//===-- PhiElimination.cpp - Eliminate PHI nodes by inserting copies ------===//
//
+// The LLVM Compiler Infrastructure
+//
+// This file was developed by the LLVM research group and 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/LiveVariables.h"
+#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/SSARegMap.h"
-#include "llvm/CodeGen/LiveVariables.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
-#include "llvm/Support/CFG.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/Statistic.h"
+#include <set>
+#include <algorithm>
+using namespace llvm;
namespace {
+ Statistic<> NumAtomic("phielim", "Number of atomic phis lowered");
+ Statistic<> NumSimple("phielim", "Number of simple phis lowered");
+
struct PNE : public MachineFunctionPass {
bool runOnMachineFunction(MachineFunction &Fn) {
bool Changed = false;
// 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);
+ Changed |= EliminatePHINodes(Fn, *I);
- //std::cerr << "AFTER PHI NODE ELIM:\n";
- //Fn.dump();
return Changed;
}
/// in predecessor basic blocks.
///
bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB);
+ void LowerAtomicPHINode(MachineBasicBlock &MBB,
+ MachineBasicBlock::iterator AfterPHIsIt,
+ DenseMap<unsigned, VirtReg2IndexFunctor> &VUC);
};
RegisterPass<PNE> X("phi-node-elimination",
- "Eliminate PHI nodes for register allocation");
+ "Eliminate PHI nodes for register allocation");
}
-const PassInfo *PHIEliminationID = X.getPassInfo();
+
+const PassInfo *llvm::PHIEliminationID = X.getPassInfo();
/// 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...
+ if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI)
+ return false; // Quick exit for basic blocks without PHIs.
- LiveVariables *LV = getAnalysisToUpdate<LiveVariables>();
- const TargetInstrInfo &MII = MF.getTarget().getInstrInfo();
+ // VRegPHIUseCount - Keep track of the number of times each virtual register
+ // is used by PHI nodes in successors of this block.
+ DenseMap<unsigned, VirtReg2IndexFunctor> VRegPHIUseCount;
+ VRegPHIUseCount.grow(MF.getSSARegMap()->getLastVirtReg());
+
+ for (MachineBasicBlock::pred_iterator PI = MBB.pred_begin(),
+ E = MBB.pred_end(); PI != E; ++PI)
+ for (MachineBasicBlock::succ_iterator SI = (*PI)->succ_begin(),
+ E = (*PI)->succ_end(); SI != E; ++SI)
+ for (MachineBasicBlock::iterator BBI = (*SI)->begin(), E = (*SI)->end();
+ BBI != E && BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI)
+ for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
+ VRegPHIUseCount[BBI->getOperand(i).getReg()]++;
+
+ // 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.begin();
+ while (AfterPHIsIt != MBB.end() &&
+ AfterPHIsIt->getOpcode() == TargetInstrInfo::PHI)
+ ++AfterPHIsIt; // Skip over all of the PHI nodes...
+
+ while (MBB.front().getOpcode() == TargetInstrInfo::PHI) {
+ LowerAtomicPHINode(MBB, AfterPHIsIt, VRegPHIUseCount);
+ }
+ return true;
+}
+
+/// LowerAtomicPHINode - Lower the PHI node at the top of the specified block,
+/// 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 PNE::LowerAtomicPHINode(MachineBasicBlock &MBB,
+ MachineBasicBlock::iterator AfterPHIsIt,
+ DenseMap<unsigned, VirtReg2IndexFunctor> &VRegPHIUseCount) {
+ // Unlink the PHI node from the basic block, but don't delete the PHI yet.
+ MachineInstr *MPhi = MBB.remove(MBB.begin());
+
+ unsigned DestReg = MPhi->getOperand(0).getReg();
+
+ // Create a new register for the incoming PHI arguments/
+ MachineFunction &MF = *MBB.getParent();
+ 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.
+ //
const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo();
+ RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);
+
+ // Update live variable information if there is any...
+ LiveVariables *LV = getAnalysisToUpdate<LiveVariables>();
+ if (LV) {
+ MachineInstr *PHICopy = prior(AfterPHIsIt);
- 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());
+ // Add information to LiveVariables to know that the incoming value is
+ // 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.
+ //
+ LV->removeVirtualRegistersKilled(MPhi);
+
+ // If the result is dead, update LV.
+ if (LV->RegisterDefIsDead(MPhi, DestReg)) {
+ LV->addVirtualRegisterDead(DestReg, PHICopy);
+ LV->removeVirtualRegistersDead(MPhi);
+ }
+ }
- assert(MI->getOperand(0).isVirtualRegister() &&
- "PHI node doesn't write virt reg?");
+ // Adjust the VRegPHIUseCount map to account for the removal of this PHI
+ // node.
+ unsigned NumPreds = (MPhi->getNumOperands()-1)/2;
+ for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2)
+ VRegPHIUseCount[MPhi->getOperand(i).getReg()] -= NumPreds;
- unsigned DestReg = MI->getOperand(0).getAllocatedRegNum();
+ // Now loop over all of the incoming arguments, changing them to copy into
+ // the IncomingReg register in the corresponding predecessor basic block.
+ //
+ std::set<MachineBasicBlock*> MBBsInsertedInto;
+ for (int i = MPhi->getNumOperands() - 1; i >= 2; i-=2) {
+ unsigned SrcReg = MPhi->getOperand(i-1).getReg();
+ assert(MRegisterInfo::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).getMachineBasicBlock();
+
+ // 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).second)
+ continue; // If the copy has already been emitted, we're done.
+
+ // Get an iterator pointing to the first terminator in the block (or end()).
+ // This is the point where we can insert a copy if we'd like to.
+ MachineBasicBlock::iterator I = opBlock.getFirstTerminator();
- // Create a new register for the incoming PHI arguments
- const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg);
- unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC);
+ // Insert the copy.
+ RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, 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.
+ // 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.
//
- MachineBasicBlock::iterator AfterPHIsIt = MBB.begin();
- while (AfterPHIsIt != MBB.end() &&
- (*AfterPHIsIt)->getOpcode() == TargetInstrInfo::PHI)
- ++AfterPHIsIt; // Skip over all of the PHI nodes...
- RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);
+ 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[SrcReg] != 0;
+
+ std::vector<MachineBasicBlock*> OpSuccBlocks;
- // Update live variable information if there is any...
- if (LV) {
- MachineInstr *PHICopy = *(AfterPHIsIt-1);
-
- // Add information to LiveVariables to know that the incoming value is
- // 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, &MBB, 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);
- std::vector<std::pair<MachineInstr*, unsigned> > Range;
- if (RKs.first != RKs.second) {
- // Copy the range into a vector...
- Range.assign(RKs.first, RKs.second);
-
- // Delete the range...
- LV->removeVirtualRegistersKilled(RKs.first, RKs.second);
-
- // Add all of the kills back, which will update the appropriate info...
- for (unsigned i = 0, e = Range.size(); i != e; ++i)
- LV->addVirtualRegisterKilled(Range[i].second, &MBB, PHICopy);
- }
+ // 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;
- RKs = LV->dead_range(MI);
- if (RKs.first != RKs.second) {
- // Works as above...
- Range.assign(RKs.first, RKs.second);
- LV->removeVirtualRegistersDead(RKs.first, RKs.second);
- for (unsigned i = 0, e = Range.size(); i != e; ++i)
- LV->addVirtualRegisterDead(Range[i].second, &MBB, PHICopy);
+ // Is it alive in this successor?
+ unsigned SuccIdx = SuccMBB->getNumber();
+ if (SuccIdx < InRegVI.AliveBlocks.size() &&
+ InRegVI.AliveBlocks[SuccIdx]) {
+ ValueIsLive = true;
+ break;
}
+
+ OpSuccBlocks.push_back(SuccMBB);
}
- // 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;
- }
-
- // 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;
- break;
- }
- }
+ // 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;
}
-
- if (HaveNotEmitted) { // If the copy has not already been emitted, do it.
- assert(opVal.isVirtualRegister() &&
- "Machine PHI Operands must all be virtual registers!");
- unsigned SrcReg = opVal.getReg();
- RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC);
-
- // Now update live variable information if we have it.
- if (LV) {
- // 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.
- //
- bool ValueIsLive = false;
- BasicBlock *BB = opBlock.getBasicBlock();
- for (succ_iterator SI = succ_begin(BB), E = succ_end(BB);
- SI != E; ++SI) {
- const std::pair<MachineBasicBlock*, unsigned> &
- SuccInfo = LV->getBasicBlockInfo(*SI);
-
- // Is it alive in this successor?
- unsigned SuccIdx = SuccInfo.second;
- if (SuccIdx < InRegVI.AliveBlocks.size() &&
- InRegVI.AliveBlocks[SuccIdx]) {
- ValueIsLive = true;
- break;
- }
-
- // Is it killed in this successor?
- MachineBasicBlock *MBB = SuccInfo.first;
- for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
- if (InRegVI.Kills[i].first == MBB) {
- ValueIsLive = true;
- 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;
}
-
- // Okay, if we now know that the value is not live out of the block,
- // we can add a kill marker to the copy we inserted saying that it
- // kills the incoming value!
- //
- if (!ValueIsLive) {
- // One more complication to worry about. There may actually be
- // multiple PHI nodes using this value on this branch. If we aren't
- // careful, the first PHI node will end up killing the value, not
- // letting it get the to the copy for the final PHI node in the
- // block. Therefore we have to check to see if there is already a
- // kill in this block, and if so, extend the lifetime to our new
- // copy.
- //
- for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
- if (InRegVI.Kills[i].first == &opBlock) {
- std::pair<LiveVariables::killed_iterator,
- LiveVariables::killed_iterator> Range
- = LV->killed_range(InRegVI.Kills[i].second);
- LV->removeVirtualRegistersKilled(Range.first, Range.second);
- break;
- }
-
- LV->addVirtualRegisterKilled(SrcReg, &opBlock, *(I-1));
+ 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;
}
- }
}
+ }
+
+ // Okay, if we now know that the value is not live out of the block,
+ // we can add a kill marker to the copy we inserted saying that it
+ // kills the incoming value!
+ //
+ if (!ValueIsLive) {
+ MachineBasicBlock::iterator Prev = prior(I);
+ LV->addVirtualRegisterKilled(SrcReg, Prev);
+
+ // This vreg no longer lives all of the way through opBlock.
+ unsigned opBlockNum = opBlock.getNumber();
+ if (opBlockNum < InRegVI.AliveBlocks.size())
+ InRegVI.AliveBlocks[opBlockNum] = false;
}
-
- // really delete the PHI instruction now!
- delete MI;
}
-
- return true;
+
+ // Really delete the PHI instruction now!
+ delete MPhi;
+ ++NumAtomic;
}
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