forked from OSchip/llvm-project
130 lines
4.2 KiB
C++
130 lines
4.2 KiB
C++
//===- DCE.cpp - Code to perform dead code elimination --------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements dead inst elimination and dead code elimination.
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//
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// Dead Inst Elimination performs a single pass over the function removing
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// instructions that are obviously dead. Dead Code Elimination is similar, but
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// it rechecks instructions that were used by removed instructions to see if
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// they are newly dead.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Instruction.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/InstIterator.h"
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#include "Support/Statistic.h"
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#include <set>
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namespace llvm {
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namespace {
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Statistic<> DIEEliminated("die", "Number of insts removed");
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Statistic<> DCEEliminated("dce", "Number of insts removed");
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//===--------------------------------------------------------------------===//
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// DeadInstElimination pass implementation
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//
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struct DeadInstElimination : public BasicBlockPass {
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virtual bool runOnBasicBlock(BasicBlock &BB) {
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bool Changed = false;
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for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); )
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if (dceInstruction(DI)) {
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Changed = true;
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++DIEEliminated;
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} else
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++DI;
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return Changed;
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}
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesCFG();
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}
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};
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RegisterOpt<DeadInstElimination> X("die", "Dead Instruction Elimination");
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}
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Pass *createDeadInstEliminationPass() {
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return new DeadInstElimination();
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}
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//===----------------------------------------------------------------------===//
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// DeadCodeElimination pass implementation
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//
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namespace {
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struct DCE : public FunctionPass {
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virtual bool runOnFunction(Function &F);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesCFG();
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}
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};
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RegisterOpt<DCE> Y("dce", "Dead Code Elimination");
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}
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bool DCE::runOnFunction(Function &F) {
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// Start out with all of the instructions in the worklist...
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std::vector<Instruction*> WorkList(inst_begin(F), inst_end(F));
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std::set<Instruction*> DeadInsts;
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// Loop over the worklist finding instructions that are dead. If they are
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// dead make them drop all of their uses, making other instructions
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// potentially dead, and work until the worklist is empty.
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//
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while (!WorkList.empty()) {
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Instruction *I = WorkList.back();
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WorkList.pop_back();
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if (isInstructionTriviallyDead(I)) { // If the instruction is dead...
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// Loop over all of the values that the instruction uses, if there are
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// instructions being used, add them to the worklist, because they might
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// go dead after this one is removed.
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//
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for (User::use_iterator UI = I->use_begin(), UE = I->use_end();
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UI != UE; ++UI)
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if (Instruction *Used = dyn_cast<Instruction>(*UI))
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WorkList.push_back(Used);
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// Tell the instruction to let go of all of the values it uses...
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I->dropAllReferences();
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// Keep track of this instruction, because we are going to delete it later
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DeadInsts.insert(I);
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}
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}
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// If we found no dead instructions, we haven't changed the function...
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if (DeadInsts.empty()) return false;
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// Otherwise, loop over the program, removing and deleting the instructions...
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for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
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for (BasicBlock::iterator BI = I->begin(); BI != I->end(); )
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if (DeadInsts.count(BI)) { // Is this instruction dead?
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BI = I->getInstList().erase(BI); // Yup, remove and delete inst
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++DCEEliminated;
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} else { // This instruction is not dead
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++BI; // Continue on to the next one...
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}
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return true;
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}
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Pass *createDeadCodeEliminationPass() {
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return new DCE();
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}
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} // End llvm namespace
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