arguments.
+ MachineFunction &MF = *MBB.getParent();
+ unsigned IncomingReg = 0;
+ bool reusedIncoming = false; // Is IncomingReg reused from an earlier PHI?
+
+ // Insert a register to register copy at the top of the current block (but
+ // after any remaining phi nodes) which copies the new incoming register
+ // into the phi node destination.
+ const TargetInstrInfo *TII = MF.getTarget().getInstrInfo();
+ if (isSourceDefinedByImplicitDef(MPhi, MRI))
+ // If all sources of a PHI node are implicit_def, just emit an
+ // implicit_def instead of a copy.
+ BuildMI(MBB, AfterPHIsIt, MPhi->getDebugLoc(),
+ TII->get(TargetOpcode::IMPLICIT_DEF), DestReg);
+ else {
+ // Can we reuse an earlier PHI node? This only happens for critical edges,
+ // typically those created by tail duplication.
+ unsigned &entry = LoweredPHIs[MPhi];
+ if (entry) {
+ // An identical PHI node was already lowered. Reuse the incoming register.
+ IncomingReg = entry;
+ reusedIncoming = true;
+ ++NumReused;
+ DEBUG(dbgs() << "Reusing " << PrintReg(IncomingReg) << " for " << *MPhi);
+ } else {
+ const TargetRegisterClass *RC = MF.getRegInfo().getRegClass(DestReg);
+ entry = IncomingReg = MF.getRegInfo().createVirtualRegister(RC);
+ }
+ BuildMI(MBB, AfterPHIsIt, MPhi->getDebugLoc(),
+ TII->get(TargetOpcode::COPY), DestReg)
+ .addReg(IncomingReg);
+ }
+
+ // Update live variable information if there is any.
+ LiveVariables *LV = getAnalysisIfAvailable<LiveVariables>();
+ if (LV) {
+ MachineInstr *PHICopy = prior(AfterPHIsIt);
+
+ if (IncomingReg) {
+ LiveVariables::VarInfo &VI = LV->getVarInfo(IncomingReg);
+
+ // Increment use count of the newly created virtual register.
+ LV->setPHIJoin(IncomingReg);
+
+ // When we are reusing the incoming register, it may already have been
+ // killed in this block. The old kill will also have been inserted at
+ // AfterPHIsIt, so it appears before the current PHICopy.
+ if (reusedIncoming)
+ if (MachineInstr *OldKill = VI.findKill(&MBB)) {
+ DEBUG(dbgs() << "Remove old kill from " << *OldKill);
+ LV->removeVirtualRegisterKilled(IncomingReg, OldKill);
+ DEBUG(MBB.dump());
+ }
// Add information to LiveVariables to know that the incoming value is
- // dead. This says that the register is dead, not killed, because we
- // cannot use the live variable information to indicate that the variable
- // is defined in multiple entry blocks. Instead, we pretend that this
- // instruction defined it and killed it at the same time.
- //
- LV->addVirtualRegisterDead(IncomingReg, PHICopy);
+ // killed. Note that because the value is defined in several places (once
+ // each for each incoming block), the "def" block and instruction fields
+ // for the VarInfo is not filled in.
+ LV->addVirtualRegisterKilled(IncomingReg, PHICopy);
+ }
- // Since we are going to be deleting the PHI node, if it is the last use
- // of any registers, or if the value itself is dead, we need to move this
- // information over to the new copy we just inserted...
- //
- std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator>
- RKs = LV->killed_range(MI);
- if (RKs.first != RKs.second) {
- for (LiveVariables::killed_iterator I = RKs.first; I != RKs.second; ++I)
- LV->addVirtualRegisterKilled(I->second, PHICopy);
- LV->removeVirtualRegistersKilled(RKs.first, RKs.second);
- }
+ // Since we are going to be deleting the PHI node, if it is the last use of
+ // any registers, or if the value itself is dead, we need to move this
+ // information over to the new copy we just inserted.
+ LV->removeVirtualRegistersKilled(MPhi);
- RKs = LV->dead_range(MI);
- if (RKs.first != RKs.second) {
- for (LiveVariables::killed_iterator I = RKs.first; I != RKs.second; ++I)
- LV->addVirtualRegisterDead(I->second, PHICopy);
- LV->removeVirtualRegistersDead(RKs.first, RKs.second);
+ // If the result is dead, update LV.
+ if (isDead) {
+ LV->addVirtualRegisterDead(DestReg, PHICopy);
+ LV->removeVirtualRegisterDead(DestReg, MPhi);
+ }
+ }
+
+ // Adjust the VRegPHIUseCount map to account for the removal of this PHI node.
+ for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2)
+ --VRegPHIUseCount[BBVRegPair(MPhi->getOperand(i+1).getMBB()->getNumber(),
+ MPhi->getOperand(i).getReg())];
+
+ // Now loop over all of the incoming arguments, changing them to copy into the
+ // IncomingReg register in the corresponding predecessor basic block.
+ SmallPtrSet<MachineBasicBlock*, 8> MBBsInsertedInto;
+ for (int i = NumSrcs - 1; i >= 0; --i) {
+ unsigned SrcReg = MPhi->getOperand(i*2+1).getReg();
+ unsigned SrcSubReg = MPhi->getOperand(i*2+1).getSubReg();
+ bool SrcUndef = MPhi->getOperand(i*2+1).isUndef() ||
+ isImplicitlyDefined(SrcReg, MRI);
+ assert(TargetRegisterInfo::isVirtualRegister(SrcReg) &&
+ "Machine PHI Operands must all be virtual registers!");
+
+ // Get the MachineBasicBlock equivalent of the BasicBlock that is the source
+ // path the PHI.
+ MachineBasicBlock &opBlock = *MPhi->getOperand(i*2+2).getMBB();
+
+ // Check to make sure we haven't already emitted the copy for this block.
+ // This can happen because PHI nodes may have multiple entries for the same
+ // basic block.
+ if (!MBBsInsertedInto.insert(&opBlock))
+ continue; // If the copy has already been emitted, we're done.
+
+ // Find a safe location to insert the copy, this may be the first terminator
+ // in the block (or end()).
+ MachineBasicBlock::iterator InsertPos =
+ findPHICopyInsertPoint(&opBlock, &MBB, SrcReg);
+
+ // Insert the copy.
+ if (!reusedIncoming && IncomingReg) {
+ if (SrcUndef) {
+ // The source register is undefined, so there is no need for a real
+ // COPY, but we still need to ensure joint dominance by defs.
+ // Insert an IMPLICIT_DEF instruction.
+ BuildMI(opBlock, InsertPos, MPhi->getDebugLoc(),
+ TII->get(TargetOpcode::IMPLICIT_DEF), IncomingReg);
+
+ // Clean up the old implicit-def, if there even was one.
+ if (MachineInstr *DefMI = MRI->getVRegDef(SrcReg))
+ if (DefMI->isImplicitDef())
+ ImpDefs.insert(DefMI);
+ } else {
+ BuildMI(opBlock, InsertPos, MPhi->getDebugLoc(),
+ TII->get(TargetOpcode::COPY), IncomingReg)
+ .addReg(SrcReg, 0, SrcSubReg);
}
}
- // Now loop over all of the incoming arguments, changing them to copy into
- // the IncomingReg register in the corresponding predecessor basic block.
- //
- for (int i = MI->getNumOperands() - 1; i >= 2; i-=2) {
- MachineOperand &opVal = MI->getOperand(i-1);
-
- // Get the MachineBasicBlock equivalent of the BasicBlock that is the
- // source path the PHI.
- MachineBasicBlock &opBlock = *MI->getOperand(i).getMachineBasicBlock();
-
- // Figure out where to insert the copy, which is at the end of the
- // predecessor basic block, but before any terminator/branch
- // instructions...
- MachineBasicBlock::iterator I = opBlock.end();
- if (I != opBlock.begin()) { // Handle empty blocks
- --I;
- // must backtrack over ALL the branches in the previous block
- while (MII.isTerminatorInstr((*I)->getOpcode()) &&
- I != opBlock.begin())
- --I;
-
- // move back to the first branch instruction so new instructions
- // are inserted right in front of it and not in front of a non-branch
- if (!MII.isTerminatorInstr((*I)->getOpcode()))
- ++I;
+ // Now update live variable information if we have it. Otherwise we're done
+ if (SrcUndef || !LV) continue;
+
+ // We want to be able to insert a kill of the register if this PHI (aka, the
+ // copy we just inserted) is the last use of the source value. Live
+ // variable analysis conservatively handles this by saying that the value is
+ // live until the end of the block the PHI entry lives in. If the value
+ // really is dead at the PHI copy, there will be no successor blocks which
+ // have the value live-in.
+
+ // Also check to see if this register is in use by another PHI node which
+ // has not yet been eliminated. If so, it will be killed at an appropriate
+ // point later.
+
+ // Is it used by any PHI instructions in this block?
+ bool ValueIsUsed = VRegPHIUseCount[BBVRegPair(opBlock.getNumber(), SrcReg)];
+
+ // Okay, if we now know that the value is not live out of the block, we can
+ // add a kill marker in this block saying that it kills the incoming value!
+ if (!ValueIsUsed && !LV->isLiveOut(SrcReg, opBlock)) {
+ // In our final twist, we have to decide which instruction kills the
+ // register. In most cases this is the copy, however, terminator
+ // instructions at the end of the block may also use the value. In this
+ // case, we should mark the last such terminator as being the killing
+ // block, not the copy.
+ MachineBasicBlock::iterator KillInst = opBlock.end();
+ MachineBasicBlock::iterator FirstTerm = opBlock.getFirstTerminator();
+ for (MachineBasicBlock::iterator Term = FirstTerm;
+ Term != opBlock.end(); ++Term) {
+ if (Term->readsRegister(SrcReg))
+ KillInst = Term;
}
-
- // Check to make sure we haven't already emitted the copy for this block.
- // This can happen because PHI nodes may have multiple entries for the
- // same basic block. It doesn't matter which entry we use though, because
- // all incoming values are guaranteed to be the same for a particular bb.
- //
- // If we emitted a copy for this basic block already, it will be right
- // where we want to insert one now. Just check for a definition of the
- // register we are interested in!
- //
- bool HaveNotEmitted = true;
-
- if (I != opBlock.begin()) {
- MachineInstr *PrevInst = *(I-1);
- for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) {
- MachineOperand &MO = PrevInst->getOperand(i);
- if (MO.isVirtualRegister() && MO.getReg() == IncomingReg)
- if (MO.opIsDef() || MO.opIsDefAndUse()) {
- HaveNotEmitted = false;
+
+ if (KillInst == opBlock.end()) {
+ // No terminator uses the register.
+
+ if (reusedIncoming || !IncomingReg) {
+ // We may have to rewind a bit if we didn't insert a copy this time.
+ KillInst = FirstTerm;
+ while (KillInst != opBlock.begin()) {
+ --KillInst;
+ if (KillInst->isDebugValue())
+ continue;
+ if (KillInst->readsRegister(SrcReg))
break;
- }
+ }
+ } else {
+ // We just inserted this copy.
+ KillInst = prior(InsertPos);
}
}
+ assert(KillInst->readsRegister(SrcReg) && "Cannot find kill instruction");
- if (HaveNotEmitted) {
- assert(opVal.isVirtualRegister() &&
- "Machine PHI Operands must all be virtual registers!");
- RegInfo->copyRegToReg(opBlock, I, IncomingReg, opVal.getReg(), RC);
- }
+ // Finally, mark it killed.
+ LV->addVirtualRegisterKilled(SrcReg, KillInst);
+
+ // This vreg no longer lives all of the way through opBlock.
+ unsigned opBlockNum = opBlock.getNumber();
+ LV->getVarInfo(SrcReg).AliveBlocks.reset(opBlockNum);
}
-
- // really delete the PHI instruction now!
- delete MI;
}
- return true;
+ // Really delete the PHI instruction now, if it is not in the LoweredPHIs map.
+ if (reusedIncoming || !IncomingReg)
+ MF.DeleteMachineInstr(MPhi);
+}
+
+/// analyzePHINodes - Gather information about the PHI nodes in here. In
+/// particular, we want to map the number of uses of a virtual register which is
+/// used in a PHI node. We map that to the BB the vreg is coming from. This is
+/// used later to determine when the vreg is killed in the BB.
+///
+void PHIElimination::analyzePHINodes(const MachineFunction& MF) {
+ for (MachineFunction::const_iterator I = MF.begin(), E = MF.end();
+ I != E; ++I)
+ for (MachineBasicBlock::const_iterator BBI = I->begin(), BBE = I->end();
+ BBI != BBE && BBI->isPHI(); ++BBI)
+ for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
+ ++VRegPHIUseCount[BBVRegPair(BBI->getOperand(i+1).getMBB()->getNumber(),
+ BBI->getOperand(i).getReg())];
+}
+
+bool PHIElimination::SplitPHIEdges(MachineFunction &MF,
+ MachineBasicBlock &MBB,
+ LiveVariables &LV,
+ MachineLoopInfo *MLI) {
+ if (MBB.empty() || !MBB.front().isPHI() || MBB.isLandingPad())
+ return false; // Quick exit for basic blocks without PHIs.
+
+ const MachineLoop *CurLoop = MLI ? MLI->getLoopFor(&MBB) : 0;
+ bool IsLoopHeader = CurLoop && &MBB == CurLoop->getHeader();
+
+ bool Changed = false;
+ for (MachineBasicBlock::iterator BBI = MBB.begin(), BBE = MBB.end();
+ BBI != BBE && BBI->isPHI(); ++BBI) {
+ for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) {
+ unsigned Reg = BBI->getOperand(i).getReg();
+ MachineBasicBlock *PreMBB = BBI->getOperand(i+1).getMBB();
+ // Is there a critical edge from PreMBB to MBB?
+ if (PreMBB->succ_size() == 1)
+ continue;
+
+ // Avoid splitting backedges of loops. It would introduce small
+ // out-of-line blocks into the loop which is very bad for code placement.
+ if (PreMBB == &MBB)
+ continue;
+ const MachineLoop *PreLoop = MLI ? MLI->getLoopFor(PreMBB) : 0;
+ if (IsLoopHeader && PreLoop == CurLoop)
+ continue;
+
+ // LV doesn't consider a phi use live-out, so isLiveOut only returns true
+ // when the source register is live-out for some other reason than a phi
+ // use. That means the copy we will insert in PreMBB won't be a kill, and
+ // there is a risk it may not be coalesced away.
+ //
+ // If the copy would be a kill, there is no need to split the edge.
+ if (!LV.isLiveOut(Reg, *PreMBB))
+ continue;
+
+ DEBUG(dbgs() << PrintReg(Reg) << " live-out before critical edge BB#"
+ << PreMBB->getNumber() << " -> BB#" << MBB.getNumber()
+ << ": " << *BBI);
+
+ // If Reg is not live-in to MBB, it means it must be live-in to some
+ // other PreMBB successor, and we can avoid the interference by splitting
+ // the edge.
+ //
+ // If Reg *is* live-in to MBB, the interference is inevitable and a copy
+ // is likely to be left after coalescing. If we are looking at a loop
+ // exiting edge, split it so we won't insert code in the loop, otherwise
+ // don't bother.
+ bool ShouldSplit = !LV.isLiveIn(Reg, MBB);
+
+ // Check for a loop exiting edge.
+ if (!ShouldSplit && CurLoop != PreLoop) {
+ DEBUG({
+ dbgs() << "Split wouldn't help, maybe avoid loop copies?\n";
+ if (PreLoop) dbgs() << "PreLoop: " << *PreLoop;
+ if (CurLoop) dbgs() << "CurLoop: " << *CurLoop;
+ });
+ // This edge could be entering a loop, exiting a loop, or it could be
+ // both: Jumping directly form one loop to the header of a sibling
+ // loop.
+ // Split unless this edge is entering CurLoop from an outer loop.
+ ShouldSplit = PreLoop && !PreLoop->contains(CurLoop);
+ }
+ if (!ShouldSplit)
+ continue;
+ if (!PreMBB->SplitCriticalEdge(&MBB, this)) {
+ DEBUG(dbgs() << "Failed to split ciritcal edge.\n");
+ continue;
+ }
+ Changed = true;
+ ++NumCriticalEdgesSplit;
+ }
+ }
+ return Changed;
}
projects / oota-llvm.git / blobdiff