ConstantProp.cpp revision 794fd75c67a2cdc128d67342c6d88a504d186896 
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#define DEBUG_TYPE "constprop"
22#include "llvm/Transforms/Scalar.h"
23#include "llvm/Analysis/ConstantFolding.h"
24#include "llvm/Constant.h"
25#include "llvm/Instruction.h"
26#include "llvm/Pass.h"
27#include "llvm/Support/Compiler.h"
28#include "llvm/Support/InstIterator.h"
29#include "llvm/ADT/Statistic.h"
30#include <set>
31using namespace llvm;
32
33STATISTIC(NumInstKilled, "Number of instructions killed");
34
35namespace {
36  struct VISIBILITY_HIDDEN ConstantPropagation : public FunctionPass {
37    static const int ID; // Pass identifcation, replacement for typeid
38    ConstantPropagation() : FunctionPass((intptr_t)&ID) {}
39
40    bool runOnFunction(Function &F);
41
42    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
43      AU.setPreservesCFG();
44    }
45  };
46
47  const int ConstantPropagation::ID = 0;
48  RegisterPass<ConstantPropagation> X("constprop",
49                                      "Simple constant propagation");
50}
51
52FunctionPass *llvm::createConstantPropagationPass() {
53  return new ConstantPropagation();
54}
55
56
57bool ConstantPropagation::runOnFunction(Function &F) {
58  // Initialize the worklist to all of the instructions ready to process...
59  std::set<Instruction*> WorkList;
60  for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) {
61      WorkList.insert(&*i);
62  }
63  bool Changed = false;
64
65  while (!WorkList.empty()) {
66    Instruction *I = *WorkList.begin();
67    WorkList.erase(WorkList.begin());    // Get an element from the worklist...
68
69    if (!I->use_empty())                 // Don't muck with dead instructions...
70      if (Constant *C = ConstantFoldInstruction(I)) {
71        // Add all of the users of this instruction to the worklist, they might
72        // be constant propagatable now...
73        for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
74             UI != UE; ++UI)
75          WorkList.insert(cast<Instruction>(*UI));
76
77        // Replace all of the uses of a variable with uses of the constant.
78        I->replaceAllUsesWith(C);
79
80        // Remove the dead instruction.
81        WorkList.erase(I);
82        I->getParent()->getInstList().erase(I);
83
84        // We made a change to the function...
85        Changed = true;
86        ++NumInstKilled;
87      }
88  }
89  return Changed;
90}
91