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[AMDGPU] Improve MIR pattern for FMinFMaxLegacy combine. NFC. #90968

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May 5, 2024
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[AMDGPU] Improve MIR pattern for FMinFMaxLegacy combine. NFC. #90968

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5 changes: 3 additions & 2 deletions llvm/lib/Target/AMDGPU/AMDGPUCombine.td
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Original file line number Diff line number Diff line change
Expand Up @@ -15,8 +15,9 @@ def fmin_fmax_legacy_matchdata : GIDefMatchData<"FMinFMaxLegacyInfo">;
let Predicates = [HasFminFmaxLegacy] in
def fcmp_select_to_fmin_fmax_legacy : GICombineRule<
(defs root:$select, fmin_fmax_legacy_matchdata:$matchinfo),
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Am I supposed to list $fcmp in the defs since I am passing it into my match function? Or doesn't that count as an "external" use of $fcmp?

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No. I prefer to only list the $root. defs is probably definitions and not def in SSA.

(match (wip_match_opcode G_SELECT):$select,
[{ return matchFMinFMaxLegacy(*${select}, ${matchinfo}); }]),
(match (G_FCMP $cond, $pred, $lhs, $rhs):$fcmp,
(G_SELECT f32:$dst, $cond, $true, $false):$select,
[{ return matchFMinFMaxLegacy(*${select}, *${fcmp}, ${matchinfo}); }]),
(apply [{ applySelectFCmpToFMinToFMaxLegacy(*${select}, ${matchinfo}); }])>;


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18 changes: 8 additions & 10 deletions llvm/lib/Target/AMDGPU/AMDGPUPostLegalizerCombiner.cpp
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Expand Up @@ -70,7 +70,8 @@ class AMDGPUPostLegalizerCombinerImpl : public Combiner {
};

// TODO: Make sure fmin_legacy/fmax_legacy don't canonicalize
bool matchFMinFMaxLegacy(MachineInstr &MI, FMinFMaxLegacyInfo &Info) const;
bool matchFMinFMaxLegacy(MachineInstr &MI, MachineInstr &FCmp,
FMinFMaxLegacyInfo &Info) const;
void applySelectFCmpToFMinToFMaxLegacy(MachineInstr &MI,
const FMinFMaxLegacyInfo &Info) const;

Expand Down Expand Up @@ -159,17 +160,14 @@ bool AMDGPUPostLegalizerCombinerImpl::tryCombineAll(MachineInstr &MI) const {
}

bool AMDGPUPostLegalizerCombinerImpl::matchFMinFMaxLegacy(
MachineInstr &MI, FMinFMaxLegacyInfo &Info) const {
// FIXME: Type predicate on pattern
if (MRI.getType(MI.getOperand(0).getReg()) != LLT::scalar(32))
return false;

Register Cond = MI.getOperand(1).getReg();
if (!MRI.hasOneNonDBGUse(Cond) ||
!mi_match(Cond, MRI,
m_GFCmp(m_Pred(Info.Pred), m_Reg(Info.LHS), m_Reg(Info.RHS))))
MachineInstr &MI, MachineInstr &FCmp, FMinFMaxLegacyInfo &Info) const {
if (!MRI.hasOneNonDBGUse(FCmp.getOperand(0).getReg()))
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Is there a way to do this check in the MIR pattern?

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No. Other uses could have sneaked in.

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Sneaked in after the autogenerated matcher runs but before this match function is called? That doesn't sound right.

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You could:

GFCmp *Cmp = cast<GFCmp>(&FCmp);

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The G_FCMP could be used by other MIs.

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The G_FCMP could be used by other MIs.

I know. I would like to write something in the pattern to check for this, instead of writing C++ to check for this.

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Open an issue?

return false;

Info.Pred =
static_cast<CmpInst::Predicate>(FCmp.getOperand(1).getPredicate());
Info.LHS = FCmp.getOperand(2).getReg();
Info.RHS = FCmp.getOperand(3).getReg();
Register True = MI.getOperand(2).getReg();
Register False = MI.getOperand(3).getReg();

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