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/IR/CFG.h"
  15 #include "llvm/IR/ValueHandle.h"
  16 #include "llvm/Pass.h"
  17 #include "llvm/Transforms/Scalar.h"
  18 #include "llvm/Transforms/Utils/Local.h"
  19
  20 using namespace llvm;
  21
  22 #define DEBUG_TYPE "flattencfg"
  23
  24 namespace {
  25 struct FlattenCFGPass : public FunctionPass {
  26   static char ID; // Pass identification, replacement for typeid
  27 public:
  28   FlattenCFGPass() : FunctionPass(ID) {
  29     initializeFlattenCFGPassPass(*PassRegistry::getPassRegistry());
  30   }
  31   bool runOnFunction(Function &F) override;
  32
  33   void getAnalysisUsage(AnalysisUsage &AU) const override {
  34     AU.addRequired<AAResultsWrapperPass>();
  35   }
  36
  37 private:
  38   AliasAnalysis *AA;
  39 };
  40 }
  41
  42 char FlattenCFGPass::ID = 0;
  43 INITIALIZE_PASS_BEGIN(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
  44                       false)
  45 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
  46 INITIALIZE_PASS_END(FlattenCFGPass, "flattencfg", "Flatten the CFG", false,
  47                     false)
  48
  49 // Public interface to the FlattenCFG pass
  50 FunctionPass *llvm::createFlattenCFGPass() { return new FlattenCFGPass(); }
  51
  52 /// iterativelyFlattenCFG - Call FlattenCFG on all the blocks in the function,
  53 /// iterating until no more changes are made.
  54 static bool iterativelyFlattenCFG(Function &F, AliasAnalysis *AA) {
  55   bool Changed = false;
  56   bool LocalChange = true;
  57
  58   // Use block handles instead of iterating over function blocks directly
  59   // to avoid using iterators invalidated by erasing blocks.
  60   std::vector<WeakVH> Blocks;
  61   Blocks.reserve(F.size());
  62   for (auto &BB : F)
  63     Blocks.push_back(&BB);
  64
  65   while (LocalChange) {
  66     LocalChange = false;
  67
  68     // Loop over all of the basic blocks and try to flatten them.
  69     for (WeakVH &BlockHandle : Blocks) {
  70       // Skip blocks erased by FlattenCFG.
  71       if (auto *BB = cast_or_null<BasicBlock>(BlockHandle))
  72         if (FlattenCFG(BB, AA))
  73           LocalChange = true;
  74     }
  75     Changed |= LocalChange;
  76   }
  77   return Changed;
  78 }
  79
  80 bool FlattenCFGPass::runOnFunction(Function &F) {
  81   AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
  82   bool EverChanged = false;
  83   // iterativelyFlattenCFG can make some blocks dead.
  84   while (iterativelyFlattenCFG(F, AA)) {
  85     removeUnreachableBlocks(F);
  86     EverChanged = true;
  87   }
  88   return EverChanged;
  89 }