/// LowerAtomicPHINode - Lower the PHI node at the top of the specified block,
-/// under the assuption that it needs to be lowered in a way that supports
+/// under the assumption that it needs to be lowered in a way that supports
/// atomic execution of PHIs. This lowering method is always correct all of the
/// time.
///
// add a kill marker in this block saying that it kills the incoming value!
if (!ValueIsUsed && !LV->isLiveOut(SrcReg, opBlock)) {
// In our final twist, we have to decide which instruction kills the
- // register. In most cases this is the copy, however, the first
- // terminator instruction at the end of the block may also use the value.
- // In this case, we should mark *it* as being the killing block, not the
- // copy.
- MachineBasicBlock::iterator KillInst;
- MachineBasicBlock::iterator Term = opBlock.getFirstTerminator();
- if (Term != opBlock.end() && Term->readsRegister(SrcReg)) {
- KillInst = Term;
-
- // Check that no other terminators use values.
-#ifndef NDEBUG
- for (MachineBasicBlock::iterator TI = llvm::next(Term);
- TI != opBlock.end(); ++TI) {
- if (TI->isDebugValue())
- continue;
- assert(!TI->readsRegister(SrcReg) &&
- "Terminator instructions cannot use virtual registers unless"
- "they are the first terminator in a block!");
- }
-#endif
- } else if (reusedIncoming || !IncomingReg) {
- // We may have to rewind a bit if we didn't insert a copy this time.
- KillInst = Term;
- while (KillInst != opBlock.begin()) {
- --KillInst;
- if (KillInst->isDebugValue())
- continue;
- if (KillInst->readsRegister(SrcReg))
- break;
+ // register. In most cases this is the copy, however, terminator
+ // instructions at the end of the block may also use the value. In this
+ // case, we should mark the last such terminator as being the killing
+ // block, not the copy.
+ MachineBasicBlock::iterator KillInst = opBlock.end();
+ MachineBasicBlock::iterator FirstTerm = opBlock.getFirstTerminator();
+ for (MachineBasicBlock::iterator Term = FirstTerm;
+ Term != opBlock.end(); ++Term) {
+ if (Term->readsRegister(SrcReg))
+ KillInst = Term;
+ }
+
+ if (KillInst == opBlock.end()) {
+ // No terminator uses the register.
+
+ if (reusedIncoming || !IncomingReg) {
+ // We may have to rewind a bit if we didn't insert a copy this time.
+ KillInst = FirstTerm;
+ while (KillInst != opBlock.begin()) {
+ --KillInst;
+ if (KillInst->isDebugValue())
+ continue;
+ if (KillInst->readsRegister(SrcReg))
+ break;
+ }
+ } else {
+ // We just inserted this copy.
+ KillInst = prior(InsertPos);
}
- } else {
- // We just inserted this copy.
- KillInst = prior(InsertPos);
}
assert(KillInst->readsRegister(SrcReg) && "Cannot find kill instruction");
if (MBB.empty() || !MBB.front().isPHI() || MBB.isLandingPad())
return false; // Quick exit for basic blocks without PHIs.
+ const MachineLoop *CurLoop = MLI ? MLI->getLoopFor(&MBB) : 0;
+ bool IsLoopHeader = CurLoop && &MBB == CurLoop->getHeader();
+
bool Changed = false;
for (MachineBasicBlock::iterator BBI = MBB.begin(), BBE = MBB.end();
BBI != BBE && BBI->isPHI(); ++BBI) {
for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) {
unsigned Reg = BBI->getOperand(i).getReg();
MachineBasicBlock *PreMBB = BBI->getOperand(i+1).getMBB();
- // We break edges when registers are live out from the predecessor block
- // (not considering PHI nodes). If the register is live in to this block
- // anyway, we would gain nothing from splitting.
+ // Is there a critical edge from PreMBB to MBB?
+ if (PreMBB->succ_size() == 1)
+ continue;
+
// Avoid splitting backedges of loops. It would introduce small
// out-of-line blocks into the loop which is very bad for code placement.
- if (PreMBB != &MBB &&
- !LV.isLiveIn(Reg, MBB) && LV.isLiveOut(Reg, *PreMBB)) {
- if (!MLI ||
- !(MLI->getLoopFor(PreMBB) == MLI->getLoopFor(&MBB) &&
- MLI->isLoopHeader(&MBB))) {
- if (PreMBB->SplitCriticalEdge(&MBB, this)) {
- Changed = true;
- ++NumCriticalEdgesSplit;
- }
- }
+ if (PreMBB == &MBB)
+ continue;
+ const MachineLoop *PreLoop = MLI ? MLI->getLoopFor(PreMBB) : 0;
+ if (IsLoopHeader && PreLoop == CurLoop)
+ continue;
+
+ // LV doesn't consider a phi use live-out, so isLiveOut only returns true
+ // when the source register is live-out for some other reason than a phi
+ // use. That means the copy we will insert in PreMBB won't be a kill, and
+ // there is a risk it may not be coalesced away.
+ //
+ // If the copy would be a kill, there is no need to split the edge.
+ if (!LV.isLiveOut(Reg, *PreMBB))
+ continue;
+
+ DEBUG(dbgs() << PrintReg(Reg) << " live-out before critical edge BB#"
+ << PreMBB->getNumber() << " -> BB#" << MBB.getNumber()
+ << ": " << *BBI);
+
+ // If Reg is not live-in to MBB, it means it must be live-in to some
+ // other PreMBB successor, and we can avoid the interference by splitting
+ // the edge.
+ //
+ // If Reg *is* live-in to MBB, the interference is inevitable and a copy
+ // is likely to be left after coalescing. If we are looking at a loop
+ // exiting edge, split it so we won't insert code in the loop, otherwise
+ // don't bother.
+ bool ShouldSplit = !LV.isLiveIn(Reg, MBB);
+
+ // Check for a loop exiting edge.
+ if (!ShouldSplit && CurLoop != PreLoop) {
+ DEBUG({
+ dbgs() << "Split wouldn't help, maybe avoid loop copies?\n";
+ if (PreLoop) dbgs() << "PreLoop: " << *PreLoop;
+ if (CurLoop) dbgs() << "CurLoop: " << *CurLoop;
+ });
+ // This edge could be entering a loop, exiting a loop, or it could be
+ // both: Jumping directly form one loop to the header of a sibling
+ // loop.
+ // Split unless this edge is entering CurLoop from an outer loop.
+ ShouldSplit = PreLoop && !PreLoop->contains(CurLoop);
+ }
+ if (!ShouldSplit)
+ continue;
+ if (!PreMBB->SplitCriticalEdge(&MBB, this)) {
+ DEBUG(dbgs() << "Failed to split ciritcal edge.\n");
+ continue;
}
+ Changed = true;
+ ++NumCriticalEdgesSplit;
}
}
return Changed;
projects / oota-llvm.git / blobdiff