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 int UnifyFunctionExitNodes::stub;
33 Pass *llvm::createUnifyFunctionExitNodesPass() {
34 return new UnifyFunctionExitNodes();
37 void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
38 // We preserve the non-critical-edgeness property
39 AU.addPreservedID(BreakCriticalEdgesID);
40 // This is a cluster of orthogonal Transforms
41 AU.addPreservedID(PromoteMemoryToRegisterID);
42 AU.addPreservedID(LowerSwitchID);
45 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
46 // BasicBlock, and converting all returns to unconditional branches to this
47 // new basic block. The singular exit node is returned.
49 // If there are no return stmts in the Function, a null pointer is returned.
51 bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
52 // Loop over all of the blocks in a function, tracking all of the blocks that
55 std::vector<BasicBlock*> ReturningBlocks;
56 std::vector<BasicBlock*> UnwindingBlocks;
57 std::vector<BasicBlock*> UnreachableBlocks;
58 for(Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
59 if (isa<ReturnInst>(I->getTerminator()))
60 ReturningBlocks.push_back(I);
61 else if (isa<UnwindInst>(I->getTerminator()))
62 UnwindingBlocks.push_back(I);
63 else if (isa<UnreachableInst>(I->getTerminator()))
64 UnreachableBlocks.push_back(I);
66 // Handle unwinding blocks first.
67 if (UnwindingBlocks.empty()) {
69 } else if (UnwindingBlocks.size() == 1) {
70 UnwindBlock = UnwindingBlocks.front();
72 UnwindBlock = new BasicBlock("UnifiedUnwindBlock", &F);
73 new UnwindInst(UnwindBlock);
75 for (std::vector<BasicBlock*>::iterator I = UnwindingBlocks.begin(),
76 E = UnwindingBlocks.end(); I != E; ++I) {
78 BB->getInstList().pop_back(); // Remove the unwind insn
79 new BranchInst(UnwindBlock, BB);
83 // Then unreachable blocks.
84 if (UnreachableBlocks.empty()) {
86 } else if (UnreachableBlocks.size() == 1) {
87 UnreachableBlock = UnreachableBlocks.front();
89 UnreachableBlock = new BasicBlock("UnifiedUnreachableBlock", &F);
90 new UnreachableInst(UnreachableBlock);
92 for (std::vector<BasicBlock*>::iterator I = UnreachableBlocks.begin(),
93 E = UnreachableBlocks.end(); I != E; ++I) {
95 BB->getInstList().pop_back(); // Remove the unreachable inst.
96 new BranchInst(UnreachableBlock, BB);
100 // Now handle return blocks.
101 if (ReturningBlocks.empty()) {
103 return false; // No blocks return
104 } else if (ReturningBlocks.size() == 1) {
105 ReturnBlock = ReturningBlocks.front(); // Already has a single return block
109 // Otherwise, we need to insert a new basic block into the function, add a PHI
110 // nodes (if the function returns values), and convert all of the return
111 // instructions into unconditional branches.
113 BasicBlock *NewRetBlock = new BasicBlock("UnifiedReturnBlock", &F);
115 SmallVector<Value *, 4> Phis;
116 unsigned NumRetVals = ReturningBlocks[0]->getTerminator()->getNumOperands();
118 new ReturnInst(NULL, NewRetBlock);
119 else if (const StructType *STy = dyn_cast<StructType>(F.getReturnType())) {
120 Instruction *InsertPt = NewRetBlock->getFirstNonPHI();
121 for (unsigned i = 0; i < NumRetVals; ++i) {
122 PHINode *PN = new PHINode(STy->getElementType(i), "UnifiedRetVal."
123 + utostr(i), InsertPt);
126 new ReturnInst(&Phis[0], NumRetVals);
129 // If the function doesn't return void... add a PHI node to the block...
130 PHINode *PN = new PHINode(F.getReturnType(), "UnifiedRetVal");
131 NewRetBlock->getInstList().push_back(PN);
133 new ReturnInst(PN, NewRetBlock);
136 // Loop over all of the blocks, replacing the return instruction with an
137 // unconditional branch.
139 for (std::vector<BasicBlock*>::iterator I = ReturningBlocks.begin(),
140 E = ReturningBlocks.end(); I != E; ++I) {
143 // Add an incoming element to the PHI node for every return instruction that
144 // is merging into this new block...
146 for (unsigned i = 0; i < NumRetVals; ++i)
147 cast<PHINode>(Phis[i])->addIncoming(BB->getTerminator()->getOperand(i),
151 BB->getInstList().pop_back(); // Remove the return insn
152 new BranchInst(NewRetBlock, BB);
154 ReturnBlock = NewRetBlock;