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[LV] Ignore user-specified interleave count when unsafe. #153009

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Merged
merged 6 commits into from
Oct 22, 2025
Merged

[LV] Ignore user-specified interleave count when unsafe. #153009

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938a0e3
[LV] Ignore user-specified interleave count when unsafe.
kmclaughlin-arm Aug 7, 2025
31c6578
- Reworded diagnostic message
kmclaughlin-arm Aug 11, 2025
20fe702
- Handle UserIC as part of selectInterleaveCount
kmclaughlin-arm Aug 12, 2025
17eda94
- Moved UserIC back out of selectInterleaveCount
kmclaughlin-arm Oct 9, 2025
444e58c
- Only set UserIC to 1 if an interleave count > 1 was requested
kmclaughlin-arm Oct 21, 2025
5ad8fb2
- Add an assert for !isSafeForAnyVectorWidth()
kmclaughlin-arm Oct 21, 2025
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10 changes: 8 additions & 2 deletions llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
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Expand Up @@ -9842,7 +9842,7 @@ bool LoopVectorizePass::processLoop(Loop *L) {

// Get user vectorization factor and interleave count.
ElementCount UserVF = Hints.getWidth();
unsigned UserIC = Hints.getInterleave();
unsigned UserIC = LVL.isSafeForAnyVectorWidth() ? Hints.getInterleave() : 1;
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If the user didn't specify an interleave hint then Hints.getInterleave() actually returns 0. If LVL.isSafeForAnyVectorWidth returns false and there is no hint you'd be setting UserIC to 1 instead of 0. I wonder if it's better to rewrite this as:

  unsigned UserIC = Hints.getInterleave();
  if (UserIC > 1 && !LVL.isSafeForAnyVectorWidth())
    UserIC = 1;


// Plan how to best vectorize.
LVP.plan(UserVF, UserIC);
Expand Down Expand Up @@ -9907,7 +9907,13 @@ bool LoopVectorizePass::processLoop(Loop *L) {
VectorizeLoop = false;
}

if (!LVP.hasPlanWithVF(VF.Width) && UserIC > 1) {
if (UserIC == 1 && Hints.getInterleave() > 1) {
LLVM_DEBUG(dbgs() << "LV: Ignoring user-specified interleave count.\n");
IntDiagMsg = {"InterleavingUnsafe",
"Ignoring user-specified interleave count due to possibly "
"unsafe dependencies in the loop."};
InterleaveLoop = false;
} else if (!LVP.hasPlanWithVF(VF.Width) && UserIC > 1) {
// Tell the user interleaving was avoided up-front, despite being explicitly
// requested.
LLVM_DEBUG(dbgs() << "LV: Ignoring UserIC, because vectorization and "
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14 changes: 5 additions & 9 deletions llvm/test/Transforms/LoopVectorize/AArch64/scalable-reductions.ll
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Expand Up @@ -417,21 +417,17 @@ for.end: ; preds = %for.body, %entry

; Note: This test was added to ensure we always check the legality of reductions (end emit a warning if necessary) before checking for memory dependencies
; CHECK-REMARK: Scalable vectorization not supported for the reduction operations found in this loop.
; CHECK-REMARK: vectorized loop (vectorization width: 4, interleaved count: 2)
; CHECK-REMARK: Ignoring user-specified interleave count due to possibly unsafe dependencies in the loop.
; CHECK-REMARK: vectorized loop (vectorization width: 4, interleaved count: 1)
define i32 @memory_dependence(ptr noalias nocapture %a, ptr noalias nocapture readonly %b, i64 %n) {
; CHECK-LABEL: @memory_dependence
; CHECK: vector.body:
; CHECK: %[[LOAD1:.*]] = load <4 x i32>
; CHECK: %[[LOAD2:.*]] = load <4 x i32>
; CHECK: %[[LOAD3:.*]] = load <4 x i32>
; CHECK: %[[LOAD4:.*]] = load <4 x i32>
; CHECK: %[[ADD1:.*]] = add nsw <4 x i32> %[[LOAD3]], %[[LOAD1]]
; CHECK: %[[ADD2:.*]] = add nsw <4 x i32> %[[LOAD4]], %[[LOAD2]]
; CHECK: %[[MUL1:.*]] = mul <4 x i32> %[[LOAD3]]
; CHECK: %[[MUL2:.*]] = mul <4 x i32> %[[LOAD4]]
; CHECK: %[[ADD1:.*]] = add nsw <4 x i32> %[[LOAD2]], %[[LOAD1]]
; CHECK: %[[MUL1:.*]] = mul <4 x i32> %[[LOAD2]]
; CHECK: middle.block:
; CHECK: %[[RDX:.*]] = mul <4 x i32> %[[MUL2]], %[[MUL1]]
; CHECK: call i32 @llvm.vector.reduce.mul.v4i32(<4 x i32> %[[RDX]])
; CHECK: call i32 @llvm.vector.reduce.mul.v4i32(<4 x i32> %[[MUL1]])
entry:
br label %for.body

Expand Down
31 changes: 31 additions & 0 deletions llvm/test/Transforms/LoopVectorize/unsafe-ic-hint-remark.ll
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@@ -0,0 +1,31 @@
; RUN: opt -passes=loop-vectorize -pass-remarks-analysis=loop-vectorize -S < %s 2>&1 | FileCheck %s

; Make sure the unsafe user specified interleave count is ignored.

; CHECK: remark: <unknown>:0:0: Ignoring user-specified interleave count due to possibly unsafe dependencies in the loop.
; CHECK-LABEL: @loop_distance_4
define void @loop_distance_4(i64 %N, ptr %a, ptr %b) {
entry:
%cmp10 = icmp sgt i64 %N, 4
br i1 %cmp10, label %for.body, label %for.end
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this shouldn't be needed, you could probably just use a constant %N below?


for.body:
%indvars.iv = phi i64 [ 4, %entry ], [ %indvars.iv.next, %for.body ]
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Suggested change
%indvars.iv = phi i64 [ 4, %entry ], [ %indvars.iv.next, %for.body ]
%iv = phi i64 [ 4, %entry ], [ %iv.next, %loop ]

for consistency with other tests

%0 = getelementptr i32, ptr %b, i64 %indvars.iv
%arrayidx = getelementptr i8, ptr %0, i64 -16
%1 = load i32, ptr %arrayidx, align 4
%arrayidx2 = getelementptr inbounds nuw i32, ptr %a, i64 %indvars.iv
%2 = load i32, ptr %arrayidx2, align 4
%add = add nsw i32 %2, %1
store i32 %add, ptr %0, align 4
%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.end, label %for.body, !llvm.loop !1

for.end:
ret void
}

!1 = !{!1, !2, !3}
!2 = !{!"llvm.loop.interleave.count", i32 4}
!3 = !{!"llvm.loop.vectorize.width", i32 4}