1 //===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This pass is used to ensure that functions have at most one return 11 // instruction in them. Additionally, it keeps track of which node is the new 12 // exit node of the CFG. If there are no exit nodes in the CFG, the getExitNode 13 // method will return a null pointer. 14 // 15 //===----------------------------------------------------------------------===// 16 17 #include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h" 18 #include "llvm/ADT/StringExtras.h" 19 #include "llvm/IR/BasicBlock.h" 20 #include "llvm/IR/Function.h" 21 #include "llvm/IR/Instructions.h" 22 #include "llvm/IR/Type.h" 23 #include "llvm/Transforms/Scalar.h" 24 using namespace llvm; 25 26 char UnifyFunctionExitNodes::ID = 0; 27 INITIALIZE_PASS(UnifyFunctionExitNodes, "mergereturn", 28 "Unify function exit nodes", false, false) 29 30 Pass *llvm::createUnifyFunctionExitNodesPass() { 31 return new UnifyFunctionExitNodes(); 32 } 33 34 void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{ 35 // We preserve the non-critical-edgeness property 36 AU.addPreservedID(BreakCriticalEdgesID); 37 // This is a cluster of orthogonal Transforms 38 AU.addPreservedID(LowerSwitchID); 39 } 40 41 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new 42 // BasicBlock, and converting all returns to unconditional branches to this 43 // new basic block. The singular exit node is returned. 44 // 45 // If there are no return stmts in the Function, a null pointer is returned. 46 // 47 bool UnifyFunctionExitNodes::runOnFunction(Function &F) { 48 // Loop over all of the blocks in a function, tracking all of the blocks that 49 // return. 50 // 51 std::vector<BasicBlock*> ReturningBlocks; 52 std::vector<BasicBlock*> UnreachableBlocks; 53 for (BasicBlock &I : F) 54 if (isa<ReturnInst>(I.getTerminator())) 55 ReturningBlocks.push_back(&I); 56 else if (isa<UnreachableInst>(I.getTerminator())) 57 UnreachableBlocks.push_back(&I); 58 59 // Then unreachable blocks. 60 if (UnreachableBlocks.empty()) { 61 UnreachableBlock = nullptr; 62 } else if (UnreachableBlocks.size() == 1) { 63 UnreachableBlock = UnreachableBlocks.front(); 64 } else { 65 UnreachableBlock = BasicBlock::Create(F.getContext(), 66 "UnifiedUnreachableBlock", &F); 67 new UnreachableInst(F.getContext(), UnreachableBlock); 68 69 for (std::vector<BasicBlock*>::iterator I = UnreachableBlocks.begin(), 70 E = UnreachableBlocks.end(); I != E; ++I) { 71 BasicBlock *BB = *I; 72 BB->getInstList().pop_back(); // Remove the unreachable inst. 73 BranchInst::Create(UnreachableBlock, BB); 74 } 75 } 76 77 // Now handle return blocks. 78 if (ReturningBlocks.empty()) { 79 ReturnBlock = nullptr; 80 return false; // No blocks return 81 } else if (ReturningBlocks.size() == 1) { 82 ReturnBlock = ReturningBlocks.front(); // Already has a single return block 83 return false; 84 } 85 86 // Otherwise, we need to insert a new basic block into the function, add a PHI 87 // nodes (if the function returns values), and convert all of the return 88 // instructions into unconditional branches. 89 // 90 BasicBlock *NewRetBlock = BasicBlock::Create(F.getContext(), 91 "UnifiedReturnBlock", &F); 92 93 PHINode *PN = nullptr; 94 if (F.getReturnType()->isVoidTy()) { 95 ReturnInst::Create(F.getContext(), nullptr, NewRetBlock); 96 } else { 97 // If the function doesn't return void... add a PHI node to the block... 98 PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(), 99 "UnifiedRetVal"); 100 NewRetBlock->getInstList().push_back(PN); 101 ReturnInst::Create(F.getContext(), PN, NewRetBlock); 102 } 103 104 // Loop over all of the blocks, replacing the return instruction with an 105 // unconditional branch. 106 // 107 for (std::vector<BasicBlock*>::iterator I = ReturningBlocks.begin(), 108 E = ReturningBlocks.end(); I != E; ++I) { 109 BasicBlock *BB = *I; 110 111 // Add an incoming element to the PHI node for every return instruction that 112 // is merging into this new block... 113 if (PN) 114 PN->addIncoming(BB->getTerminator()->getOperand(0), BB); 115 116 BB->getInstList().pop_back(); // Remove the return insn 117 BranchInst::Create(NewRetBlock, BB); 118 } 119 ReturnBlock = NewRetBlock; 120 return true; 121 } 122
