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/InitializePasses.h" 17 #include "llvm/Pass.h" 18 #include "llvm/Transforms/Scalar.h" 19 #include "llvm/Transforms/Utils/Local.h" 20 21 using namespace llvm; 22 23 #define DEBUG_TYPE "flattencfg" 24 25 namespace { 26 struct FlattenCFGPass : public FunctionPass { 27 static char ID; // Pass identification, replacement for typeid 28 public: 29 FlattenCFGPass() : FunctionPass(ID) { 30 initializeFlattenCFGPassPass(*PassRegistry::getPassRegistry()); 31 } 32 bool runOnFunction(Function &F) override; 33 34 void getAnalysisUsage(AnalysisUsage &AU) const override { 35 AU.addRequired<AAResultsWrapperPass>(); 36 } 37 38 private: 39 AliasAnalysis *AA; 40 }; 41 } 42 43 char FlattenCFGPass::ID = 0; 44 INITIALIZE_PASS_BEGIN(FlattenCFGPass, "flattencfg", "Flatten the CFG", false, 45 false) 46 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass) 47 INITIALIZE_PASS_END(FlattenCFGPass, "flattencfg", "Flatten the CFG", false, 48 false) 49 50 // Public interface to the FlattenCFG pass 51 FunctionPass *llvm::createFlattenCFGPass() { return new FlattenCFGPass(); } 52 53 /// iterativelyFlattenCFG - Call FlattenCFG on all the blocks in the function, 54 /// iterating until no more changes are made. 55 static bool iterativelyFlattenCFG(Function &F, AliasAnalysis *AA) { 56 bool Changed = false; 57 bool LocalChange = true; 58 59 // Use block handles instead of iterating over function blocks directly 60 // to avoid using iterators invalidated by erasing blocks. 61 std::vector<WeakVH> Blocks; 62 Blocks.reserve(F.size()); 63 for (auto &BB : F) 64 Blocks.push_back(&BB); 65 66 while (LocalChange) { 67 LocalChange = false; 68 69 // Loop over all of the basic blocks and try to flatten them. 70 for (WeakVH &BlockHandle : Blocks) { 71 // Skip blocks erased by FlattenCFG. 72 if (auto *BB = cast_or_null<BasicBlock>(BlockHandle)) 73 if (FlattenCFG(BB, AA)) 74 LocalChange = true; 75 } 76 Changed |= LocalChange; 77 } 78 return Changed; 79 } 80 81 bool FlattenCFGPass::runOnFunction(Function &F) { 82 AA = &getAnalysis<AAResultsWrapperPass>().getAAResults(); 83 bool EverChanged = false; 84 // iterativelyFlattenCFG can make some blocks dead. 85 while (iterativelyFlattenCFG(F, AA)) { 86 removeUnreachableBlocks(F); 87 EverChanged = true; 88 } 89 return EverChanged; 90 } 91