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 } 80