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//===- OptimizePHIs.cpp - Optimize machine instruction PHIs ---------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This pass optimizes machine instruction PHIs to take advantage of
// opportunities created during DAG legalization.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include <cassert>
using namespace llvm;
#define DEBUG_TYPE "opt-phis"
STATISTIC(NumPHICycles, "Number of PHI cycles replaced");
STATISTIC(NumDeadPHICycles, "Number of dead PHI cycles");
namespace {
class OptimizePHIs : public MachineFunctionPass {
MachineRegisterInfo *MRI;
const TargetInstrInfo *TII;
public:
static char ID; // Pass identification
OptimizePHIs() : MachineFunctionPass(ID) {
initializeOptimizePHIsPass(*PassRegistry::getPassRegistry());
}
bool runOnMachineFunction(MachineFunction &Fn) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
MachineFunctionPass::getAnalysisUsage(AU);
}
private:
using InstrSet = SmallPtrSet<MachineInstr *, 16>;
using InstrSetIterator = SmallPtrSetIterator<MachineInstr *>;
bool IsSingleValuePHICycle(MachineInstr *MI, unsigned &SingleValReg,
InstrSet &PHIsInCycle);
bool IsDeadPHICycle(MachineInstr *MI, InstrSet &PHIsInCycle);
bool OptimizeBB(MachineBasicBlock &MBB);
};
} // end anonymous namespace
char OptimizePHIs::ID = 0;
char &llvm::OptimizePHIsID = OptimizePHIs::ID;
INITIALIZE_PASS(OptimizePHIs, DEBUG_TYPE,
"Optimize machine instruction PHIs", false, false)
bool OptimizePHIs::runOnMachineFunction(MachineFunction &Fn) {
if (skipFunction(Fn.getFunction()))
return false;
MRI = &Fn.getRegInfo();
TII = Fn.getSubtarget().getInstrInfo();
// Find dead PHI cycles and PHI cycles that can be replaced by a single
// value. InstCombine does these optimizations, but DAG legalization may
// introduce new opportunities, e.g., when i64 values are split up for
// 32-bit targets.
bool Changed = false;
for (MachineBasicBlock &MBB : Fn)
Changed |= OptimizeBB(MBB);
return Changed;
}
/// IsSingleValuePHICycle - Check if MI is a PHI where all the source operands
/// are copies of SingleValReg, possibly via copies through other PHIs. If
/// SingleValReg is zero on entry, it is set to the register with the single
/// non-copy value. PHIsInCycle is a set used to keep track of the PHIs that
/// have been scanned. PHIs may be grouped by cycle, several cycles or chains.
bool OptimizePHIs::IsSingleValuePHICycle(MachineInstr *MI,
unsigned &SingleValReg,
InstrSet &PHIsInCycle) {
assert(MI->isPHI() && "IsSingleValuePHICycle expects a PHI instruction");
Register DstReg = MI->getOperand(0).getReg();
// See if we already saw this register.
if (!PHIsInCycle.insert(MI).second)
return true;
// Don't scan crazily complex things.
if (PHIsInCycle.size() == 16)
return false;
// Scan the PHI operands.
for (unsigned i = 1; i != MI->getNumOperands(); i += 2) {
Register SrcReg = MI->getOperand(i).getReg();
if (SrcReg == DstReg)
continue;
MachineInstr *SrcMI = MRI->getVRegDef(SrcReg);
// Skip over register-to-register moves.
if (SrcMI && SrcMI->isCopy() && !SrcMI->getOperand(0).getSubReg() &&
!SrcMI->getOperand(1).getSubReg() &&
SrcMI->getOperand(1).getReg().isVirtual()) {
SrcReg = SrcMI->getOperand(1).getReg();
SrcMI = MRI->getVRegDef(SrcReg);
}
if (!SrcMI)
return false;
if (SrcMI->isPHI()) {
if (!IsSingleValuePHICycle(SrcMI, SingleValReg, PHIsInCycle))
return false;
} else {
// Fail if there is more than one non-phi/non-move register.
if (SingleValReg != 0 && SingleValReg != SrcReg)
return false;
SingleValReg = SrcReg;
}
}
return true;
}
/// IsDeadPHICycle - Check if the register defined by a PHI is only used by
/// other PHIs in a cycle.
bool OptimizePHIs::IsDeadPHICycle(MachineInstr *MI, InstrSet &PHIsInCycle) {
assert(MI->isPHI() && "IsDeadPHICycle expects a PHI instruction");
Register DstReg = MI->getOperand(0).getReg();
assert(DstReg.isVirtual() && "PHI destination is not a virtual register");
// See if we already saw this register.
if (!PHIsInCycle.insert(MI).second)
return true;
// Don't scan crazily complex things.
if (PHIsInCycle.size() == 16)
return false;
for (MachineInstr &UseMI : MRI->use_nodbg_instructions(DstReg)) {
if (!UseMI.isPHI() || !IsDeadPHICycle(&UseMI, PHIsInCycle))
return false;
}
return true;
}
/// OptimizeBB - Remove dead PHI cycles and PHI cycles that can be replaced by
/// a single value.
bool OptimizePHIs::OptimizeBB(MachineBasicBlock &MBB) {
bool Changed = false;
for (MachineBasicBlock::iterator
MII = MBB.begin(), E = MBB.end(); MII != E; ) {
MachineInstr *MI = &*MII++;
if (!MI->isPHI())
break;
// Check for single-value PHI cycles.
unsigned SingleValReg = 0;
InstrSet PHIsInCycle;
if (IsSingleValuePHICycle(MI, SingleValReg, PHIsInCycle) &&
SingleValReg != 0) {
Register OldReg = MI->getOperand(0).getReg();
if (!MRI->constrainRegClass(SingleValReg, MRI->getRegClass(OldReg)))
continue;
MRI->replaceRegWith(OldReg, SingleValReg);
MI->eraseFromParent();
// The kill flags on OldReg and SingleValReg may no longer be correct.
MRI->clearKillFlags(SingleValReg);
++NumPHICycles;
Changed = true;
continue;
}
// Check for dead PHI cycles.
PHIsInCycle.clear();
if (IsDeadPHICycle(MI, PHIsInCycle)) {
for (MachineInstr *PhiMI : PHIsInCycle) {
if (MII == PhiMI)
++MII;
PhiMI->eraseFromParent();
}
++NumDeadPHICycles;
Changed = true;
}
}
return Changed;
}
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