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"
19 static RegisterOpt<UnifyFunctionExitNodes>
20 X("mergereturn", "Unify function exit nodes");
22 void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
23 // We preserve the non-critical-edgeness property
24 AU.addPreservedID(BreakCriticalEdgesID);
27 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
28 // BasicBlock, and converting all returns to unconditional branches to this
29 // new basic block. The singular exit node is returned.
31 // If there are no return stmts in the Function, a null pointer is returned.
33 bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
34 // Loop over all of the blocks in a function, tracking all of the blocks that
37 vector<BasicBlock*> ReturningBlocks;
38 for(Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
39 if (isa<ReturnInst>(I->getTerminator()))
40 ReturningBlocks.push_back(I);
42 if (ReturningBlocks.empty()) {
44 return false; // No blocks return
45 } else if (ReturningBlocks.size() == 1) {
46 ExitNode = ReturningBlocks.front(); // Already has a single return block
50 // Otherwise, we need to insert a new basic block into the function, add a PHI
51 // node (if the function returns a value), and convert all of the return
52 // instructions into unconditional branches.
54 BasicBlock *NewRetBlock = new BasicBlock("UnifiedExitNode", &F);
57 if (F.getReturnType() != Type::VoidTy) {
58 // If the function doesn't return void... add a PHI node to the block...
59 PN = new PHINode(F.getReturnType(), "UnifiedRetVal");
60 NewRetBlock->getInstList().push_back(PN);
61 NewRetBlock->getInstList().push_back(new ReturnInst(PN));
63 // If it returns void, just add a return void instruction to the block
64 NewRetBlock->getInstList().push_back(new ReturnInst());
67 // Loop over all of the blocks, replacing the return instruction with an
68 // unconditional branch.
70 for (vector<BasicBlock*>::iterator I = ReturningBlocks.begin(),
71 E = ReturningBlocks.end(); I != E; ++I) {
74 // Add an incoming element to the PHI node for every return instruction that
75 // is merging into this new block...
76 if (PN) PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
78 BB->getInstList().pop_back(); // Remove the return insn
79 BB->getInstList().push_back(new BranchInst(NewRetBlock));
81 ExitNode = NewRetBlock;