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[LoopUtils] Consider fast-math flags derived from function attribute when getting recurrence identity #162630

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[LoopUtils] Consider fast-math flags derived from function attribute when getting recurrence identity #162630

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397178e
[Function] Add getFMFFromFnAttribute() to derive FastMathFlags from f…
Mel-Chen Oct 9, 2025
9f34121
pre-commit test
Mel-Chen Oct 9, 2025
b437fe9
[LoopUtils] Consider fast-math flags derived from function attribute …
Mel-Chen Oct 9, 2025
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3 changes: 3 additions & 0 deletions llvm/include/llvm/IR/Function.h
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Expand Up @@ -470,6 +470,9 @@ class LLVM_ABI Function : public GlobalObject, public ilist_node<Function> {
return AttributeSets.getFnStackAlignment();
}

/// Derive fast-math flags from the function attributes.
FastMathFlags getFMFFromFnAttribute() const;

/// Returns true if the function has ssp, sspstrong, or sspreq fn attrs.
bool hasStackProtectorFnAttr() const;

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6 changes: 1 addition & 5 deletions llvm/lib/Analysis/IVDescriptors.cpp
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Expand Up @@ -998,11 +998,7 @@ bool RecurrenceDescriptor::isReductionPHI(PHINode *Phi, Loop *TheLoop,
ScalarEvolution *SE) {
BasicBlock *Header = TheLoop->getHeader();
Function &F = *Header->getParent();
FastMathFlags FMF;
FMF.setNoNaNs(
F.getFnAttribute("no-nans-fp-math").getValueAsBool());
FMF.setNoSignedZeros(
F.getFnAttribute("no-signed-zeros-fp-math").getValueAsBool());
Comment on lines -1001 to -1005
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This should be deleted without replacement

FastMathFlags FMF = F.getFMFFromFnAttribute();

if (AddReductionVar(Phi, RecurKind::Add, TheLoop, FMF, RedDes, DB, AC, DT,
SE)) {
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8 changes: 8 additions & 0 deletions llvm/lib/IR/Function.cpp
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Expand Up @@ -784,6 +784,14 @@ uint64_t Function::getFnAttributeAsParsedInteger(StringRef Name,
return Result;
}

FastMathFlags Function::getFMFFromFnAttribute() const {
FastMathFlags FuncFMF;
FuncFMF.setNoNaNs(getFnAttribute("no-nans-fp-math").getValueAsBool());
FuncFMF.setNoSignedZeros(
getFnAttribute("no-signed-zeros-fp-math").getValueAsBool());
return FuncFMF;
}

/// gets the specified attribute from the list of attributes.
Attribute Function::getParamAttribute(unsigned ArgNo,
Attribute::AttrKind Kind) const {
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8 changes: 6 additions & 2 deletions llvm/lib/Transforms/Utils/LoopUtils.cpp
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Expand Up @@ -1398,8 +1398,10 @@ Value *llvm::createSimpleReduction(IRBuilderBase &Builder, Value *Src,
RecurKind RdxKind) {
auto *SrcVecEltTy = cast<VectorType>(Src->getType())->getElementType();
auto getIdentity = [&]() {
Function *Fn = Builder.GetInsertBlock()->getParent();
return getRecurrenceIdentity(RdxKind, SrcVecEltTy,
Builder.getFastMathFlags());
Builder.getFastMathFlags() |
Fn->getFMFFromFnAttribute());
};
switch (RdxKind) {
case RecurKind::AddChainWithSubs:
Expand Down Expand Up @@ -1442,7 +1444,9 @@ Value *llvm::createSimpleReduction(IRBuilderBase &Builder, Value *Src,
assert(VPReductionIntrinsic::isVPReduction(VPID) &&
"No VPIntrinsic for this reduction");
auto *EltTy = cast<VectorType>(Src->getType())->getElementType();
Value *Iden = getRecurrenceIdentity(Kind, EltTy, Builder.getFastMathFlags());
Function *Fn = Builder.GetInsertBlock()->getParent();
Value *Iden = getRecurrenceIdentity(
Kind, EltTy, Builder.getFastMathFlags() | Fn->getFMFFromFnAttribute());
Value *Ops[] = {Iden, Src, Mask, EVL};
return Builder.CreateIntrinsic(EltTy, VPID, Ops);
}
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59 changes: 59 additions & 0 deletions llvm/test/Transforms/LoopVectorize/reduction.ll
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Expand Up @@ -1335,3 +1335,62 @@ exit:
%lcssa.exit = phi i64 [ %phi.sum.next, %loop.latch ]
ret i64 %lcssa.exit
}

define float @reduction_sum_float(i64 %n, ptr %a) #0 {
; CHECK-LABEL: define float @reduction_sum_float(
; CHECK-SAME: i64 [[N:%.*]], ptr [[A:%.*]]) #[[ATTR0:[0-9]+]] {
; CHECK-NEXT: entry:
; CHECK-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 [[N]], 4
; CHECK-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; CHECK: vector.ph:
; CHECK-NEXT: [[N_VEC:%.*]] = and i64 [[N]], -4
; CHECK-NEXT: br label [[VECTOR_BODY:%.*]]
; CHECK: vector.body:
; CHECK-NEXT: [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
; CHECK-NEXT: [[VEC_PHI:%.*]] = phi <4 x float> [ <float 0.000000e+00, float -0.000000e+00, float -0.000000e+00, float -0.000000e+00>, [[VECTOR_PH]] ], [ [[TMP1:%.*]], [[VECTOR_BODY]] ]
; CHECK-NEXT: [[TMP0:%.*]] = getelementptr inbounds float, ptr [[A]], i64 [[INDEX]]
; CHECK-NEXT: [[WIDE_LOAD:%.*]] = load <4 x float>, ptr [[TMP0]], align 4
; CHECK-NEXT: [[TMP1]] = fadd <4 x float> [[VEC_PHI]], [[WIDE_LOAD]]
; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 4
; CHECK-NEXT: [[TMP2:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
; CHECK-NEXT: br i1 [[TMP2]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP27:![0-9]+]]
; CHECK: middle.block:
; CHECK-NEXT: [[TMP3:%.*]] = call float @llvm.vector.reduce.fadd.v4f32(float 0.000000e+00, <4 x float> [[TMP1]])
; CHECK-NEXT: [[CMP_N:%.*]] = icmp eq i64 [[N]], [[N_VEC]]
; CHECK-NEXT: br i1 [[CMP_N]], label [[EXIT:%.*]], label [[SCALAR_PH]]
; CHECK: scalar.ph:
; CHECK-NEXT: [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
; CHECK-NEXT: [[BC_MERGE_RDX:%.*]] = phi float [ [[TMP3]], [[MIDDLE_BLOCK]] ], [ 0.000000e+00, [[ENTRY]] ]
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV:%.*]] = phi i64 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[RDX:%.*]] = phi float [ [[BC_MERGE_RDX]], [[SCALAR_PH]] ], [ [[SUM:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[ARRAYIDX:%.*]] = getelementptr inbounds float, ptr [[A]], i64 [[IV]]
; CHECK-NEXT: [[TMP4:%.*]] = load float, ptr [[ARRAYIDX]], align 4
; CHECK-NEXT: [[SUM]] = fadd float [[RDX]], [[TMP4]]
; CHECK-NEXT: [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
; CHECK-NEXT: [[EXITCOND:%.*]] = icmp eq i64 [[IV_NEXT]], [[N]]
; CHECK-NEXT: br i1 [[EXITCOND]], label [[EXIT]], label [[LOOP]], !llvm.loop [[LOOP28:![0-9]+]]
; CHECK: exit:
; CHECK-NEXT: [[SUM_LCSSA:%.*]] = phi float [ [[SUM]], [[LOOP]] ], [ [[TMP3]], [[MIDDLE_BLOCK]] ]
; CHECK-NEXT: ret float [[SUM_LCSSA]]
;
entry:
br label %loop

loop:
%iv = phi i64 [ 0, %entry ], [ %iv.next, %loop ]
%rdx = phi float [ 0.0, %entry ], [ %sum, %loop ]
%arrayidx = getelementptr inbounds float, ptr %a, i64 %iv
%0 = load float, ptr %arrayidx, align 4
%sum = fadd float %rdx, %0
%iv.next = add nuw nsw i64 %iv, 1
%exitcond = icmp eq i64 %iv.next, %n
br i1 %exitcond, label %exit, label %loop

exit:
%sum.lcssa = phi float [ %sum, %loop ]
ret float %sum.lcssa
}

attributes #0 = { "no-signed-zeros-fp-math"="true" }