ConstantProp.cpp revision 36b56886974eae4f9c5ebc96befd3e7bfe5de338 
1//===- ConstantProp.cpp - Code to perform Simple Constant Propagation -----===//
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 constant propagation and merging:
11//
12// Specifically, this:
13//   * Converts instructions like "add int 1, 2" into 3
14//
15// Notice that:
16//   * This pass has a habit of making definitions be dead.  It is a good idea
17//     to run a DIE pass sometime after running this pass.
18//
19//===----------------------------------------------------------------------===//
20
21#define DEBUG_TYPE "constprop"
22#include "llvm/Transforms/Scalar.h"
23#include "llvm/ADT/Statistic.h"
24#include "llvm/Analysis/ConstantFolding.h"
25#include "llvm/IR/Constant.h"
26#include "llvm/IR/DataLayout.h"
27#include "llvm/IR/InstIterator.h"
28#include "llvm/IR/Instruction.h"
29#include "llvm/Pass.h"
30#include "llvm/Target/TargetLibraryInfo.h"
31#include <set>
32using namespace llvm;
33
34STATISTIC(NumInstKilled, "Number of instructions killed");
35
36namespace {
37  struct ConstantPropagation : public FunctionPass {
38    static char ID; // Pass identification, replacement for typeid
39    ConstantPropagation() : FunctionPass(ID) {
40      initializeConstantPropagationPass(*PassRegistry::getPassRegistry());
41    }
42
43    bool runOnFunction(Function &F) override;
44
45    void getAnalysisUsage(AnalysisUsage &AU) const override {
46      AU.setPreservesCFG();
47      AU.addRequired<TargetLibraryInfo>();
48    }
49  };
50}
51
52char ConstantPropagation::ID = 0;
53INITIALIZE_PASS_BEGIN(ConstantPropagation, "constprop",
54                "Simple constant propagation", false, false)
55INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
56INITIALIZE_PASS_END(ConstantPropagation, "constprop",
57                "Simple constant propagation", false, false)
58
59FunctionPass *llvm::createConstantPropagationPass() {
60  return new ConstantPropagation();
61}
62
63bool ConstantPropagation::runOnFunction(Function &F) {
64  // Initialize the worklist to all of the instructions ready to process...
65  std::set<Instruction*> WorkList;
66  for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) {
67      WorkList.insert(&*i);
68  }
69  bool Changed = false;
70  DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
71  const DataLayout *DL = DLP ? &DLP->getDataLayout() : 0;
72  TargetLibraryInfo *TLI = &getAnalysis<TargetLibraryInfo>();
73
74  while (!WorkList.empty()) {
75    Instruction *I = *WorkList.begin();
76    WorkList.erase(WorkList.begin());    // Get an element from the worklist...
77
78    if (!I->use_empty())                 // Don't muck with dead instructions...
79      if (Constant *C = ConstantFoldInstruction(I, DL, TLI)) {
80        // Add all of the users of this instruction to the worklist, they might
81        // be constant propagatable now...
82        for (User *U : I->users())
83          WorkList.insert(cast<Instruction>(U));
84
85        // Replace all of the uses of a variable with uses of the constant.
86        I->replaceAllUsesWith(C);
87
88        // Remove the dead instruction.
89        WorkList.erase(I);
90        I->eraseFromParent();
91
92        // We made a change to the function...
93        Changed = true;
94        ++NumInstKilled;
95      }
96  }
97  return Changed;
98}
99