1 //===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source 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/iTerminators.h"
22 #include "llvm/iPHINode.h"
23 #include "llvm/Type.h"
26 static RegisterOpt<UnifyFunctionExitNodes>
27 X("mergereturn", "Unify function exit nodes");
29 Pass *llvm::createUnifyFunctionExitNodesPass() {
30 return new UnifyFunctionExitNodes();
33 void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
34 // We preserve the non-critical-edgeness property
35 AU.addPreservedID(BreakCriticalEdgesID);
38 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
39 // BasicBlock, and converting all returns to unconditional branches to this
40 // new basic block. The singular exit node is returned.
42 // If there are no return stmts in the Function, a null pointer is returned.
44 bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
45 // Loop over all of the blocks in a function, tracking all of the blocks that
48 std::vector<BasicBlock*> ReturningBlocks;
49 std::vector<BasicBlock*> UnwindingBlocks;
50 for(Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
51 if (isa<ReturnInst>(I->getTerminator()))
52 ReturningBlocks.push_back(I);
53 else if (isa<UnwindInst>(I->getTerminator()))
54 UnwindingBlocks.push_back(I);
56 // Handle unwinding blocks first...
57 if (UnwindingBlocks.empty()) {
59 } else if (UnwindingBlocks.size() == 1) {
60 UnwindBlock = UnwindingBlocks.front();
62 UnwindBlock = new BasicBlock("UnifiedUnwindBlock", &F);
63 new UnwindInst(UnwindBlock);
65 for (std::vector<BasicBlock*>::iterator I = UnwindingBlocks.begin(),
66 E = UnwindingBlocks.end(); I != E; ++I) {
68 BB->getInstList().pop_back(); // Remove the return insn
69 new BranchInst(UnwindBlock, 0, 0, BB);
73 // Now handle return blocks...
74 if (ReturningBlocks.empty()) {
76 return false; // No blocks return
77 } else if (ReturningBlocks.size() == 1) {
78 ReturnBlock = ReturningBlocks.front(); // Already has a single return block
82 // Otherwise, we need to insert a new basic block into the function, add a PHI
83 // node (if the function returns a value), and convert all of the return
84 // instructions into unconditional branches.
86 BasicBlock *NewRetBlock = new BasicBlock("UnifiedReturnBlock", &F);
89 if (F.getReturnType() != Type::VoidTy) {
90 // If the function doesn't return void... add a PHI node to the block...
91 PN = new PHINode(F.getReturnType(), "UnifiedRetVal");
92 NewRetBlock->getInstList().push_back(PN);
94 new ReturnInst(PN, NewRetBlock);
96 // Loop over all of the blocks, replacing the return instruction with an
97 // unconditional branch.
99 for (std::vector<BasicBlock*>::iterator I = ReturningBlocks.begin(),
100 E = ReturningBlocks.end(); I != E; ++I) {
103 // Add an incoming element to the PHI node for every return instruction that
104 // is merging into this new block...
105 if (PN) PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
107 BB->getInstList().pop_back(); // Remove the return insn
108 new BranchInst(NewRetBlock, BB);
110 ReturnBlock = NewRetBlock;