lib/Transforms/Scalar/ConstantProp.cpp

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