#define DEBUG_TYPE "phielim"
#include "PHIElimination.h"
-#include "llvm/BasicBlock.h"
-#include "llvm/Instructions.h"
#include "llvm/CodeGen/LiveVariables.h"
#include "llvm/CodeGen/Passes.h"
-#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.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>
#include <map>
using namespace llvm;
STATISTIC(NumAtomic, "Number of atomic phis lowered");
+STATISTIC(NumSplits, "Number of critical edges split on demand");
+STATISTIC(NumReused, "Number of reused lowered phis");
char PHIElimination::ID = 0;
static RegisterPass<PHIElimination>
const PassInfo *const llvm::PHIEliminationID = &X;
void llvm::PHIElimination::getAnalysisUsage(AnalysisUsage &AU) const {
- AU.addPreserved<LiveVariables>();
- AU.addPreservedID(MachineLoopInfoID);
- AU.addPreservedID(MachineDominatorsID);
- MachineFunctionPass::getAnalysisUsage(AU);
- }
+ AU.addPreserved<LiveVariables>();
+ AU.addPreserved<MachineDominatorTree>();
+ // rdar://7401784 This would be nice:
+ // AU.addPreservedID(MachineLoopInfoID);
+ MachineFunctionPass::getAnalysisUsage(AU);
+}
bool llvm::PHIElimination::runOnMachineFunction(MachineFunction &Fn) {
MRI = &Fn.getRegInfo();
- analyzePHINodes(Fn);
-
+ PHIDefs.clear();
+ PHIKills.clear();
bool Changed = false;
+ // Split critical edges to help the coalescer
+ if (LiveVariables *LV = getAnalysisIfAvailable<LiveVariables>())
+ for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
+ Changed |= SplitPHIEdges(Fn, *I, *LV);
+
+ // Populate VRegPHIUseCount
+ analyzePHINodes(Fn);
+
// 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);
DefMI->eraseFromParent();
}
+ // Clean up the lowered PHI instructions.
+ for (LoweredPHIMap::iterator I = LoweredPHIs.begin(), E = LoweredPHIs.end();
+ I != E; ++I)
+ Fn.DeleteMachineInstr(I->first);
+ LoweredPHIs.clear();
+
ImpDefs.clear();
VRegPHIUseCount.clear();
return Changed;
}
-
/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
/// predecessor basic blocks.
///
return true;
}
-// FindCopyInsertPoint - Find a safe place in MBB to insert a copy from SrcReg.
-// This needs to be after any def or uses of SrcReg, but before any subsequent
-// point where control flow might jump out of the basic block.
+// FindCopyInsertPoint - Find a safe place in MBB to insert a copy from SrcReg
+// when following the CFG edge to SuccMBB. This needs to be after any def of
+// SrcReg, but before any subsequent point where control flow might jump out of
+// the basic block.
MachineBasicBlock::iterator
llvm::PHIElimination::FindCopyInsertPoint(MachineBasicBlock &MBB,
+ MachineBasicBlock &SuccMBB,
unsigned SrcReg) {
// Handle the trivial case trivially.
if (MBB.empty())
return MBB.begin();
- // If this basic block does not contain an invoke, then control flow always
- // reaches the end of it, so place the copy there. The logic below works in
- // this case too, but is more expensive.
- if (!isa<InvokeInst>(MBB.getBasicBlock()->getTerminator()))
+ // Usually, we just want to insert the copy before the first terminator
+ // instruction. However, for the edge going to a landing pad, we must insert
+ // the copy before the call/invoke instruction.
+ if (!SuccMBB.isLandingPad())
return MBB.getFirstTerminator();
- // Discover any definition/uses in this basic block.
+ // Discover any defs/uses in this basic block.
SmallPtrSet<MachineInstr*, 8> DefUsesInMBB;
for (MachineRegisterInfo::reg_iterator RI = MRI->reg_begin(SrcReg),
- RE = MRI->reg_end(); RI != RE; ++RI) {
+ RE = MRI->reg_end(); RI != RE; ++RI) {
MachineInstr *DefUseMI = &*RI;
if (DefUseMI->getParent() == &MBB)
DefUsesInMBB.insert(DefUseMI);
MachineBasicBlock::iterator InsertPoint;
if (DefUsesInMBB.empty()) {
- // No def/uses. Insert the copy at the start of the basic block.
+ // No defs. Insert the copy at the start of the basic block.
InsertPoint = MBB.begin();
} else if (DefUsesInMBB.size() == 1) {
- // Insert the copy immediately after the definition/use.
+ // Insert the copy immediately after the def/use.
InsertPoint = *DefUsesInMBB.begin();
++InsertPoint;
} else {
- // Insert the copy immediately after the last definition/use.
+ // Insert the copy immediately after the last def/use.
InsertPoint = MBB.end();
while (!DefUsesInMBB.count(&*--InsertPoint)) {}
++InsertPoint;
/// under the assuption 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 llvm::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());
MachineFunction &MF = *MBB.getParent();
const TargetRegisterClass *RC = MF.getRegInfo().getRegClass(DestReg);
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
BuildMI(MBB, AfterPHIsIt, MPhi->getDebugLoc(),
TII->get(TargetInstrInfo::IMPLICIT_DEF), DestReg);
else {
- IncomingReg = MF.getRegInfo().createVirtualRegister(RC);
+ // 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(errs() << "Reusing %reg" << IncomingReg << " for " << *MPhi);
+ } else {
+ entry = IncomingReg = MF.getRegInfo().createVirtualRegister(RC);
+ }
TII->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC, RC);
}
// Record PHI def.
- //assert(!hasPHIDef(DestReg) && "Vreg has multiple phi-defs?");
- //PHIDefs[DestReg] = &MBB;
+ assert(!hasPHIDef(DestReg) && "Vreg has multiple phi-defs?");
+ PHIDefs[DestReg] = &MBB;
// Update live variable information if there is any.
LiveVariables *LV = getAnalysisIfAvailable<LiveVariables>();
MachineInstr *PHICopy = prior(AfterPHIsIt);
if (IncomingReg) {
+ LiveVariables::VarInfo &VI = LV->getVarInfo(IncomingReg);
+
// Increment use count of the newly created virtual register.
- LV->getVarInfo(IncomingReg).NumUses++;
+ VI.NumUses++;
+
+ // 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(errs() << "Remove old kill from " << *OldKill);
+ LV->removeVirtualRegisterKilled(IncomingReg, OldKill);
+ DEBUG(MBB.dump());
+ }
// Add information to LiveVariables to know that the incoming value is
// killed. Note that because the value is defined in several places (once
// 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(),
+ --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
MachineBasicBlock &opBlock = *MPhi->getOperand(i*2+2).getMBB();
// Record the kill.
- //PHIKills[SrcReg].insert(&opBlock);
+ PHIKills[SrcReg].insert(&opBlock);
// If source is defined by an implicit def, there is no need to insert a
// copy.
// 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 = FindCopyInsertPoint(opBlock, SrcReg);
+ MachineBasicBlock::iterator InsertPos =
+ FindCopyInsertPoint(opBlock, MBB, SrcReg);
// Insert the copy.
- TII->copyRegToReg(opBlock, InsertPos, IncomingReg, SrcReg, RC, RC);
+ if (!reusedIncoming && IncomingReg)
+ TII->copyRegToReg(opBlock, InsertPos, IncomingReg, SrcReg, RC, RC);
// Now update live variable information if we have it. Otherwise we're done
if (!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.
- //
- // Check to see if the copy is the last use, and if so, update the live
- // variables information so that it knows the copy source instruction kills
- // the incoming value.
- LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg);
-
- // Loop over all of the successors of the basic block, checking to see if
- // the value is either live in the block, or if it is killed in the block.
+
// 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 ValueIsLive = VRegPHIUseCount[BBVRegPair(&opBlock, SrcReg)] != 0;
-
- std::vector<MachineBasicBlock*> OpSuccBlocks;
-
- // Otherwise, scan successors, including the BB the PHI node lives in.
- for (MachineBasicBlock::succ_iterator SI = opBlock.succ_begin(),
- E = opBlock.succ_end(); SI != E && !ValueIsLive; ++SI) {
- MachineBasicBlock *SuccMBB = *SI;
-
- // Is it alive in this successor?
- unsigned SuccIdx = SuccMBB->getNumber();
- if (InRegVI.AliveBlocks.test(SuccIdx)) {
- ValueIsLive = true;
- break;
- }
-
- OpSuccBlocks.push_back(SuccMBB);
- }
-
- // Check to see if this value is live because there is a use in a successor
- // that kills it.
- if (!ValueIsLive) {
- switch (OpSuccBlocks.size()) {
- case 1: {
- MachineBasicBlock *MBB = OpSuccBlocks[0];
- for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
- if (InRegVI.Kills[i]->getParent() == MBB) {
- ValueIsLive = true;
- break;
- }
- break;
- }
- case 2: {
- MachineBasicBlock *MBB1 = OpSuccBlocks[0], *MBB2 = OpSuccBlocks[1];
- for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
- if (InRegVI.Kills[i]->getParent() == MBB1 ||
- InRegVI.Kills[i]->getParent() == MBB2) {
- ValueIsLive = true;
- break;
- }
- break;
- }
- default:
- std::sort(OpSuccBlocks.begin(), OpSuccBlocks.end());
- for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
- if (std::binary_search(OpSuccBlocks.begin(), OpSuccBlocks.end(),
- InRegVI.Kills[i]->getParent())) {
- ValueIsLive = true;
- break;
- }
- }
- }
+ 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 (!ValueIsLive) {
+ 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, the first
// terminator instruction at the end of the block may also use the value.
// In this case, we should mark *it* as being the killing block, not the
// copy.
- MachineBasicBlock::iterator KillInst = prior(InsertPos);
+ MachineBasicBlock::iterator KillInst;
MachineBasicBlock::iterator Term = opBlock.getFirstTerminator();
- if (Term != opBlock.end()) {
- if (Term->readsRegister(SrcReg))
- KillInst = Term;
-
+ if (Term != opBlock.end() && Term->readsRegister(SrcReg)) {
+ KillInst = Term;
+
// Check that no other terminators use values.
#ifndef NDEBUG
- for (MachineBasicBlock::iterator TI = next(Term); TI != opBlock.end();
- ++TI) {
+ for (MachineBasicBlock::iterator TI = llvm::next(Term);
+ TI != opBlock.end(); ++TI) {
assert(!TI->readsRegister(SrcReg) &&
"Terminator instructions cannot use virtual registers unless"
"they are the first terminator in a block!");
}
#endif
+ } else if (reusedIncoming || !IncomingReg) {
+ // We may have to rewind a bit if we didn't insert a copy this time.
+ KillInst = Term;
+ while (KillInst != opBlock.begin())
+ if ((--KillInst)->readsRegister(SrcReg))
+ break;
+ } else {
+ // We just inserted this copy.
+ KillInst = prior(InsertPos);
}
-
+ assert(KillInst->readsRegister(SrcReg) && "Cannot find kill instruction");
+
// Finally, mark it killed.
LV->addVirtualRegisterKilled(SrcReg, KillInst);
// This vreg no longer lives all of the way through opBlock.
unsigned opBlockNum = opBlock.getNumber();
- InRegVI.AliveBlocks.reset(opBlockNum);
+ LV->getVarInfo(SrcReg).AliveBlocks.reset(opBlockNum);
}
}
-
- // Really delete the PHI instruction now!
- MF.DeleteMachineInstr(MPhi);
- ++NumAtomic;
+
+ // 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
for (MachineBasicBlock::const_iterator BBI = I->begin(), BBE = I->end();
BBI != BBE && BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI)
for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
- ++VRegPHIUseCount[BBVRegPair(BBI->getOperand(i + 1).getMBB(),
+ ++VRegPHIUseCount[BBVRegPair(BBI->getOperand(i+1).getMBB()->getNumber(),
BBI->getOperand(i).getReg())];
}
+
+bool llvm::PHIElimination::SplitPHIEdges(MachineFunction &MF,
+ MachineBasicBlock &MBB,
+ LiveVariables &LV) {
+ if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI)
+ return false; // Quick exit for basic blocks without PHIs.
+
+ for (MachineBasicBlock::const_iterator BBI = MBB.begin(), BBE = MBB.end();
+ BBI != BBE && BBI->getOpcode() == TargetInstrInfo::PHI; ++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();
+ // We break edges when registers are live out from the predecessor block
+ // (not considering PHI nodes). If the register is live in to this block
+ // anyway, we would gain nothing from splitting.
+ if (!LV.isLiveIn(Reg, MBB) && LV.isLiveOut(Reg, *PreMBB))
+ SplitCriticalEdge(PreMBB, &MBB);
+ }
+ }
+ return true;
+}
+
+MachineBasicBlock *PHIElimination::SplitCriticalEdge(MachineBasicBlock *A,
+ MachineBasicBlock *B) {
+ assert(A && B && "Missing MBB end point");
+
+ MachineFunction *MF = A->getParent();
+
+ // We may need to update A's terminator, but we can't do that if AnalyzeBranch
+ // fails. If A uses a jump table, we won't touch it.
+ const TargetInstrInfo *TII = MF->getTarget().getInstrInfo();
+ MachineBasicBlock *TBB = 0, *FBB = 0;
+ SmallVector<MachineOperand, 4> Cond;
+ if (TII->AnalyzeBranch(*A, TBB, FBB, Cond))
+ return NULL;
+
+ ++NumSplits;
+
+ MachineBasicBlock *NMBB = MF->CreateMachineBasicBlock();
+ MF->insert(llvm::next(MachineFunction::iterator(A)), NMBB);
+ DEBUG(errs() << "PHIElimination splitting critical edge:"
+ " BB#" << A->getNumber()
+ << " -- BB#" << NMBB->getNumber()
+ << " -- BB#" << B->getNumber() << '\n');
+
+ A->ReplaceUsesOfBlockWith(B, NMBB);
+ A->updateTerminator();
+
+ // Insert unconditional "jump B" instruction in NMBB if necessary.
+ NMBB->addSuccessor(B);
+ if (!NMBB->isLayoutSuccessor(B)) {
+ Cond.clear();
+ MF->getTarget().getInstrInfo()->InsertBranch(*NMBB, B, NULL, Cond);
+ }
+
+ // Fix PHI nodes in B so they refer to NMBB instead of A
+ for (MachineBasicBlock::iterator i = B->begin(), e = B->end();
+ i != e && i->getOpcode() == TargetInstrInfo::PHI; ++i)
+ for (unsigned ni = 1, ne = i->getNumOperands(); ni != ne; ni += 2)
+ if (i->getOperand(ni+1).getMBB() == A)
+ i->getOperand(ni+1).setMBB(NMBB);
+
+ if (LiveVariables *LV=getAnalysisIfAvailable<LiveVariables>())
+ LV->addNewBlock(NMBB, A, B);
+
+ if (MachineDominatorTree *MDT=getAnalysisIfAvailable<MachineDominatorTree>())
+ MDT->addNewBlock(NMBB, A);
+
+ return NMBB;
+}
+
+unsigned
+PHIElimination::PHINodeTraits::getHashValue(const MachineInstr *MI) {
+ if (!MI || MI==getEmptyKey() || MI==getTombstoneKey())
+ return DenseMapInfo<MachineInstr*>::getHashValue(MI);
+ unsigned hash = 0;
+ for (unsigned ni = 1, ne = MI->getNumOperands(); ni != ne; ni += 2)
+ hash = hash*37 + DenseMapInfo<BBVRegPair>::
+ getHashValue(BBVRegPair(MI->getOperand(ni+1).getMBB()->getNumber(),
+ MI->getOperand(ni).getReg()));
+ return hash;
+}
+
+bool PHIElimination::PHINodeTraits::isEqual(const MachineInstr *LHS,
+ const MachineInstr *RHS) {
+ const MachineInstr *EmptyKey = getEmptyKey();
+ const MachineInstr *TombstoneKey = getTombstoneKey();
+ if (!LHS || !RHS || LHS==EmptyKey || RHS==EmptyKey ||
+ LHS==TombstoneKey || RHS==TombstoneKey)
+ return LHS==RHS;
+
+ unsigned ne = LHS->getNumOperands();
+ if (ne != RHS->getNumOperands())
+ return false;
+ // Ignore operand 0, the defined register.
+ for (unsigned ni = 1; ni != ne; ni += 2)
+ if (LHS->getOperand(ni).getReg() != RHS->getOperand(ni).getReg() ||
+ LHS->getOperand(ni+1).getMBB() != RHS->getOperand(ni+1).getMBB())
+ return false;
+ return true;
+}