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//===-- OptimizeCmps.cpp - Optimize comparison instrs ---------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs optimization of comparison instructions. For instance, in
// this code:
//
// sub r1, 1
// cmp r1, 0
// bz L1
//
// If the "sub" instruction all ready sets (or could be modified to set) the
// same flag that the "cmp" instruction sets and that "bz" uses, then we can
// eliminate the "cmp" instruction.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "opt-compares"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;
STATISTIC(NumEliminated, "Number of compares eliminated");
static cl::opt<bool>
EnableOptCmps("enable-optimize-cmps", cl::init(false), cl::Hidden);
namespace {
class OptimizeCmps : public MachineFunctionPass {
const TargetMachine *TM;
const TargetInstrInfo *TII;
MachineRegisterInfo *MRI;
bool OptimizeCmpInstr(MachineInstr *MI, MachineBasicBlock *MBB);
public:
static char ID; // Pass identification
OptimizeCmps() : MachineFunctionPass(ID) {}
virtual bool runOnMachineFunction(MachineFunction &MF);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
MachineFunctionPass::getAnalysisUsage(AU);
}
};
}
char OptimizeCmps::ID = 0;
INITIALIZE_PASS(OptimizeCmps, "opt-cmps",
"Optimize comparison instrs", false, false);
FunctionPass *llvm::createOptimizeCmpsPass() { return new OptimizeCmps(); }
/// OptimizeCmpInstr - If the instruction is a compare and the previous
/// instruction it's comparing against all ready sets (or could be modified to
/// set) the same flag as the compare, then we can remove the comparison and use
/// the flag from the previous instruction.
bool OptimizeCmps::OptimizeCmpInstr(MachineInstr *MI, MachineBasicBlock *MBB) {
// If this instruction is a comparison against zero and isn't comparing a
// physical register, we can try to optimize it.
unsigned SrcReg;
int CmpValue;
if (!TII->isCompareInstr(MI, SrcReg, CmpValue) ||
TargetRegisterInfo::isPhysicalRegister(SrcReg) ||
CmpValue != 0)
return false;
MachineRegisterInfo::def_iterator DI = MRI->def_begin(SrcReg);
if (llvm::next(DI) != MRI->def_end())
// Only support one definition.
return false;
// Attempt to convert the defining instruction to set the "zero" flag.
if (TII->convertToSetZeroFlag(&*DI, MI)) {
++NumEliminated;
return true;
}
return false;
}
bool OptimizeCmps::runOnMachineFunction(MachineFunction &MF) {
TM = &MF.getTarget();
TII = TM->getInstrInfo();
MRI = &MF.getRegInfo();
if (!EnableOptCmps) return false;
bool Changed = false;
for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
MachineBasicBlock *MBB = &*I;
for (MachineBasicBlock::iterator
MII = MBB->begin(), ME = MBB->end(); MII != ME; ) {
MachineInstr *MI = &*MII++;
Changed |= OptimizeCmpInstr(MI, MBB);
}
}
return Changed;
}
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