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/Transforms/Scalar.h" 19 #include "llvm/BasicBlock.h" 20 #include "llvm/Function.h" 21 #include "llvm/Instructions.h" 22 #include "llvm/Type.h" 23 #include "llvm/ADT/StringExtras.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.addPreserved("mem2reg"); 39 AU.addPreservedID(LowerSwitchID); 40 } 41 42 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new 43 // BasicBlock, and converting all returns to unconditional branches to this 44 // new basic block. The singular exit node is returned. 45 // 46 // If there are no return stmts in the Function, a null pointer is returned. 47 // 48 bool UnifyFunctionExitNodes::runOnFunction(Function &F) { 49 // Loop over all of the blocks in a function, tracking all of the blocks that 50 // return. 51 // 52 std::vector<BasicBlock*> ReturningBlocks; 53 std::vector<BasicBlock*> UnwindingBlocks; 54 std::vector<BasicBlock*> UnreachableBlocks; 55 for(Function::iterator I = F.begin(), E = F.end(); I != E; ++I) 56 if (isa<ReturnInst>(I->getTerminator())) 57 ReturningBlocks.push_back(I); 58 else if (isa<UnwindInst>(I->getTerminator())) 59 UnwindingBlocks.push_back(I); 60 else if (isa<UnreachableInst>(I->getTerminator())) 61 UnreachableBlocks.push_back(I); 62 63 // Handle unwinding blocks first. 64 if (UnwindingBlocks.empty()) { 65 UnwindBlock = 0; 66 } else if (UnwindingBlocks.size() == 1) { 67 UnwindBlock = UnwindingBlocks.front(); 68 } else { 69 UnwindBlock = BasicBlock::Create(F.getContext(), "UnifiedUnwindBlock", &F); 70 new UnwindInst(F.getContext(), UnwindBlock); 71 72 for (std::vector<BasicBlock*>::iterator I = UnwindingBlocks.begin(), 73 E = UnwindingBlocks.end(); I != E; ++I) { 74 BasicBlock *BB = *I; 75 BB->getInstList().pop_back(); // Remove the unwind insn 76 BranchInst::Create(UnwindBlock, BB); 77 } 78 } 79 80 // Then unreachable blocks. 81 if (UnreachableBlocks.empty()) { 82 UnreachableBlock = 0; 83 } else if (UnreachableBlocks.size() == 1) { 84 UnreachableBlock = UnreachableBlocks.front(); 85 } else { 86 UnreachableBlock = BasicBlock::Create(F.getContext(), 87 "UnifiedUnreachableBlock", &F); 88 new UnreachableInst(F.getContext(), UnreachableBlock); 89 90 for (std::vector<BasicBlock*>::iterator I = UnreachableBlocks.begin(), 91 E = UnreachableBlocks.end(); I != E; ++I) { 92 BasicBlock *BB = *I; 93 BB->getInstList().pop_back(); // Remove the unreachable inst. 94 BranchInst::Create(UnreachableBlock, BB); 95 } 96 } 97 98 // Now handle return blocks. 99 if (ReturningBlocks.empty()) { 100 ReturnBlock = 0; 101 return false; // No blocks return 102 } else if (ReturningBlocks.size() == 1) { 103 ReturnBlock = ReturningBlocks.front(); // Already has a single return block 104 return false; 105 } 106 107 // Otherwise, we need to insert a new basic block into the function, add a PHI 108 // nodes (if the function returns values), and convert all of the return 109 // instructions into unconditional branches. 110 // 111 BasicBlock *NewRetBlock = BasicBlock::Create(F.getContext(), 112 "UnifiedReturnBlock", &F); 113 114 PHINode *PN = 0; 115 if (F.getReturnType()->isVoidTy()) { 116 ReturnInst::Create(F.getContext(), NULL, NewRetBlock); 117 } else { 118 // If the function doesn't return void... add a PHI node to the block... 119 PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(), 120 "UnifiedRetVal"); 121 NewRetBlock->getInstList().push_back(PN); 122 ReturnInst::Create(F.getContext(), PN, NewRetBlock); 123 } 124 125 // Loop over all of the blocks, replacing the return instruction with an 126 // unconditional branch. 127 // 128 for (std::vector<BasicBlock*>::iterator I = ReturningBlocks.begin(), 129 E = ReturningBlocks.end(); I != E; ++I) { 130 BasicBlock *BB = *I; 131 132 // Add an incoming element to the PHI node for every return instruction that 133 // is merging into this new block... 134 if (PN) 135 PN->addIncoming(BB->getTerminator()->getOperand(0), BB); 136 137 BB->getInstList().pop_back(); // Remove the return insn 138 BranchInst::Create(NewRetBlock, BB); 139 } 140 ReturnBlock = NewRetBlock; 141 return true; 142 } 143
