/// If we have a SelectInst that will likely profit from branch prediction,
/// turn it into a branch.
bool CodeGenPrepare::OptimizeSelectInst(SelectInst *SI) {
bool VectorCond = !SI->getCondition()->getType()->isIntegerTy(1);
// Can we convert the 'select' to CF ?
if (DisableSelectToBranch || OptSize || !TLI || VectorCond)
return false;
TargetLowering::SelectSupportKind SelectKind;
if (VectorCond)
SelectKind = TargetLowering::VectorMaskSelect;
else if (SI->getType()->isVectorTy())
SelectKind = TargetLowering::ScalarCondVectorVal;
else
SelectKind = TargetLowering::ScalarValSelect;
// Do we have efficient codegen support for this kind of 'selects' ?
if (TLI->isSelectSupported(SelectKind)) {
// We have efficient codegen support for the select instruction.
// Check if it is profitable to keep this 'select'.
if (!TLI->isPredictableSelectExpensive() ||
!isFormingBranchFromSelectProfitable(SI))
return false;
}
ModifiedDT = true;
// First, we split the block containing the select into 2 blocks.
BasicBlock *StartBlock = SI->getParent();
BasicBlock::iterator SplitPt = ++(BasicBlock::iterator(SI));
BasicBlock *NextBlock = StartBlock->splitBasicBlock(SplitPt, "select.end");
// Create a new block serving as the landing pad for the branch.
BasicBlock *SmallBlock = BasicBlock::Create(SI->getContext(), "select.mid",
NextBlock->getParent(), NextBlock);
// Move the unconditional branch from the block with the select in it into our
// landing pad block.
StartBlock->getTerminator()->eraseFromParent();
BranchInst::Create(NextBlock, SmallBlock);
// Insert the real conditional branch based on the original condition.
BranchInst::Create(NextBlock, SmallBlock, SI->getCondition(), SI);
// The select itself is replaced with a PHI Node.
PHINode *PN = PHINode::Create(SI->getType(), 2, "", NextBlock->begin());
PN->takeName(SI);
PN->addIncoming(SI->getTrueValue(), StartBlock);
PN->addIncoming(SI->getFalseValue(), SmallBlock);
SI->replaceAllUsesWith(PN);
SI->eraseFromParent();
// Instruct OptimizeBlock to skip to the next block.
CurInstIterator = StartBlock->end();
++NumSelectsExpanded;
return true;
}
bool CodeGenPrepare::OptimizeSelectInst(SelectInst *SI) {
// If we have a SelectInst that will likely profit from branch prediction,
// turn it into a branch.
if (DisableSelectToBranch || OptSize || !TLI ||
!TLI->isPredictableSelectExpensive())
return false;
if (!SI->getCondition()->getType()->isIntegerTy(1) ||
!isFormingBranchFromSelectProfitable(SI))
return false;
ModifiedDT = true;
// First, we split the block containing the select into 2 blocks.
BasicBlock *StartBlock = SI->getParent();
BasicBlock::iterator SplitPt = ++(BasicBlock::iterator(SI));
BasicBlock *NextBlock = StartBlock->splitBasicBlock(SplitPt, "select.end");
// Create a new block serving as the landing pad for the branch.
BasicBlock *SmallBlock = BasicBlock::Create(SI->getContext(), "select.mid",
NextBlock->getParent(), NextBlock);
// Move the unconditional branch from the block with the select in it into our
// landing pad block.
StartBlock->getTerminator()->eraseFromParent();
BranchInst::Create(NextBlock, SmallBlock);
// Insert the real conditional branch based on the original condition.
BranchInst::Create(NextBlock, SmallBlock, SI->getCondition(), SI);
// The select itself is replaced with a PHI Node.
PHINode *PN = PHINode::Create(SI->getType(), 2, "", NextBlock->begin());
PN->takeName(SI);
PN->addIncoming(SI->getTrueValue(), StartBlock);
PN->addIncoming(SI->getFalseValue(), SmallBlock);
SI->replaceAllUsesWith(PN);
SI->eraseFromParent();
// Instruct OptimizeBlock to skip to the next block.
CurInstIterator = StartBlock->end();
++NumSelectsExpanded;
return true;
}
/// RewriteLoopExitValues - Check to see if this loop has a computable
/// loop-invariant execution count. If so, this means that we can compute the
/// final value of any expressions that are recurrent in the loop, and
/// substitute the exit values from the loop into any instructions outside of
/// the loop that use the final values of the current expressions.
///
/// This is mostly redundant with the regular IndVarSimplify activities that
/// happen later, except that it's more powerful in some cases, because it's
/// able to brute-force evaluate arbitrary instructions as long as they have
/// constant operands at the beginning of the loop.
void IndVarSimplify::RewriteLoopExitValues(Loop *L,
SCEVExpander &Rewriter) {
// Verify the input to the pass in already in LCSSA form.
assert(L->isLCSSAForm(*DT));
SmallVector<BasicBlock*, 8> ExitBlocks;
L->getUniqueExitBlocks(ExitBlocks);
// Find all values that are computed inside the loop, but used outside of it.
// Because of LCSSA, these values will only occur in LCSSA PHI Nodes. Scan
// the exit blocks of the loop to find them.
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
BasicBlock *ExitBB = ExitBlocks[i];
// If there are no PHI nodes in this exit block, then no values defined
// inside the loop are used on this path, skip it.
PHINode *PN = dyn_cast<PHINode>(ExitBB->begin());
if (!PN) continue;
unsigned NumPreds = PN->getNumIncomingValues();
// Iterate over all of the PHI nodes.
BasicBlock::iterator BBI = ExitBB->begin();
while ((PN = dyn_cast<PHINode>(BBI++))) {
if (PN->use_empty())
continue; // dead use, don't replace it
// SCEV only supports integer expressions for now.
if (!PN->getType()->isIntegerTy() && !PN->getType()->isPointerTy())
continue;
// It's necessary to tell ScalarEvolution about this explicitly so that
// it can walk the def-use list and forget all SCEVs, as it may not be
// watching the PHI itself. Once the new exit value is in place, there
// may not be a def-use connection between the loop and every instruction
// which got a SCEVAddRecExpr for that loop.
SE->forgetValue(PN);
// Iterate over all of the values in all the PHI nodes.
for (unsigned i = 0; i != NumPreds; ++i) {
// If the value being merged in is not integer or is not defined
// in the loop, skip it.
Value *InVal = PN->getIncomingValue(i);
if (!isa<Instruction>(InVal))
continue;
// If this pred is for a subloop, not L itself, skip it.
if (LI->getLoopFor(PN->getIncomingBlock(i)) != L)
continue; // The Block is in a subloop, skip it.
// Check that InVal is defined in the loop.
Instruction *Inst = cast<Instruction>(InVal);
if (!L->contains(Inst))
continue;
// Okay, this instruction has a user outside of the current loop
// and varies predictably *inside* the loop. Evaluate the value it
// contains when the loop exits, if possible.
const SCEV *ExitValue = SE->getSCEVAtScope(Inst, L->getParentLoop());
if (!ExitValue->isLoopInvariant(L))
continue;
Changed = true;
++NumReplaced;
Value *ExitVal = Rewriter.expandCodeFor(ExitValue, PN->getType(), Inst);
DEBUG(dbgs() << "INDVARS: RLEV: AfterLoopVal = " << *ExitVal << '\n'
<< " LoopVal = " << *Inst << "\n");
PN->setIncomingValue(i, ExitVal);
// If this instruction is dead now, delete it.
RecursivelyDeleteTriviallyDeadInstructions(Inst);
if (NumPreds == 1) {
// Completely replace a single-pred PHI. This is safe, because the
// NewVal won't be variant in the loop, so we don't need an LCSSA phi
// node anymore.
PN->replaceAllUsesWith(ExitVal);
RecursivelyDeleteTriviallyDeadInstructions(PN);
}
}
if (NumPreds != 1) {
// Clone the PHI and delete the original one. This lets IVUsers and
// any other maps purge the original user from their records.
PHINode *NewPN = cast<PHINode>(PN->clone());
NewPN->takeName(PN);
NewPN->insertBefore(PN);
PN->replaceAllUsesWith(NewPN);
PN->eraseFromParent();
}
}
}
}
