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