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#include "llvm/Transforms/Scalar.h"
18#include "llvm/ADT/DepthFirstIterator.h"
19#include "llvm/ADT/SmallPtrSet.h"
20#include "llvm/ADT/SmallVector.h"
21#include "llvm/ADT/Statistic.h"
22#include "llvm/IR/BasicBlock.h"
23#include "llvm/IR/CFG.h"
24#include "llvm/IR/InstIterator.h"
25#include "llvm/IR/Instructions.h"
26#include "llvm/IR/IntrinsicInst.h"
27#include "llvm/Pass.h"
28using namespace llvm;
29
30#define DEBUG_TYPE "adce"
31
32STATISTIC(NumRemoved, "Number of instructions removed");
33
34namespace {
35  struct ADCE : public FunctionPass {
36    static char ID; // Pass identification, replacement for typeid
37    ADCE() : FunctionPass(ID) {
38      initializeADCEPass(*PassRegistry::getPassRegistry());
39    }
40
41    bool runOnFunction(Function& F) override;
42
43    void getAnalysisUsage(AnalysisUsage& AU) const override {
44      AU.setPreservesCFG();
45    }
46
47  };
48}
49
50char ADCE::ID = 0;
51INITIALIZE_PASS(ADCE, "adce", "Aggressive Dead Code Elimination", false, false)
52
53bool ADCE::runOnFunction(Function& F) {
54  if (skipOptnoneFunction(F))
55    return false;
56
57  SmallPtrSet<Instruction*, 128> alive;
58  SmallVector<Instruction*, 128> worklist;
59
60  // Collect the set of "root" instructions that are known live.
61  for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
62    if (isa<TerminatorInst>(I.getInstructionIterator()) ||
63        isa<DbgInfoIntrinsic>(I.getInstructionIterator()) ||
64        isa<LandingPadInst>(I.getInstructionIterator()) ||
65        I->mayHaveSideEffects()) {
66      alive.insert(I.getInstructionIterator());
67      worklist.push_back(I.getInstructionIterator());
68    }
69
70  // Propagate liveness backwards to operands.
71  while (!worklist.empty()) {
72    Instruction* curr = worklist.pop_back_val();
73    for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end();
74         OI != OE; ++OI)
75      if (Instruction* Inst = dyn_cast<Instruction>(OI))
76        if (alive.insert(Inst))
77          worklist.push_back(Inst);
78  }
79
80  // The inverse of the live set is the dead set.  These are those instructions
81  // which have no side effects and do not influence the control flow or return
82  // value of the function, and may therefore be deleted safely.
83  // NOTE: We reuse the worklist vector here for memory efficiency.
84  for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
85    if (!alive.count(I.getInstructionIterator())) {
86      worklist.push_back(I.getInstructionIterator());
87      I->dropAllReferences();
88    }
89
90  for (SmallVectorImpl<Instruction *>::iterator I = worklist.begin(),
91       E = worklist.end(); I != E; ++I) {
92    ++NumRemoved;
93    (*I)->eraseFromParent();
94  }
95
96  return !worklist.empty();
97}
98
99FunctionPass *llvm::createAggressiveDCEPass() {
100  return new ADCE();
101}
102