ADCE.cpp revision 7af1c78b98d2df7d0ab9154461ca3d835706716e 
1//===- DCE.cpp - Code to perform dead code elimination --------------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
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.
14//
15//===----------------------------------------------------------------------===//
16
17#define DEBUG_TYPE "adce"
18#include "llvm/Transforms/Scalar.h"
19#include "llvm/BasicBlock.h"
20#include "llvm/Instructions.h"
21#include "llvm/IntrinsicInst.h"
22#include "llvm/Pass.h"
23#include "llvm/Support/CFG.h"
24#include "llvm/Support/Compiler.h"
25#include "llvm/Support/InstIterator.h"
26#include "llvm/ADT/DepthFirstIterator.h"
27#include "llvm/ADT/SmallPtrSet.h"
28#include "llvm/ADT/SmallVector.h"
29#include "llvm/ADT/Statistic.h"
30
31using namespace llvm;
32
33STATISTIC(NumRemoved, "Number of instructions removed");
34
35namespace {
36  struct VISIBILITY_HIDDEN ADCE : public FunctionPass {
37    static char ID; // Pass identification, replacement for typeid
38    ADCE() : FunctionPass(&ID) {}
39
40    virtual bool runOnFunction(Function& F);
41
42    virtual void getAnalysisUsage(AnalysisUsage& AU) const {
43      AU.setPreservesCFG();
44    }
45
46  };
47}
48
49char ADCE::ID = 0;
50static RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination");
51
52bool ADCE::runOnFunction(Function& F) {
53  SmallPtrSet<Instruction*, 128> alive;
54  SmallVector<Instruction*, 128> worklist;
55
56  // Collect the set of "root" instructions that are known live.
57  for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
58    if (isa<TerminatorInst>(I.getInstructionIterator()) ||
59        isa<DbgInfoIntrinsic>(I.getInstructionIterator()) ||
60        I->mayHaveSideEffects()) {
61      alive.insert(I.getInstructionIterator());
62      worklist.push_back(I.getInstructionIterator());
63    }
64
65  // Propagate liveness backwards to operands.
66  while (!worklist.empty()) {
67    Instruction* curr = worklist.back();
68    worklist.pop_back();
69
70    for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end();
71         OI != OE; ++OI)
72      if (Instruction* Inst = dyn_cast<Instruction>(OI))
73        if (alive.insert(Inst))
74          worklist.push_back(Inst);
75  }
76
77  // The inverse of the live set is the dead set.  These are those instructions
78  // which have no side effects and do not influence the control flow or return
79  // value of the function, and may therefore be deleted safely.
80  // NOTE: We reuse the worklist vector here for memory efficiency.
81  for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
82    if (!alive.count(I.getInstructionIterator())) {
83      worklist.push_back(I.getInstructionIterator());
84      I->dropAllReferences();
85    }
86
87  for (SmallVector<Instruction*, 1024>::iterator I = worklist.begin(),
88       E = worklist.end(); I != E; ++I) {
89    NumRemoved++;
90    (*I)->eraseFromParent();
91  }
92
93  return !worklist.empty();
94}
95
96FunctionPass *llvm::createAggressiveDCEPass() {
97  return new ADCE();
98}
99