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