ConstantProp.cpp revision b6ac8bc586f7d7a2c07e3d3141a77c8e0cc1bb5d 
1//===- ConstantProp.cpp - Code to perform Simple Constant Propagation -----===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file was developed by the LLVM research group and is distributed under
6// the University of Illinois Open Source 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 to run a DIE pass sometime after running this pass.
18//
19//===----------------------------------------------------------------------===//
20
21#include "llvm/Transforms/Scalar.h"
22#include "llvm/Transforms/Utils/Local.h"
23#include "llvm/Constant.h"
24#include "llvm/Instruction.h"
25#include "llvm/Pass.h"
26#include "llvm/Support/InstIterator.h"
27#include "Support/Statistic.h"
28#include <set>
29using namespace llvm;
30
31namespace {
32  Statistic<> NumInstKilled("constprop", "Number of instructions killed");
33
34  struct ConstantPropagation : public FunctionPass {
35    bool runOnFunction(Function &F);
36
37    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
38      AU.setPreservesCFG();
39    }
40  };
41
42  RegisterOpt<ConstantPropagation> X("constprop","Simple constant propagation");
43}
44
45Pass *llvm::createConstantPropagationPass() {
46  return new ConstantPropagation();
47}
48
49
50bool ConstantPropagation::runOnFunction(Function &F) {
51  // Initialize the worklist to all of the instructions ready to process...
52  std::set<Instruction*> WorkList(inst_begin(F), inst_end(F));
53  bool Changed = false;
54
55  while (!WorkList.empty()) {
56    Instruction *I = *WorkList.begin();
57    WorkList.erase(WorkList.begin());    // Get an element from the worklist...
58
59    if (!I->use_empty())                 // Don't muck with dead instructions...
60      if (Constant *C = ConstantFoldInstruction(I)) {
61        // Add all of the users of this instruction to the worklist, they might
62        // be constant propagatable now...
63        for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
64             UI != UE; ++UI)
65          WorkList.insert(cast<Instruction>(*UI));
66
67        // Replace all of the uses of a variable with uses of the constant.
68        I->replaceAllUsesWith(C);
69
70        // We made a change to the function...
71        Changed = true;
72        ++NumInstKilled;
73      }
74  }
75  return Changed;
76}
77