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