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[InstCombine] Handle scalable splats of constants in getMinimumFPType #132960

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Merged
merged 4 commits into from
Mar 27, 2025
Merged

[InstCombine] Handle scalable splats of constants in getMinimumFPType #132960

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4b9c243
Precommit tests
lukel97 Mar 25, 2025
01be82c
[InstCombine] Handle scalable splats of constants in getMinimumFPType
lukel97 Mar 25, 2025
d26e672
Add fixed vector test
lukel97 Mar 27, 2025
8c7f8c6
Also handle fixed-length splats, as a sort of fast-path. Should be NFC?
lukel97 Mar 27, 2025
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11 changes: 6 additions & 5 deletions llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1685,11 +1685,12 @@ static Type *getMinimumFPType(Value *V, bool PreferBFloat) {
return T;

// We can only correctly find a minimum type for a scalable vector when it is
// a splat. For splats of constant values the fpext is wrapped up as a
// ConstantExpr.
if (auto *FPCExt = dyn_cast<ConstantExpr>(V))
if (FPCExt->getOpcode() == Instruction::FPExt)
return FPCExt->getOperand(0)->getType();
// a splat.
if (auto *FPCE = dyn_cast<ConstantExpr>(V))
if (isa<ScalableVectorType>(V->getType()))
if (auto *Splat = dyn_cast<ConstantFP>(FPCE->getSplatValue()))
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In InstCombinerImpl::visitCallInst there is a similar example of using splats:

    // Handle mul by one:
    if (Constant *CV1 = dyn_cast<Constant>(Arg1))
      if (ConstantInt *Splat =
              dyn_cast_or_null<ConstantInt>(CV1->getSplatValue()))

and I wondered if it was worth doing the same here. Perhaps I'm wrong, but I thought splats aren't restricted to scalable vectors I think and would apply to fixed-width too? For example,

  if (auto *CFP = dyn_cast<Constant>(V))
    if (auto *Splat = dyn_cast_or_null<ConstantFP>(CFP->getSplatValue()))

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Fixed-width vectors are handled by shrinkFPConstantVector below.

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Yeah, shrinkFPConstantVector will handle splatted and non-splatted fixed vectors too.

But I think it should be NFC if we also handle fixed splats here, I guess it would act as a sort of fast path to prevent having to check shrinkFPConstant on each element? I've added it to this PR but let me know if you want to leave it out to keep it closer to the original

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Thanks for this. I personally think it looks neater to handle all splats in one place.

if (Type *T = shrinkFPConstant(Splat, PreferBFloat))
return T;

// Try to shrink a vector of FP constants. This returns nullptr on scalable
// vectors
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13 changes: 13 additions & 0 deletions llvm/test/Transforms/InstCombine/scalable-const-fp-splat.ll
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Expand Up @@ -13,3 +13,16 @@ define <vscale x 2 x float> @shrink_splat_scalable_extend(<vscale x 2 x float> %
%5 = fptrunc <vscale x 2 x double> %4 to <vscale x 2 x float>
ret <vscale x 2 x float> %5
}

define <vscale x 2 x float> @shrink_splat_scalable_extend_rhs_constexpr(<vscale x 2 x float> %a) {
; CHECK-LABEL: define <vscale x 2 x float> @shrink_splat_scalable_extend_rhs_constexpr(
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Would be good to have a fixed-width variant of this too, if there isn't one already?

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Added to fpextend.ll in d26e672

; CHECK-SAME: <vscale x 2 x float> [[A:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = fptrunc <vscale x 2 x double> splat (double -1.000000e+00) to <vscale x 2 x float>
; CHECK-NEXT: [[TMP3:%.*]] = fadd <vscale x 2 x float> [[A]], [[TMP1]]
; CHECK-NEXT: ret <vscale x 2 x float> [[TMP3]]
;
%2 = fpext <vscale x 2 x float> %a to <vscale x 2 x double>
%4 = fadd <vscale x 2 x double> %2, splat (double -1.000000e+00)
%5 = fptrunc <vscale x 2 x double> %4 to <vscale x 2 x float>
ret <vscale x 2 x float> %5
}
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