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[LoopVectorizer][AArch64] Move getMinTripCountTailFoldingThreshold later. #132170

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merged 5 commits into from
Mar 26, 2025
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[LoopVectorizer][AArch64] Move getMinTripCountTailFoldingThreshold later. #132170

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6fb8134
[AArch64][LoopVectorizer] Add a test for VF=4 low-trip-count vectoriz…
davemgreen Mar 19, 2025
d189001
[LoopVectorizer][AArch64] Move getMinTripCountTailFoldingThreshold la…
davemgreen Mar 20, 2025
9ad5ec3
Address comments
davemgreen Mar 24, 2025
b06ca2e
Remove simplify-cfg
davemgreen Mar 26, 2025
76bf30d
ScalarEpilogueNeeded -> NoScalarEpilogueNeeded
davemgreen Mar 26, 2025
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66 changes: 40 additions & 26 deletions llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4025,11 +4025,8 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
MaxPowerOf2RuntimeVF = std::nullopt; // Stick with tail-folding for now.
}

if (MaxPowerOf2RuntimeVF && *MaxPowerOf2RuntimeVF > 0) {
assert((UserVF.isNonZero() || isPowerOf2_32(*MaxPowerOf2RuntimeVF)) &&
"MaxFixedVF must be a power of 2");
unsigned MaxVFtimesIC =
UserIC ? *MaxPowerOf2RuntimeVF * UserIC : *MaxPowerOf2RuntimeVF;
auto ScalarEpilogueNeeded = [this, &UserIC](unsigned MaxVF) {
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@fhahn fhahn Mar 26, 2025

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The function returns true if no scalar epilogue is needed, so the name should be adjusted?

Suggested change
auto ScalarEpilogueNeeded = [this, &UserIC](unsigned MaxVF) {
auto NoScalarEpilogueNeeded = [this, &UserIC](unsigned MaxVF) {

unsigned MaxVFtimesIC = UserIC ? MaxVF * UserIC : MaxVF;
ScalarEvolution *SE = PSE.getSE();
// Currently only loops with countable exits are vectorized, but calling
// getSymbolicMaxBackedgeTakenCount allows enablement work for loops with
Expand All @@ -4043,13 +4040,41 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
const SCEV *Rem = SE->getURemExpr(
SE->applyLoopGuards(ExitCount, TheLoop),
SE->getConstant(BackedgeTakenCount->getType(), MaxVFtimesIC));
if (Rem->isZero()) {
return Rem->isZero();
};

if (MaxPowerOf2RuntimeVF > 0) {
assert((UserVF.isNonZero() || isPowerOf2_32(*MaxPowerOf2RuntimeVF)) &&
"MaxFixedVF must be a power of 2");
if (ScalarEpilogueNeeded(*MaxPowerOf2RuntimeVF)) {
// Accept MaxFixedVF if we do not have a tail.
LLVM_DEBUG(dbgs() << "LV: No tail will remain for any chosen VF.\n");
return MaxFactors;
}
}

auto ExpectedTC = getSmallBestKnownTC(PSE, TheLoop);
if (ExpectedTC && ExpectedTC <= TTI.getMinTripCountTailFoldingThreshold()) {
if (MaxPowerOf2RuntimeVF > 0) {
// If we have a low-trip-count, and the fixed-width VF is known to divide
// the trip count but the scalable factor does not, use the fixed-width
// factor in preference to allow the generation of a non-predicated loop.
if (ScalarEpilogueStatus == CM_ScalarEpilogueNotAllowedLowTripLoop &&
ScalarEpilogueNeeded(MaxFactors.FixedVF.getFixedValue())) {
LLVM_DEBUG(dbgs() << "LV: Picking a fixed-width so that no tail will "
"remain for any chosen VF.\n");
MaxFactors.ScalableVF = ElementCount::getScalable(0);
return MaxFactors;
}
}

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@david-arm david-arm Mar 20, 2025

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Can you re-add the debug output that we had before, i.e. something like:

        LLVM_DEBUG(dbgs() << "The target considers the trip count too "
                             "small to consider vectorizing with tail-folding.\n");

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@davemgreen davemgreen Mar 24, 2025

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reportVectorizationFailure will print the message it is reporting to dbgs() too. It didn't seem necessary to print the same info twice.

It will print

LV: Not vectorizing: The trip count is below the minial threshold value.

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@david-arm david-arm Mar 26, 2025

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OK yeah fair enough, but I think we're still missing the original Hints.emitRemarkWithHints(); call? I think LoopVectorizationCostModel does have access to a Hints pointer.

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@davemgreen davemgreen Mar 26, 2025

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It looks like it gets emitted in one of the parent functions. None of the other returns from this function use
emitRemarkWithHints, which was the reason I didn't more it over.

reportVectorizationFailure will emit the -Rpass=analysis output. The -Rpass=missed remark will be emitted by the VF now being scalar

remark: the cost-model indicates that vectorization is not beneficial [-Rpass-missed=loop-vectorize]

reportVectorizationFailure(
"The trip count is below the minial threshold value.",
"loop trip count is too low, avoiding vectorization", "LowTripCount",
ORE, TheLoop);
return FixedScalableVFPair::getNone();
}

// If we don't know the precise trip count, or if the trip count that we
// found modulo the vectorization factor is not zero, try to fold the tail
// by masking.
Expand Down Expand Up @@ -10597,26 +10622,15 @@ bool LoopVectorizePass::processLoop(Loop *L) {
if (Hints.getForce() == LoopVectorizeHints::FK_Enabled)
LLVM_DEBUG(dbgs() << " But vectorizing was explicitly forced.\n");
else {
if (*ExpectedTC > TTI->getMinTripCountTailFoldingThreshold()) {
LLVM_DEBUG(dbgs() << "\n");
// Predicate tail-folded loops are efficient even when the loop
// iteration count is low. However, setting the epilogue policy to
// `CM_ScalarEpilogueNotAllowedLowTripLoop` prevents vectorizing loops
// with runtime checks. It's more effective to let
// `isOutsideLoopWorkProfitable` determine if vectorization is
// beneficial for the loop.
if (SEL != CM_ScalarEpilogueNotNeededUsePredicate)
SEL = CM_ScalarEpilogueNotAllowedLowTripLoop;
} else {
LLVM_DEBUG(dbgs() << " But the target considers the trip count too "
"small to consider vectorizing.\n");
reportVectorizationFailure(
"The trip count is below the minial threshold value.",
"loop trip count is too low, avoiding vectorization",
"LowTripCount", ORE, L);
Hints.emitRemarkWithHints();
return false;
}
LLVM_DEBUG(dbgs() << "\n");
// Predicate tail-folded loops are efficient even when the loop
// iteration count is low. However, setting the epilogue policy to
// `CM_ScalarEpilogueNotAllowedLowTripLoop` prevents vectorizing loops
// with runtime checks. It's more effective to let
// `isOutsideLoopWorkProfitable` determine if vectorization is
// beneficial for the loop.
if (SEL != CM_ScalarEpilogueNotNeededUsePredicate)
SEL = CM_ScalarEpilogueNotAllowedLowTripLoop;
}
}

Expand Down
102 changes: 101 additions & 1 deletion llvm/test/Transforms/LoopVectorize/AArch64/low_trip_count_predicates.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -18,7 +18,7 @@ target triple = "aarch64-unknown-linux-gnu"

; DEBUG-LABEL: LV: Checking a loop in 'trip_count_too_small'
; DEBUG: LV: Found a loop with a very small trip count. This loop is worth vectorizing only if no scalar iteration overheads are incurred.
; DEBUG: LV: Not vectorizing: The trip count is below the minial threshold value..
; DEBUG: LV: Not vectorizing: Runtime SCEV check is required with -Os/-Oz.

; DEBUG-LABEL: LV: Checking a loop in 'too_many_runtime_checks'
; DEBUG: LV: Found trip count: 0
Expand Down Expand Up @@ -490,9 +490,103 @@ while.end:
ret void
}

; This has a trip-count of 4, and should vectorize with vf==4.
define i32 @tc4(ptr noundef readonly captures(none) %tmp) vscale_range(1,16) {
; CHECK-LABEL: define i32 @tc4(
; CHECK-SAME: ptr noundef readonly captures(none) [[TMP:%.*]]) #[[ATTR1]] {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: br i1 false, label %[[SCALAR_PH:.*]], label %[[VECTOR_PH:.*]]
; CHECK: [[VECTOR_PH]]:
; 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 i32> [ zeroinitializer, %[[VECTOR_PH]] ], [ [[TMP3:%.*]], %[[VECTOR_BODY]] ]
; CHECK-NEXT: [[ARRAYIDX1:%.*]] = getelementptr inbounds nuw [4 x i32], ptr [[TMP]], i64 0, i64 [[INDEX]]
; CHECK-NEXT: [[TMP2:%.*]] = getelementptr inbounds nuw i32, ptr [[ARRAYIDX1]], i32 0
; CHECK-NEXT: [[WIDE_LOAD:%.*]] = load <4 x i32>, ptr [[TMP2]], align 4
; CHECK-NEXT: [[TMP3]] = add <4 x i32> [[VEC_PHI]], [[WIDE_LOAD]]
; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 4
; CHECK-NEXT: br i1 true, label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP7:![0-9]+]]
; CHECK: [[MIDDLE_BLOCK]]:
; CHECK-NEXT: [[TMP4:%.*]] = call i32 @llvm.vector.reduce.add.v4i32(<4 x i32> [[TMP3]])
; CHECK-NEXT: br i1 true, label %[[FOR_COND_CLEANUP:.*]], label %[[SCALAR_PH]]
; CHECK: [[SCALAR_PH]]:
; CHECK-NEXT: [[BC_RESUME_VAL:%.*]] = phi i64 [ 4, %[[MIDDLE_BLOCK]] ], [ 0, %[[ENTRY]] ]
; CHECK-NEXT: [[BC_MERGE_RDX:%.*]] = phi i32 [ [[TMP4]], %[[MIDDLE_BLOCK]] ], [ 0, %[[ENTRY]] ]
; CHECK-NEXT: br label %[[FOR_BODY:.*]]
; CHECK: [[FOR_COND_CLEANUP]]:
; CHECK-NEXT: [[ADD_LCSSA:%.*]] = phi i32 [ [[ADD:%.*]], %[[FOR_BODY]] ], [ [[TMP4]], %[[MIDDLE_BLOCK]] ]
; CHECK-NEXT: ret i32 [[ADD_LCSSA]]
; CHECK: [[FOR_BODY]]:
; CHECK-NEXT: [[INDVARS_IV:%.*]] = phi i64 [ [[BC_RESUME_VAL]], %[[SCALAR_PH]] ], [ [[INDVARS_IV_NEXT:%.*]], %[[FOR_BODY]] ]
; CHECK-NEXT: [[SUM_0179:%.*]] = phi i32 [ [[BC_MERGE_RDX]], %[[SCALAR_PH]] ], [ [[ADD]], %[[FOR_BODY]] ]
; CHECK-NEXT: [[ARRAYIDX2:%.*]] = getelementptr inbounds nuw [4 x i32], ptr [[TMP]], i64 0, i64 [[INDVARS_IV]]
; CHECK-NEXT: [[TMP5:%.*]] = load i32, ptr [[ARRAYIDX2]], align 4
; CHECK-NEXT: [[ADD]] = add i32 [[SUM_0179]], [[TMP5]]
; CHECK-NEXT: [[INDVARS_IV_NEXT]] = add nuw nsw i64 [[INDVARS_IV]], 1
; CHECK-NEXT: [[EXITCOND_NOT:%.*]] = icmp eq i64 [[INDVARS_IV_NEXT]], 4
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label %[[FOR_COND_CLEANUP]], label %[[FOR_BODY]], !llvm.loop [[LOOP8:![0-9]+]]
;
entry:
br label %for.body

for.cond.cleanup: ; preds = %for.body
%add.lcssa = phi i32 [ %add, %for.body ]
ret i32 %add.lcssa

for.body: ; preds = %entry, %for.body
%indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
%sum.0179 = phi i32 [ 0, %entry ], [ %add, %for.body ]
%arrayidx1 = getelementptr inbounds nuw [4 x i32], ptr %tmp, i64 0, i64 %indvars.iv
%0 = load i32, ptr %arrayidx1, align 4
%add = add i32 %sum.0179, %0
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
%exitcond.not = icmp eq i64 %indvars.iv.next, 4
br i1 %exitcond.not, label %for.cond.cleanup, label %for.body
}

; This has a trip-count of 4 from a profile.
define i32 @tc4_from_profile(ptr noundef readonly captures(none) %tmp, i64 %N) vscale_range(1,16) {
; CHECK-LABEL: define i32 @tc4_from_profile(
; CHECK-SAME: ptr noundef readonly captures(none) [[TMP:%.*]], i64 [[N:%.*]]) #[[ATTR1]] {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: br label %[[FOR_BODY:.*]]
; CHECK: [[FOR_COND_CLEANUP:.*]]:
; CHECK-NEXT: [[TMP4:%.*]] = phi i32 [ [[ADD:%.*]], %[[FOR_BODY]] ]
; CHECK-NEXT: ret i32 [[TMP4]]
; CHECK: [[FOR_BODY]]:
; CHECK-NEXT: [[INDVARS_IV:%.*]] = phi i64 [ 0, %[[ENTRY]] ], [ [[INDVARS_IV_NEXT:%.*]], %[[FOR_BODY]] ]
; CHECK-NEXT: [[SUM_0179:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ [[ADD]], %[[FOR_BODY]] ]
; CHECK-NEXT: [[ARRAYIDX1:%.*]] = getelementptr inbounds nuw [4 x i32], ptr [[TMP]], i64 0, i64 [[INDVARS_IV]]
; CHECK-NEXT: [[TMP0:%.*]] = load i32, ptr [[ARRAYIDX1]], align 4
; CHECK-NEXT: [[ADD]] = add i32 [[SUM_0179]], [[TMP0]]
; CHECK-NEXT: [[INDVARS_IV_NEXT]] = add nuw nsw i64 [[INDVARS_IV]], 1
; CHECK-NEXT: [[EXITCOND_NOT:%.*]] = icmp eq i64 [[INDVARS_IV_NEXT]], [[N]]
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label %[[FOR_COND_CLEANUP]], label %[[FOR_BODY]], !prof [[PROF9:![0-9]+]]
;
entry:
br label %for.body

for.cond.cleanup: ; preds = %for.body
%add.lcssa = phi i32 [ %add, %for.body ]
ret i32 %add.lcssa

for.body: ; preds = %entry, %for.body
%indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
%sum.0179 = phi i32 [ 0, %entry ], [ %add, %for.body ]
%arrayidx1 = getelementptr inbounds nuw [4 x i32], ptr %tmp, i64 0, i64 %indvars.iv
%0 = load i32, ptr %arrayidx1, align 4
%add = add i32 %sum.0179, %0
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
%exitcond.not = icmp eq i64 %indvars.iv.next, %N
br i1 %exitcond.not, label %for.cond.cleanup, label %for.body, !prof !2
}


!0 = distinct !{!0, !1}
!1 = !{!"llvm.loop.vectorize.predicate.enable", i1 true}
!2 = !{!"branch_weights", i32 10, i32 30}

;.
; CHECK-VS1: [[LOOP0]] = distinct !{[[LOOP0]], [[META1:![0-9]+]], [[META2:![0-9]+]]}
; CHECK-VS1: [[META1]] = !{!"llvm.loop.isvectorized", i32 1}
Expand All @@ -501,6 +595,9 @@ while.end:
; CHECK-VS1: [[LOOP4]] = distinct !{[[LOOP4]], [[META1]]}
; CHECK-VS1: [[LOOP5]] = distinct !{[[LOOP5]], [[META1]], [[META2]]}
; CHECK-VS1: [[LOOP6]] = distinct !{[[LOOP6]], [[META1]]}
; CHECK-VS1: [[LOOP7]] = distinct !{[[LOOP7]], [[META1]], [[META2]]}
; CHECK-VS1: [[LOOP8]] = distinct !{[[LOOP8]], [[META2]], [[META1]]}
; CHECK-VS1: [[PROF9]] = !{!"branch_weights", i32 10, i32 30}
;.
; CHECK-VS2: [[LOOP0]] = distinct !{[[LOOP0]], [[META1:![0-9]+]], [[META2:![0-9]+]]}
; CHECK-VS2: [[META1]] = !{!"llvm.loop.isvectorized", i32 1}
Expand All @@ -509,4 +606,7 @@ while.end:
; CHECK-VS2: [[LOOP4]] = distinct !{[[LOOP4]], [[META1]]}
; CHECK-VS2: [[LOOP5]] = distinct !{[[LOOP5]], [[META1]], [[META2]]}
; CHECK-VS2: [[LOOP6]] = distinct !{[[LOOP6]], [[META1]]}
; CHECK-VS2: [[LOOP7]] = distinct !{[[LOOP7]], [[META1]], [[META2]]}
; CHECK-VS2: [[LOOP8]] = distinct !{[[LOOP8]], [[META2]], [[META1]]}
; CHECK-VS2: [[PROF9]] = !{!"branch_weights", i32 10, i32 30}
;.
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