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96 lines
3.2 KiB
C++
96 lines
3.2 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 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 file implements the Aggressive Dead Code Elimination pass. This pass
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// optimistically assumes that all instructions are dead until proven otherwise,
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// allowing it to eliminate dead computations that other DCE passes do not
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// catch, particularly involving loop computations.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "adce"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/BasicBlock.h"
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#include "llvm/Instructions.h"
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#include "llvm/IntrinsicInst.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Support/InstIterator.h"
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#include "llvm/ADT/DepthFirstIterator.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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using namespace llvm;
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STATISTIC(NumRemoved, "Number of instructions removed");
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namespace {
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struct ADCE : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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ADCE() : FunctionPass(&ID) {}
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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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}
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char ADCE::ID = 0;
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static RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination");
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bool ADCE::runOnFunction(Function& F) {
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SmallPtrSet<Instruction*, 128> alive;
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SmallVector<Instruction*, 128> worklist;
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// Collect the set of "root" instructions that are known live.
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for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
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if (isa<TerminatorInst>(I.getInstructionIterator()) ||
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isa<DbgInfoIntrinsic>(I.getInstructionIterator()) ||
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I->mayHaveSideEffects()) {
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alive.insert(I.getInstructionIterator());
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worklist.push_back(I.getInstructionIterator());
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}
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// Propagate liveness backwards to operands.
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while (!worklist.empty()) {
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Instruction* curr = worklist.pop_back_val();
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for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end();
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OI != OE; ++OI)
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if (Instruction* Inst = dyn_cast<Instruction>(OI))
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if (alive.insert(Inst))
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worklist.push_back(Inst);
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}
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// The inverse of the live set is the dead set. These are those instructions
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// which have no side effects and do not influence the control flow or return
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// value of the function, and may therefore be deleted safely.
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// NOTE: We reuse the worklist vector here for memory efficiency.
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for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
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if (!alive.count(I.getInstructionIterator())) {
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worklist.push_back(I.getInstructionIterator());
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I->dropAllReferences();
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}
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for (SmallVector<Instruction*, 1024>::iterator I = worklist.begin(),
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E = worklist.end(); I != E; ++I) {
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NumRemoved++;
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(*I)->eraseFromParent();
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}
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return !worklist.empty();
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}
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FunctionPass *llvm::createAggressiveDCEPass() {
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return new ADCE();
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}
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