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/Compiler.h"
  28 #include "llvm/Support/InstIterator.h"
  29 #include "llvm/ADT/Statistic.h"
  30 #include <set>
  31 using namespace llvm;
  32
  33 STATISTIC(NumInstKilled, "Number of instructions killed");
  34
  35 namespace {
  36   struct VISIBILITY_HIDDEN ConstantPropagation : public FunctionPass {
  37     static char ID; // Pass identification, replacement for typeid
  38     ConstantPropagation() : FunctionPass(&ID) {}
  39
  40     bool runOnFunction(Function &F);
  41
  42     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  43       AU.setPreservesCFG();
  44     }
  45   };
  46 }
  47
  48 char ConstantPropagation::ID = 0;
  49 static RegisterPass<ConstantPropagation>
  50 X("constprop", "Simple constant propagation");
  51
  52 FunctionPass *llvm::createConstantPropagationPass() {
  53   return new ConstantPropagation();
  54 }
  55
  56
  57 bool 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->eraseFromParent();
  83
  84         // We made a change to the function...
  85         Changed = true;
  86         ++NumInstKilled;
  87       }
  88   }
  89   return Changed;
  90 }