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/PassManager.h" 15 #include "llvm/IR/ValueHandle.h" 16 #include "llvm/InitializePasses.h" 17 #include "llvm/Pass.h" 18 #include "llvm/Transforms/Scalar.h" 19 #include "llvm/Transforms/Scalar/FlattenCFG.h" 20 #include "llvm/Transforms/Utils/Local.h" 21 22 using namespace llvm; 23 24 #define DEBUG_TYPE "flattencfg" 25 26 namespace { 27 struct FlattenCFGLegacyPass : public FunctionPass { 28 static char ID; // Pass identification, replacement for typeid 29 public: 30 FlattenCFGLegacyPass() : FunctionPass(ID) { 31 initializeFlattenCFGLegacyPassPass(*PassRegistry::getPassRegistry()); 32 } 33 bool runOnFunction(Function &F) override; 34 35 void getAnalysisUsage(AnalysisUsage &AU) const override { 36 AU.addRequired<AAResultsWrapperPass>(); 37 } 38 39 private: 40 AliasAnalysis *AA; 41 }; 42 43 /// iterativelyFlattenCFG - Call FlattenCFG on all the blocks in the function, 44 /// iterating until no more changes are made. 45 bool iterativelyFlattenCFG(Function &F, AliasAnalysis *AA) { 46 bool Changed = false; 47 bool LocalChange = true; 48 49 // Use block handles instead of iterating over function blocks directly 50 // to avoid using iterators invalidated by erasing blocks. 51 std::vector<WeakVH> Blocks; 52 Blocks.reserve(F.size()); 53 for (auto &BB : F) 54 Blocks.push_back(&BB); 55 56 while (LocalChange) { 57 LocalChange = false; 58 59 // Loop over all of the basic blocks and try to flatten them. 60 for (WeakVH &BlockHandle : Blocks) { 61 // Skip blocks erased by FlattenCFG. 62 if (auto *BB = cast_or_null<BasicBlock>(BlockHandle)) 63 if (FlattenCFG(BB, AA)) 64 LocalChange = true; 65 } 66 Changed |= LocalChange; 67 } 68 return Changed; 69 } 70 } // namespace 71 72 char FlattenCFGLegacyPass::ID = 0; 73 74 INITIALIZE_PASS_BEGIN(FlattenCFGLegacyPass, "flattencfg", "Flatten the CFG", 75 false, false) 76 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass) 77 INITIALIZE_PASS_END(FlattenCFGLegacyPass, "flattencfg", "Flatten the CFG", 78 false, false) 79 80 // Public interface to the FlattenCFG pass 81 FunctionPass *llvm::createFlattenCFGPass() { 82 return new FlattenCFGLegacyPass(); 83 } 84 85 bool FlattenCFGLegacyPass::runOnFunction(Function &F) { 86 AA = &getAnalysis<AAResultsWrapperPass>().getAAResults(); 87 bool EverChanged = false; 88 // iterativelyFlattenCFG can make some blocks dead. 89 while (iterativelyFlattenCFG(F, AA)) { 90 removeUnreachableBlocks(F); 91 EverChanged = true; 92 } 93 return EverChanged; 94 } 95 96 PreservedAnalyses FlattenCFGPass::run(Function &F, 97 FunctionAnalysisManager &AM) { 98 bool EverChanged = false; 99 AliasAnalysis *AA = &AM.getResult<AAManager>(F); 100 // iterativelyFlattenCFG can make some blocks dead. 101 while (iterativelyFlattenCFG(F, AA)) { 102 removeUnreachableBlocks(F); 103 EverChanged = true; 104 } 105 return EverChanged ? PreservedAnalyses::none() : PreservedAnalyses::all(); 106 } 107