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Set sub-register <undef> flags more accurately.

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When widening an existing <def,reads-undef> operand to a super-register,
it may be necessary to clear the <undef> flag because the wider register
is now read-modify-write through the instruction.

Conversely, it may be necessary to add an <undef> flag when the
coalescer turns a full-register def into a sub-register def, but the
larger register wasn't live before the instruction.

This happens in test/CodeGen/ARM/coalesce-subregs.ll, but the test
is too small for the <undef> flags to affect the generated code.

llvm-svn: 156951
This commit is contained in:
Jakob Stoklund Olesen 2012-05-16 21:22:35 +00:00
parent 8bc0051492
commit 92778c5641
1 changed files with 11 additions and 7 deletions
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18
lib/CodeGen/RegisterCoalescer.cpp
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@ -911,6 +911,7 @@ void RegisterCoalescer::updateRegDefsUses(unsigned SrcReg,
unsigned DstReg,
unsigned SubIdx) {
bool DstIsPhys = TargetRegisterInfo::isPhysicalRegister(DstReg);
LiveInterval &DstInt = LIS->getInterval(DstReg);
// Update LiveDebugVariables.
LDV->renameRegister(SrcReg, DstReg, SubIdx);
@ -934,17 +935,20 @@ void RegisterCoalescer::updateRegDefsUses(unsigned SrcReg,
bool Reads, Writes;
tie(Reads, Writes) = UseMI->readsWritesVirtualRegister(SrcReg, &Ops);
// 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)
Reads = DstInt.liveAt(LIS->getInstructionIndex(UseMI));
// Replace SrcReg with DstReg in all UseMI operands.
for (unsigned i = 0, e = Ops.size(); i != e; ++i) {
MachineOperand &MO = UseMI->getOperand(Ops[i]);
// Make sure we don't create read-modify-write defs accidentally. We
// assume here that a SrcReg def cannot be joined into a live DstReg. If
// RegisterCoalescer starts tracking partially live registers, we will
// need to check the actual LiveInterval to determine if DstReg is live
// here.
if (SubIdx && !Reads)
MO.setIsUndef();
// 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())
MO.setIsUndef(!Reads);
if (DstIsPhys)
MO.substPhysReg(DstReg, *TRI);