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//===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass is used to ensure that functions have at most one return
// instruction in them. Additionally, it keeps track of which node is the new
// exit node of the CFG. If there are no exit nodes in the CFG, the getExitNode
// method will return a null pointer.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/BasicBlock.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Type.h"
using namespace llvm;
char UnifyFunctionExitNodes::ID = 0;
static RegisterPass<UnifyFunctionExitNodes>
X("mergereturn", "Unify function exit nodes");
int UnifyFunctionExitNodes::stub;
Pass *llvm::createUnifyFunctionExitNodesPass() {
return new UnifyFunctionExitNodes();
}
void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
// We preserve the non-critical-edgeness property
AU.addPreservedID(BreakCriticalEdgesID);
// This is a cluster of orthogonal Transforms
AU.addPreservedID(PromoteMemoryToRegisterID);
AU.addPreservedID(LowerSelectID);
AU.addPreservedID(LowerSwitchID);
}
// UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
// BasicBlock, and converting all returns to unconditional branches to this
// new basic block. The singular exit node is returned.
//
// If there are no return stmts in the Function, a null pointer is returned.
//
bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
// Loop over all of the blocks in a function, tracking all of the blocks that
// return.
//
std::vector<BasicBlock*> ReturningBlocks;
std::vector<BasicBlock*> UnwindingBlocks;
std::vector<BasicBlock*> UnreachableBlocks;
for(Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
if (isa<ReturnInst>(I->getTerminator()))
ReturningBlocks.push_back(I);
else if (isa<UnwindInst>(I->getTerminator()))
UnwindingBlocks.push_back(I);
else if (isa<UnreachableInst>(I->getTerminator()))
UnreachableBlocks.push_back(I);
// Handle unwinding blocks first.
if (UnwindingBlocks.empty()) {
UnwindBlock = 0;
} else if (UnwindingBlocks.size() == 1) {
UnwindBlock = UnwindingBlocks.front();
} else {
UnwindBlock = new BasicBlock("UnifiedUnwindBlock", &F);
new UnwindInst(UnwindBlock);
for (std::vector<BasicBlock*>::iterator I = UnwindingBlocks.begin(),
E = UnwindingBlocks.end(); I != E; ++I) {
BasicBlock *BB = *I;
BB->getInstList().pop_back(); // Remove the unwind insn
new BranchInst(UnwindBlock, BB);
}
}
// Then unreachable blocks.
if (UnreachableBlocks.empty()) {
UnreachableBlock = 0;
} else if (UnreachableBlocks.size() == 1) {
UnreachableBlock = UnreachableBlocks.front();
} else {
UnreachableBlock = new BasicBlock("UnifiedUnreachableBlock", &F);
new UnreachableInst(UnreachableBlock);
for (std::vector<BasicBlock*>::iterator I = UnreachableBlocks.begin(),
E = UnreachableBlocks.end(); I != E; ++I) {
BasicBlock *BB = *I;
BB->getInstList().pop_back(); // Remove the unreachable inst.
new BranchInst(UnreachableBlock, BB);
}
}
// Now handle return blocks.
if (ReturningBlocks.empty()) {
ReturnBlock = 0;
return false; // No blocks return
} else if (ReturningBlocks.size() == 1) {
ReturnBlock = ReturningBlocks.front(); // Already has a single return block
return false;
}
// Otherwise, we need to insert a new basic block into the function, add a PHI
// node (if the function returns a value), and convert all of the return
// instructions into unconditional branches.
//
BasicBlock *NewRetBlock = new BasicBlock("UnifiedReturnBlock", &F);
PHINode *PN = 0;
if (F.getReturnType() != Type::VoidTy) {
// If the function doesn't return void... add a PHI node to the block...
PN = new PHINode(F.getReturnType(), "UnifiedRetVal");
NewRetBlock->getInstList().push_back(PN);
}
new ReturnInst(PN, NewRetBlock);
// Loop over all of the blocks, replacing the return instruction with an
// unconditional branch.
//
for (std::vector<BasicBlock*>::iterator I = ReturningBlocks.begin(),
E = ReturningBlocks.end(); I != E; ++I) {
BasicBlock *BB = *I;
// Add an incoming element to the PHI node for every return instruction that
// is merging into this new block...
if (PN) PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
BB->getInstList().pop_back(); // Remove the return insn
new BranchInst(NewRetBlock, BB);
}
ReturnBlock = NewRetBlock;
return true;
}
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