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Never clear <undef> flags on already joined copies.

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RegisterCoalescer set <undef> flags on all operands of copy instructions
that are scheduled to be removed. This is so they won't affect
shrinkToUses() by introducing false register reads.

Make sure those <undef> flags are never cleared, or shrinkToUses() could
cause live intervals to end at instructions about to be deleted.

This would be a lot simpler if RegisterCoalescer could just erase joined
copies immediately instead of keeping all the to-be-deleted instructions
around.

This fixes PR12862. Unfortunately, bugpoint can't create a sane test
case for this. Like many other coalescer problems, this failure depends
of a very fragile series of events.

<rdar://problem/11474428>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157001 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Jakob Stoklund Olesen 2012-05-17 18:32:42 +00:00
parent e3da8c6286
commit 94e58f3a7c
1 changed files with 6 additions and 4 deletions
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10
lib/CodeGen/RegisterCoalescer.cpp
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@ -918,6 +918,8 @@ void RegisterCoalescer::updateRegDefsUses(unsigned SrcReg,
for (MachineRegisterInfo::reg_iterator I = MRI->reg_begin(SrcReg);
MachineInstr *UseMI = I.skipInstruction();) {
bool AlreadyJoined = JoinedCopies.count(UseMI);
// A PhysReg copy that won't be coalesced can perhaps be rematerialized
// instead.
if (DstIsPhys) {
@ -925,7 +927,7 @@ void RegisterCoalescer::updateRegDefsUses(unsigned SrcReg,
UseMI->getOperand(1).getReg() == SrcReg &&
UseMI->getOperand(0).getReg() != SrcReg &&
UseMI->getOperand(0).getReg() != DstReg &&
!JoinedCopies.count(UseMI) &&
!AlreadyJoined &&
reMaterializeTrivialDef(LIS->getInterval(SrcReg), false,
UseMI->getOperand(0).getReg(), UseMI))
continue;
@ -937,7 +939,7 @@ void RegisterCoalescer::updateRegDefsUses(unsigned SrcReg,
// If SrcReg wasn't read, it may still be the case that DstReg is live-in
// because SrcReg is a sub-register.
if (!Reads && SubIdx)
if (!Reads && SubIdx && !AlreadyJoined)
Reads = DstInt.liveAt(LIS->getInstructionIndex(UseMI));
// Replace SrcReg with DstReg in all UseMI operands.
@ -947,7 +949,7 @@ void RegisterCoalescer::updateRegDefsUses(unsigned SrcReg,
// Adjust <undef> flags in case of sub-register joins. We don't want to
// turn a full def into a read-modify-write sub-register def and vice
// versa.
if (SubIdx && MO.isDef())
if (SubIdx && !AlreadyJoined && MO.isDef())
MO.setIsUndef(!Reads);
if (DstIsPhys)
@ -957,7 +959,7 @@ void RegisterCoalescer::updateRegDefsUses(unsigned SrcReg,
}
// This instruction is a copy that will be removed.
if (JoinedCopies.count(UseMI))
if (AlreadyJoined)
continue;
DEBUG({