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[LLVM][InstCombine] Preserve vector types when shrinking FP constants. #163598

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Oct 21, 2025
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[LLVM][InstCombine] Preserve vector types when shrinking FP constants. #163598

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4e936bb
[LLVM][InstCombine] Preserve vector types when shrinking FP constants.
paulwalker-arm Oct 15, 2025
b6765f8
Use result of shrinkFPConstant directly and shorten PPC_FP128 check.
paulwalker-arm Oct 16, 2025
db09782
Prevent APFloat copy when calling shrinkFPConstant, fix vector return…
paulwalker-arm Oct 17, 2025
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54 changes: 33 additions & 21 deletions llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1643,33 +1643,46 @@ Instruction *InstCombinerImpl::visitSExt(SExtInst &Sext) {

/// Return a Constant* for the specified floating-point constant if it fits
/// in the specified FP type without changing its value.
static bool fitsInFPType(ConstantFP *CFP, const fltSemantics &Sem) {
static bool fitsInFPType(APFloat F, const fltSemantics &Sem) {
bool losesInfo;
APFloat F = CFP->getValueAPF();
(void)F.convert(Sem, APFloat::rmNearestTiesToEven, &losesInfo);
return !losesInfo;
}

static Type *shrinkFPConstant(ConstantFP *CFP, bool PreferBFloat) {
if (CFP->getType() == Type::getPPC_FP128Ty(CFP->getContext()))
return nullptr; // No constant folding of this.
static Type *shrinkFPConstant(LLVMContext &Ctx, const APFloat &F,
bool PreferBFloat) {
// See if the value can be truncated to bfloat and then reextended.
if (PreferBFloat && fitsInFPType(CFP, APFloat::BFloat()))
return Type::getBFloatTy(CFP->getContext());
if (PreferBFloat && fitsInFPType(F, APFloat::BFloat()))
return Type::getBFloatTy(Ctx);
// See if the value can be truncated to half and then reextended.
if (!PreferBFloat && fitsInFPType(CFP, APFloat::IEEEhalf()))
return Type::getHalfTy(CFP->getContext());
if (!PreferBFloat && fitsInFPType(F, APFloat::IEEEhalf()))
return Type::getHalfTy(Ctx);
// See if the value can be truncated to float and then reextended.
if (fitsInFPType(CFP, APFloat::IEEEsingle()))
return Type::getFloatTy(CFP->getContext());
if (CFP->getType()->isDoubleTy())
return nullptr; // Won't shrink.
if (fitsInFPType(CFP, APFloat::IEEEdouble()))
return Type::getDoubleTy(CFP->getContext());
if (fitsInFPType(F, APFloat::IEEEsingle()))
return Type::getFloatTy(Ctx);
if (&F.getSemantics() == &APFloat::IEEEdouble())
return nullptr; // Won't shrink.
// See if the value can be truncated to double and then reextended.
if (fitsInFPType(F, APFloat::IEEEdouble()))
return Type::getDoubleTy(Ctx);
// Don't try to shrink to various long double types.
return nullptr;
}

static Type *shrinkFPConstant(ConstantFP *CFP, bool PreferBFloat) {
Type *Ty = CFP->getType();
if (Ty->getScalarType()->isPPC_FP128Ty())
return nullptr; // No constant folding of this.

Type *ShrinkTy =
shrinkFPConstant(CFP->getContext(), CFP->getValueAPF(), PreferBFloat);
if (ShrinkTy)
if (auto *VecTy = dyn_cast<VectorType>(Ty))
ShrinkTy = VectorType::get(ShrinkTy, VecTy);

return ShrinkTy;
}

// Determine if this is a vector of ConstantFPs and if so, return the minimal
// type we can safely truncate all elements to.
static Type *shrinkFPConstantVector(Value *V, bool PreferBFloat) {
Expand Down Expand Up @@ -1720,10 +1733,10 @@ static Type *getMinimumFPType(Value *V, bool PreferBFloat) {

// Try to shrink scalable and fixed splat vectors.
if (auto *FPC = dyn_cast<Constant>(V))
if (isa<VectorType>(V->getType()))
if (auto *VTy = dyn_cast<VectorType>(V->getType()))
if (auto *Splat = dyn_cast_or_null<ConstantFP>(FPC->getSplatValue()))
if (Type *T = shrinkFPConstant(Splat, PreferBFloat))
return T;
return VectorType::get(T, VTy);
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@nikic nikic Oct 15, 2025

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This looks right to me, but I don't really get how this worked previously. E.g. v4f32_fadd has a vector splat, why did that work?

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@paulwalker-arm paulwalker-arm Oct 16, 2025

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Only the Instruction::FRem handling uses getMinimumFPType()'s return type and even then the bug only occurs when LHS is the vector splat of which there were no tests.


// Try to shrink a vector of FP constants. This returns nullptr on scalable
// vectors
Expand Down Expand Up @@ -1796,10 +1809,9 @@ Instruction *InstCombinerImpl::visitFPTrunc(FPTruncInst &FPT) {
Type *Ty = FPT.getType();
auto *BO = dyn_cast<BinaryOperator>(FPT.getOperand(0));
if (BO && BO->hasOneUse()) {
Type *LHSMinType =
getMinimumFPType(BO->getOperand(0), /*PreferBFloat=*/Ty->isBFloatTy());
Type *RHSMinType =
getMinimumFPType(BO->getOperand(1), /*PreferBFloat=*/Ty->isBFloatTy());
bool PreferBFloat = Ty->getScalarType()->isBFloatTy();
Type *LHSMinType = getMinimumFPType(BO->getOperand(0), PreferBFloat);
Type *RHSMinType = getMinimumFPType(BO->getOperand(1), PreferBFloat);
unsigned OpWidth = BO->getType()->getFPMantissaWidth();
unsigned LHSWidth = LHSMinType->getFPMantissaWidth();
unsigned RHSWidth = RHSMinType->getFPMantissaWidth();
Expand Down
36 changes: 36 additions & 0 deletions llvm/test/Transforms/InstCombine/fpextend.ll
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt < %s -passes=instcombine -S | FileCheck %s
; RUN: opt < %s -passes=instcombine -use-constant-fp-for-fixed-length-splat -S | FileCheck %s

define float @test(float %x) nounwind {
; CHECK-LABEL: @test(
Expand Down Expand Up @@ -449,6 +450,28 @@ define bfloat @bf16_frem(bfloat %x) {
ret bfloat %t3
}

define <4 x bfloat> @v4bf16_frem_x_const(<4 x bfloat> %x) {
; CHECK-LABEL: @v4bf16_frem_x_const(
; CHECK-NEXT: [[TMP1:%.*]] = frem <4 x bfloat> [[X:%.*]], splat (bfloat 0xR40C9)
; CHECK-NEXT: ret <4 x bfloat> [[TMP1]]
;
%t1 = fpext <4 x bfloat> %x to <4 x float>
%t2 = frem <4 x float> %t1, splat(float 6.281250e+00)
%t3 = fptrunc <4 x float> %t2 to <4 x bfloat>
ret <4 x bfloat> %t3
}

define <4 x bfloat> @v4bf16_frem_const_x(<4 x bfloat> %x) {
; CHECK-LABEL: @v4bf16_frem_const_x(
; CHECK-NEXT: [[TMP1:%.*]] = frem <4 x bfloat> splat (bfloat 0xR40C9), [[X:%.*]]
; CHECK-NEXT: ret <4 x bfloat> [[TMP1]]
;
%t1 = fpext <4 x bfloat> %x to <4 x float>
%t2 = frem <4 x float> splat(float 6.281250e+00), %t1
%t3 = fptrunc <4 x float> %t2 to <4 x bfloat>
ret <4 x bfloat> %t3
}

define <4 x float> @v4f32_fadd(<4 x float> %a) {
; CHECK-LABEL: @v4f32_fadd(
; CHECK-NEXT: [[TMP1:%.*]] = fadd <4 x float> [[A:%.*]], splat (float -1.000000e+00)
Expand All @@ -459,3 +482,16 @@ define <4 x float> @v4f32_fadd(<4 x float> %a) {
%5 = fptrunc <4 x double> %4 to <4 x float>
ret <4 x float> %5
}

define <4 x float> @v4f32_fadd_const_not_shrinkable(<4 x float> %a) {
; CHECK-LABEL: @v4f32_fadd_const_not_shrinkable(
; CHECK-NEXT: [[TMP1:%.*]] = fpext <4 x float> [[A:%.*]] to <4 x double>
; CHECK-NEXT: [[TMP2:%.*]] = fadd <4 x double> [[TMP1]], splat (double -1.000000e+100)
; CHECK-NEXT: [[TMP3:%.*]] = fptrunc <4 x double> [[TMP2]] to <4 x float>
; CHECK-NEXT: ret <4 x float> [[TMP3]]
;
%2 = fpext <4 x float> %a to <4 x double>
%4 = fadd <4 x double> %2, splat (double -1.000000e+100)
%5 = fptrunc <4 x double> %4 to <4 x float>
ret <4 x float> %5
}
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