llvm-project/llvm/lib/CodeGen/PHIElimination.cpp

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//===-- 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.
//
//===----------------------------------------------------------------------===//
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//
// 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"
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#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/SSARegMap.h"
#include "llvm/Target/TargetInstrInfo.h"
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#include "llvm/Target/TargetMachine.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Compiler.h"
#include <set>
#include <algorithm>
using namespace llvm;
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namespace {
static Statistic<> NumAtomic("phielim", "Number of atomic phis lowered");
static Statistic<> NumSimple("phielim", "Number of simple phis lowered");
struct VISIBILITY_HIDDEN PNE : public MachineFunctionPass {
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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)
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Changed |= EliminatePHINodes(Fn, *I);
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return Changed;
}
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addPreserved<LiveVariables>();
MachineFunctionPass::getAnalysisUsage(AU);
}
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,
DenseMap<unsigned, VirtReg2IndexFunctor> &VUC);
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};
RegisterPass<PNE> X("phi-node-elimination",
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"Eliminate PHI nodes for register allocation");
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}
const PassInfo *llvm::PHIEliminationID = X.getPassInfo();
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/// 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 basic blocks without PHIs.
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// 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;
}
/// InstructionUsesRegister - Return true if the specified machine instr has a
/// use of the specified register.
static bool InstructionUsesRegister(MachineInstr *MI, unsigned SrcReg) {
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i)
if (MI->getOperand(i).isRegister() &&
MI->getOperand(i).getReg() == SrcReg &&
MI->getOperand(i).isUse())
return true;
return false;
}
/// 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);
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// Update live variable information if there is any...
LiveVariables *LV = getAnalysisToUpdate<LiveVariables>();
if (LV) {
MachineInstr *PHICopy = prior(AfterPHIsIt);
// 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);
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// 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.
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//
LV->removeVirtualRegistersKilled(MPhi);
// If the result is dead, update LV.
if (LV->RegisterDefIsDead(MPhi, DestReg)) {
LV->addVirtualRegisterDead(DestReg, PHICopy);
LV->removeVirtualRegistersDead(MPhi);
}
// Realize that the destination register is defined by the PHI copy now, not
// the PHI itself.
LV->getVarInfo(DestReg).DefInst = PHICopy;
}
// 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;
// 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();
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// 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();
// Insert the copy.
RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, 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.
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//
// 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[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 (SuccIdx < InRegVI.AliveBlocks.size() &&
InRegVI.AliveBlocks[SuccIdx]) {
ValueIsLive = true;
break;
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}
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;
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}
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 in this block saying that it kills the incoming
// value!
if (!ValueIsLive) {
// 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.
bool FirstTerminatorUsesValue = false;
if (I != opBlock.end()) {
FirstTerminatorUsesValue = InstructionUsesRegister(I, SrcReg);
// Check that no other terminators use values.
#ifndef NDEBUG
for (MachineBasicBlock::iterator TI = next(I); TI != opBlock.end();
++TI) {
assert(!InstructionUsesRegister(TI, SrcReg) &&
"Terminator instructions cannot use virtual registers unless"
"they are the first terminator in a block!");
}
#endif
}
MachineBasicBlock::iterator KillInst;
if (!FirstTerminatorUsesValue)
KillInst = prior(I);
else
KillInst = I;
// Finally, mark it killed.
LV->addVirtualRegisterKilled(SrcReg, KillInst);
// This vreg no longer lives all of the way through opBlock.
unsigned opBlockNum = opBlock.getNumber();
if (opBlockNum < InRegVI.AliveBlocks.size())
InRegVI.AliveBlocks[opBlockNum] = false;
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}
}
// Really delete the PHI instruction now!
delete MPhi;
++NumAtomic;
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}