1 //===- DCE.cpp - Code to perform dead code elimination --------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements the Aggressive Dead Code Elimination pass. This pass
11 // optimistically assumes that all instructions are dead until proven otherwise,
12 // allowing it to eliminate dead computations that other DCE passes do not
13 // catch, particularly involving loop computations.
15 //===----------------------------------------------------------------------===//
17 #define DEBUG_TYPE "adce"
18 #include "llvm/Transforms/Scalar.h"
19 #include "llvm/Instructions.h"
20 #include "llvm/Pass.h"
21 #include "llvm/Support/Compiler.h"
22 #include "llvm/Support/InstIterator.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/ADT/SmallPtrSet.h"
28 STATISTIC(NumRemoved, "Number of instructions removed");
31 struct VISIBILITY_HIDDEN ADCE : public FunctionPass {
32 static char ID; // Pass identification, replacement for typeid
33 ADCE() : FunctionPass((intptr_t)&ID) {}
35 virtual bool runOnFunction(Function& F);
37 virtual void getAnalysisUsage(AnalysisUsage& AU) const {
45 static RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination");
47 bool ADCE::runOnFunction(Function& F) {
48 SmallPtrSet<Instruction*, 32> alive;
49 std::vector<Instruction*> worklist;
51 // Collect the set of "root" instructions that are known live.
52 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
53 if (isa<TerminatorInst>(I.getInstructionIterator()) ||
54 I->mayWriteToMemory()) {
55 alive.insert(I.getInstructionIterator());
56 worklist.push_back(I.getInstructionIterator());
59 // Propagate liveness backwards to operands.
60 while (!worklist.empty()) {
61 Instruction* curr = worklist.back();
64 for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end();
66 if (Instruction* Inst = dyn_cast<Instruction>(OI))
67 if (alive.insert(Inst))
68 worklist.push_back(Inst);
71 // The inverse of the live set is the dead set. These are those instructions
72 // which have no side effects and do not influence the control flow or return
73 // value of the function, and may therefore be deleted safely.
74 SmallPtrSet<Instruction*, 32> dead;
75 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
76 if (!alive.count(I.getInstructionIterator())) {
77 dead.insert(I.getInstructionIterator());
78 I->dropAllReferences();
81 for (SmallPtrSet<Instruction*, 32>::iterator I = dead.begin(),
82 E = dead.end(); I != E; ++I) {
84 (*I)->eraseFromParent();
90 FunctionPass *llvm::createAggressiveDCEPass() {