1 //===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
15 //===----------------------------------------------------------------------===//
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/SmallVector.h"
24 #include "llvm/ADT/StringExtras.h"
27 char UnifyFunctionExitNodes::ID = 0;
28 static RegisterPass<UnifyFunctionExitNodes>
29 X("mergereturn", "Unify function exit nodes");
31 Pass *llvm::createUnifyFunctionExitNodesPass() {
32 return new UnifyFunctionExitNodes();
35 void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
36 // We preserve the non-critical-edgeness property
37 AU.addPreservedID(BreakCriticalEdgesID);
38 // This is a cluster of orthogonal Transforms
39 AU.addPreservedID(PromoteMemoryToRegisterID);
40 AU.addPreservedID(LowerSwitchID);
43 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
44 // BasicBlock, and converting all returns to unconditional branches to this
45 // new basic block. The singular exit node is returned.
47 // If there are no return stmts in the Function, a null pointer is returned.
49 bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
50 // Loop over all of the blocks in a function, tracking all of the blocks that
53 std::vector<BasicBlock*> ReturningBlocks;
54 std::vector<BasicBlock*> UnwindingBlocks;
55 std::vector<BasicBlock*> UnreachableBlocks;
56 for(Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
57 if (isa<ReturnInst>(I->getTerminator()))
58 ReturningBlocks.push_back(I);
59 else if (isa<UnwindInst>(I->getTerminator()))
60 UnwindingBlocks.push_back(I);
61 else if (isa<UnreachableInst>(I->getTerminator()))
62 UnreachableBlocks.push_back(I);
64 // Handle unwinding blocks first.
65 if (UnwindingBlocks.empty()) {
67 } else if (UnwindingBlocks.size() == 1) {
68 UnwindBlock = UnwindingBlocks.front();
70 UnwindBlock = BasicBlock::Create("UnifiedUnwindBlock", &F);
71 new UnwindInst(UnwindBlock);
73 for (std::vector<BasicBlock*>::iterator I = UnwindingBlocks.begin(),
74 E = UnwindingBlocks.end(); I != E; ++I) {
76 BB->getInstList().pop_back(); // Remove the unwind insn
77 BranchInst::Create(UnwindBlock, BB);
81 // Then unreachable blocks.
82 if (UnreachableBlocks.empty()) {
84 } else if (UnreachableBlocks.size() == 1) {
85 UnreachableBlock = UnreachableBlocks.front();
87 UnreachableBlock = BasicBlock::Create("UnifiedUnreachableBlock", &F);
88 new UnreachableInst(UnreachableBlock);
90 for (std::vector<BasicBlock*>::iterator I = UnreachableBlocks.begin(),
91 E = UnreachableBlocks.end(); I != E; ++I) {
93 BB->getInstList().pop_back(); // Remove the unreachable inst.
94 BranchInst::Create(UnreachableBlock, BB);
98 // Now handle return blocks.
99 if (ReturningBlocks.empty()) {
101 return false; // No blocks return
102 } else if (ReturningBlocks.size() == 1) {
103 ReturnBlock = ReturningBlocks.front(); // Already has a single return block
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.
111 BasicBlock *NewRetBlock = BasicBlock::Create("UnifiedReturnBlock", &F);
113 SmallVector<Value *, 4> Phis;
114 unsigned NumRetVals = ReturningBlocks[0]->getTerminator()->getNumOperands();
116 ReturnInst::Create(NULL, NewRetBlock);
117 else if (const StructType *STy = dyn_cast<StructType>(F.getReturnType())) {
118 Instruction *InsertPt = NULL;
120 InsertPt = NewRetBlock->getFirstNonPHI();
122 for (unsigned i = 0; i < NumRetVals; ++i) {
124 PN = PHINode::Create(STy->getElementType(i), "UnifiedRetVal."
125 + utostr(i), InsertPt);
127 PN = PHINode::Create(STy->getElementType(i), "UnifiedRetVal."
128 + utostr(i), NewRetBlock);
132 ReturnInst::Create(&Phis[0], NumRetVals, NewRetBlock);
135 // If the function doesn't return void... add a PHI node to the block...
136 PHINode *PN = PHINode::Create(F.getReturnType(), "UnifiedRetVal");
137 NewRetBlock->getInstList().push_back(PN);
139 ReturnInst::Create(PN, NewRetBlock);
142 // Loop over all of the blocks, replacing the return instruction with an
143 // unconditional branch.
145 for (std::vector<BasicBlock*>::iterator I = ReturningBlocks.begin(),
146 E = ReturningBlocks.end(); I != E; ++I) {
149 // Add an incoming element to the PHI node for every return instruction that
150 // is merging into this new block...
152 for (unsigned i = 0; i < NumRetVals; ++i)
153 cast<PHINode>(Phis[i])->addIncoming(BB->getTerminator()->getOperand(i),
157 BB->getInstList().pop_back(); // Remove the return insn
158 BranchInst::Create(NewRetBlock, BB);
160 ReturnBlock = NewRetBlock;