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[CalcSpillWeights] don't mark live intervals with spillable inlineasm ops as having infinite spill weight #70747

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Nov 3, 2023
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[CalcSpillWeights] don't mark live intervals with spillable inlineasm ops as having infinite spill weight #70747

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13 changes: 12 additions & 1 deletion llvm/lib/CodeGen/CalcSpillWeights.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -146,6 +146,17 @@ void VirtRegAuxInfo::calculateSpillWeightAndHint(LiveInterval &LI) {
LI.setWeight(Weight);
}

static bool canMemFoldInlineAsm(LiveInterval &LI,
const MachineRegisterInfo &MRI) {
for (const MachineOperand &MO : MRI.reg_operands(LI.reg())) {
const MachineInstr *MI = MO.getParent();
if (MI->isInlineAsm() && MI->mayFoldInlineAsmRegOp(MI->getOperandNo(&MO)))
return true;
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What happens if a register is both foldable and not foldable?
I.e., something like asm "rm, r", %0, %0.

Given the second constraints force %0 to stay in register, we can't fold right.

In other words, depending on the semantic that we want, we may want to check all the operands.
Now, I still approved this PR because although the resulting LI won't actually be spillable in this case, regalloc will still handle it correctly.

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@nickdesaulniers nickdesaulniers Oct 31, 2023 •
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What happens if a register is both foldable and not foldable?
I.e., something like asm "rm, r", %0, %0.

I'm not sure that can be expressed from inline asm.

Do you mean something like:

asm ("# %0 %1"::"rm"(x), "r"(x));

?

I that case, %0 would have a distinct storage location from %1. x would be copied into the inline asm twice. %0's constraint is rm (register OR memory), %1's constraint is r (register). I don't think AND NOT is expressible via the constraint language.

}

return false;
}

float VirtRegAuxInfo::weightCalcHelper(LiveInterval &LI, SlotIndex *Start,
SlotIndex *End) {
MachineRegisterInfo &MRI = MF.getRegInfo();
Expand Down Expand Up @@ -315,7 +326,7 @@ float VirtRegAuxInfo::weightCalcHelper(LiveInterval &LI, SlotIndex *Start,
// into instruction itself makes perfect sense.
if (ShouldUpdateLI && LI.isZeroLength(LIS.getSlotIndexes()) &&
!LI.isLiveAtIndexes(LIS.getRegMaskSlots()) &&
!isLiveAtStatepointVarArg(LI)) {
!isLiveAtStatepointVarArg(LI) && !canMemFoldInlineAsm(LI, MRI)) {
LI.markNotSpillable();
return -1.0;
}
Expand Down