src/llvm/lib/Transforms/Scalar/FlattenCFGPass.cpp

   1 //===- FlattenCFGPass.cpp - CFG Flatten Pass ----------------------===//
   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 flattening of CFG.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "llvm/Transforms/Scalar.h"
  15 #include "llvm/Analysis/AliasAnalysis.h"
  16 #include "llvm/IR/CFG.h"
  17 #include "llvm/Pass.h"
  18 #include "llvm/Transforms/Utils/Local.h"
  19 using namespace llvm;
  20
  21 #define DEBUG_TYPE "flattencfg"
  22
  23 namespace {
  24 struct FlattenCFGPass : public FunctionPass {
  25   static char ID; // Pass identification, replacement for typeid
  26 public:
  27   FlattenCFGPass() : FunctionPass(ID) {
  28     initializeFlattenCFGPassPass(*PassRegistry::getPassRegistry());
  29   }
  30   bool runOnFunction(Function &F) override;
  31
  32   void getAnalysisUsage(AnalysisUsage &AU) const override {
  33     AU.addRequired<AliasAnalysis>();
  34   }
  35
  36 private:
  37   AliasAnalysis *AA;
  38 };
  39 }
  40
  41 char FlattenCFGPass::ID = 0;
  42 INITIALIZE_PASS_BEGIN(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
  43                       false)
  44 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
  45 INITIALIZE_PASS_END(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
  46                     false)
  47
  48 // Public interface to the FlattenCFG pass
  49 FunctionPass *llvm::createFlattenCFGPass() { return new FlattenCFGPass(); }
  50
  51 /// iterativelyFlattenCFG - Call FlattenCFG on all the blocks in the function,
  52 /// iterating until no more changes are made.
  53 static bool iterativelyFlattenCFG(Function &F, AliasAnalysis *AA) {
  54   bool Changed = false;
  55   bool LocalChange = true;
  56   while (LocalChange) {
  57     LocalChange = false;
  58
  59     // Loop over all of the basic blocks and remove them if they are unneeded...
  60     //
  61     for (Function::iterator BBIt = F.begin(); BBIt != F.end();) {
  62       if (FlattenCFG(BBIt++, AA)) {
  63         LocalChange = true;
  64       }
  65     }
  66     Changed |= LocalChange;
  67   }
  68   return Changed;
  69 }
  70
  71 bool FlattenCFGPass::runOnFunction(Function &F) {
  72   AA = &getAnalysis<AliasAnalysis>();
  73   bool EverChanged = false;
  74   // iterativelyFlattenCFG can make some blocks dead.
  75   while (iterativelyFlattenCFG(F, AA)) {
  76     removeUnreachableBlocks(F);
  77     EverChanged = true;
  78   }
  79   return EverChanged;
  80 }