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/BasicBlock.h"
20 #include "llvm/Instructions.h"
21 #include "llvm/Pass.h"
22 #include "llvm/Support/CFG.h"
23 #include "llvm/Support/Compiler.h"
24 #include "llvm/Support/InstIterator.h"
25 #include "llvm/ADT/DepthFirstIterator.h"
26 #include "llvm/ADT/SmallPtrSet.h"
27 #include "llvm/ADT/SmallVector.h"
28 #include "llvm/ADT/Statistic.h"
33 STATISTIC(NumRemoved, "Number of instructions removed");
36 struct VISIBILITY_HIDDEN ADCE : public FunctionPass {
37 static char ID; // Pass identification, replacement for typeid
38 ADCE() : FunctionPass((intptr_t)&ID) {}
40 virtual bool runOnFunction(Function& F);
42 virtual void getAnalysisUsage(AnalysisUsage& AU) const {
50 static RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination");
52 bool ADCE::runOnFunction(Function& F) {
53 SmallPtrSet<Instruction*, 128> alive;
54 SmallVector<Instruction*, 128> worklist;
56 SmallPtrSet<BasicBlock*, 64> reachable;
57 SmallVector<BasicBlock*, 16> unreachable;
59 // First, collect the set of reachable blocks ...
60 for (df_ext_iterator<BasicBlock*, SmallPtrSet<BasicBlock*, 64> >
61 DI = df_ext_begin(&F.getEntryBlock(), reachable),
62 DE = df_ext_end(&F.getEntryBlock(), reachable); DI != DE; ++DI)
63 ; // Deliberately empty, df_ext_iterator will fill in the set.
65 // ... and then invert it into the list of unreachable ones. These
66 // blocks will be removed from the function.
67 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
68 if (!reachable.count(FI))
69 unreachable.push_back(FI);
71 // Prepare to remove blocks by removing the PHI node entries for those blocks
72 // in their successors, and remove them from reference counting.
73 for (SmallVector<BasicBlock*, 16>::iterator UI = unreachable.begin(),
74 UE = unreachable.end(); UI != UE; ++UI) {
76 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB);
78 BasicBlock* succ = *SI;
79 BasicBlock::iterator succ_inst = succ->begin();
80 while (PHINode* P = dyn_cast<PHINode>(succ_inst)) {
81 P->removeIncomingValue(BB);
86 BB->dropAllReferences();
89 // Finally, erase the unreachable blocks.
90 for (SmallVector<BasicBlock*, 16>::iterator UI = unreachable.begin(),
91 UE = unreachable.end(); UI != UE; ++UI)
92 (*UI)->eraseFromParent();
94 // Collect the set of "root" instructions that are known live.
95 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
96 if (isa<TerminatorInst>(I.getInstructionIterator()) ||
97 I->mayWriteToMemory()) {
98 alive.insert(I.getInstructionIterator());
99 worklist.push_back(I.getInstructionIterator());
102 // Propagate liveness backwards to operands.
103 while (!worklist.empty()) {
104 Instruction* curr = worklist.back();
107 for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end();
109 if (Instruction* Inst = dyn_cast<Instruction>(OI))
110 if (alive.insert(Inst))
111 worklist.push_back(Inst);
114 // The inverse of the live set is the dead set. These are those instructions
115 // which have no side effects and do not influence the control flow or return
116 // value of the function, and may therefore be deleted safely.
117 // NOTE: We reuse the worklist vector here for memory efficiency.
118 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
119 if (!alive.count(I.getInstructionIterator())) {
120 worklist.push_back(I.getInstructionIterator());
121 I->dropAllReferences();
124 for (SmallVector<Instruction*, 1024>::iterator I = worklist.begin(),
125 E = worklist.end(); I != E; ++I) {
127 (*I)->eraseFromParent();
130 return !worklist.empty();
133 FunctionPass *llvm::createAggressiveDCEPass() {