lib/Transforms/Scalar/FlattenCFGPass.cpp

   1 //===- FlattenCFGPass.cpp - CFG Flatten Pass ----------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements flattening of CFG.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/Analysis/AliasAnalysis.h"
  14 #include "llvm/Transforms/Utils/Local.h"
  15 #include "llvm/IR/CFG.h"
  16 #include "llvm/Pass.h"
  17 #include "llvm/Transforms/Scalar.h"
  18 using namespace llvm;
  19
  20 #define DEBUG_TYPE "flattencfg"
  21
  22 namespace {
  23 struct FlattenCFGPass : public FunctionPass {
  24   static char ID; // Pass identification, replacement for typeid
  25 public:
  26   FlattenCFGPass() : FunctionPass(ID) {
  27     initializeFlattenCFGPassPass(*PassRegistry::getPassRegistry());
  28   }
  29   bool runOnFunction(Function &F) override;
  30
  31   void getAnalysisUsage(AnalysisUsage &AU) const override {
  32     AU.addRequired<AAResultsWrapperPass>();
  33   }
  34
  35 private:
  36   AliasAnalysis *AA;
  37 };
  38 }
  39
  40 char FlattenCFGPass::ID = 0;
  41 INITIALIZE_PASS_BEGIN(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
  42                       false)
  43 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
  44 INITIALIZE_PASS_END(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
  45                     false)
  46
  47 // Public interface to the FlattenCFG pass
  48 FunctionPass *llvm::createFlattenCFGPass() { return new FlattenCFGPass(); }
  49
  50 /// iterativelyFlattenCFG - Call FlattenCFG on all the blocks in the function,
  51 /// iterating until no more changes are made.
  52 static bool iterativelyFlattenCFG(Function &F, AliasAnalysis *AA) {
  53   bool Changed = false;
  54   bool LocalChange = true;
  55   while (LocalChange) {
  56     LocalChange = false;
  57
  58     // Loop over all of the basic blocks and remove them if they are unneeded...
  59     //
  60     for (Function::iterator BBIt = F.begin(); BBIt != F.end();) {
  61       if (FlattenCFG(&*BBIt++, AA)) {
  62         LocalChange = true;
  63       }
  64     }
  65     Changed |= LocalChange;
  66   }
  67   return Changed;
  68 }
  69
  70 bool FlattenCFGPass::runOnFunction(Function &F) {
  71   AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
  72   bool EverChanged = false;
  73   // iterativelyFlattenCFG can make some blocks dead.
  74   while (iterativelyFlattenCFG(F, AA)) {
  75     removeUnreachableBlocks(F);
  76     EverChanged = true;
  77   }
  78   return EverChanged;
  79 }