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Implement Transforms/SimplifyCFG/return-merge.ll

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This actually causes us to turn code like:

  return C ? A : B;

into a select instruction.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12617 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2004-04-02 18:13:43 +00:00
parent 512c2481b1
commit 147af6b75f
1 changed files with 57 additions and 1 deletions
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58
lib/Transforms/Utils/SimplifyCFG.cpp
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@ -613,16 +613,23 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
// If this is a returning block with only PHI nodes in it, fold the return
// instruction into any unconditional branch predecessors.
//
// If any predecessor is a conditional branch that just selects among
// different return values, fold the replace the branch/return with a select
// and return.
if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
BasicBlock::iterator BBI = BB->getTerminator();
if (BBI == BB->begin() || isa<PHINode>(--BBI)) {
// Find predecessors that end with unconditional branches.
// Find predecessors that end with branches.
std::vector<BasicBlock*> UncondBranchPreds;
std::vector<BranchInst*> CondBranchPreds;
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
TerminatorInst *PTI = (*PI)->getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
if (BI->isUnconditional())
UncondBranchPreds.push_back(*PI);
else
CondBranchPreds.push_back(BI);
}
// If we found some, do the transformation!
@ -654,6 +661,55 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
return true;
}
// Check out all of the conditional branches going to this return
// instruction. If any of them just select between returns, change the
// branch itself into a select/return pair.
while (!CondBranchPreds.empty()) {
BranchInst *BI = CondBranchPreds.back();
CondBranchPreds.pop_back();
BasicBlock *TrueSucc = BI->getSuccessor(0);
BasicBlock *FalseSucc = BI->getSuccessor(1);
BasicBlock *OtherSucc = TrueSucc == BB ? FalseSucc : TrueSucc;
// Check to see if the non-BB successor is also a return block.
if (isa<ReturnInst>(OtherSucc->getTerminator())) {
// Check to see if there are only PHI instructions in this block.
BasicBlock::iterator OSI = OtherSucc->getTerminator();
if (OSI == OtherSucc->begin() || isa<PHINode>(--OSI)) {
// Okay, we found a branch that is going to two return nodes. If
// there is no return value for this function, just change the
// branch into a return.
if (RI->getNumOperands() == 0) {
TrueSucc->removePredecessor(BI->getParent());
FalseSucc->removePredecessor(BI->getParent());
new ReturnInst(0, BI);
BI->getParent()->getInstList().erase(BI);
return true;
}
// Otherwise, figure out what the true and false return values are
// so we can insert a new select instruction.
Value *TrueValue = TrueSucc->getTerminator()->getOperand(0);
Value *FalseValue = FalseSucc->getTerminator()->getOperand(0);
// Unwrap any PHI nodes in the return blocks.
if (PHINode *TVPN = dyn_cast<PHINode>(TrueValue))
if (TVPN->getParent() == TrueSucc)
TrueValue = TVPN->getIncomingValueForBlock(BI->getParent());
if (PHINode *FVPN = dyn_cast<PHINode>(FalseValue))
if (FVPN->getParent() == FalseSucc)
FalseValue = FVPN->getIncomingValueForBlock(BI->getParent());
// Insert a new select instruction.
Value *NewRetVal = new SelectInst(BI->getCondition(), TrueValue,
FalseValue, "retval", BI);
new ReturnInst(NewRetVal, BI);
BI->getParent()->getInstList().erase(BI);
return true;
}
}
}
}
} else if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->begin())) {
// Check to see if the first instruction in this block is just an unwind.