1 //===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===//
3 // This pass is used to ensure that functions have at most one return
4 // instruction in them. Additionally, it keeps track of which node is the new
5 // exit node of the CFG. If there are no exit nodes in the CFG, the getExitNode
6 // method will return a null pointer.
8 //===----------------------------------------------------------------------===//
10 #include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
11 #include "llvm/Transforms/Scalar.h"
12 #include "llvm/BasicBlock.h"
13 #include "llvm/Function.h"
14 #include "llvm/iTerminators.h"
15 #include "llvm/iPHINode.h"
16 #include "llvm/Type.h"
18 static RegisterOpt<UnifyFunctionExitNodes>
19 X("mergereturn", "Unify function exit nodes");
21 Pass *createUnifyFunctionExitNodesPass() {
22 return new UnifyFunctionExitNodes();
25 void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
26 // We preserve the non-critical-edgeness property
27 AU.addPreservedID(BreakCriticalEdgesID);
30 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
31 // BasicBlock, and converting all returns to unconditional branches to this
32 // new basic block. The singular exit node is returned.
34 // If there are no return stmts in the Function, a null pointer is returned.
36 bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
37 // Loop over all of the blocks in a function, tracking all of the blocks that
40 std::vector<BasicBlock*> ReturningBlocks;
41 std::vector<BasicBlock*> UnwindingBlocks;
42 for(Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
43 if (isa<ReturnInst>(I->getTerminator()))
44 ReturningBlocks.push_back(I);
45 else if (isa<UnwindInst>(I->getTerminator()))
46 UnwindingBlocks.push_back(I);
48 // Handle unwinding blocks first...
49 if (UnwindingBlocks.empty()) {
51 } else if (UnwindingBlocks.size() == 1) {
52 UnwindBlock = UnwindingBlocks.front();
54 UnwindBlock = new BasicBlock("UnifiedUnwindBlock", &F);
55 UnwindBlock->getInstList().push_back(new UnwindInst());
57 for (std::vector<BasicBlock*>::iterator I = UnwindingBlocks.begin(),
58 E = UnwindingBlocks.end(); I != E; ++I) {
60 BB->getInstList().pop_back(); // Remove the return insn
61 BB->getInstList().push_back(new BranchInst(UnwindBlock));
65 // Now handle return blocks...
66 if (ReturningBlocks.empty()) {
68 return false; // No blocks return
69 } else if (ReturningBlocks.size() == 1) {
70 ReturnBlock = ReturningBlocks.front(); // Already has a single return block
74 // Otherwise, we need to insert a new basic block into the function, add a PHI
75 // node (if the function returns a value), and convert all of the return
76 // instructions into unconditional branches.
78 BasicBlock *NewRetBlock = new BasicBlock("UnifiedReturnBlock", &F);
81 if (F.getReturnType() != Type::VoidTy) {
82 // If the function doesn't return void... add a PHI node to the block...
83 PN = new PHINode(F.getReturnType(), "UnifiedRetVal");
84 NewRetBlock->getInstList().push_back(PN);
85 NewRetBlock->getInstList().push_back(new ReturnInst(PN));
87 // If it returns void, just add a return void instruction to the block
88 NewRetBlock->getInstList().push_back(new ReturnInst());
91 // Loop over all of the blocks, replacing the return instruction with an
92 // unconditional branch.
94 for (std::vector<BasicBlock*>::iterator I = ReturningBlocks.begin(),
95 E = ReturningBlocks.end(); I != E; ++I) {
98 // Add an incoming element to the PHI node for every return instruction that
99 // is merging into this new block...
100 if (PN) PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
102 BB->getInstList().pop_back(); // Remove the return insn
103 BB->getInstList().push_back(new BranchInst(NewRetBlock));
105 ReturnBlock = NewRetBlock;