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JIT: (bug) CompareNotGreaterThan feeding a conditional select is evaluated as GreaterThan #133550
Description
Repro
using System;
using System.Runtime.CompilerServices;
using System.Runtime.Intrinsics;
using System.Runtime.Intrinsics.X86;
class Program
{
[MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
static float Test(Vector128<float> a, Vector128<float> b, Vector128<float> x, Vector128<float> y) =>
Vector128.ConditionalSelect(Sse.CompareNotGreaterThan(a, b), x, y).GetElement(0);
static void Main()
{
if (!Avx512F.IsSupported)
throw new PlatformNotSupportedException("AVX-512F is required.");
var x = Vector128.Create(111.0f);
var y = Vector128.Create(222.0f);
// 1 is not greater than 2, so the mask is all-ones and x must be selected.
Console.WriteLine(Test(Vector128.Create(1.0f), Vector128.Create(2.0f), x, y));
// 2 is not greater than 2, so the mask is all-ones and x must be selected.
Console.WriteLine(Test(Vector128.Create(2.0f), Vector128.Create(2.0f), x, y));
// 3 is greater than 2, so the mask is all-zeros and y must be selected.
Console.WriteLine(Test(Vector128.Create(3.0f), Vector128.Create(2.0f), x, y));
}
}Expected
111
111
222
Actual
222
222
111
The comparison result is inverted: every lane selects the opposite operand. Running the same
program with DOTNET_EnableAVX512=0 prints the expected values, so only the AVX-512 path is
affected.
Platform
Windows x64 with AVX-512F, local Checked build of dotnet/runtime main (ef97eb5).
Activity
dotnet-policy-service commentedon Sep 10, 2026
Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.
Fix not-greater-than predicate when lowering SIMD selects (#133589)
EgorBo commentedon Sep 18, 2026
Affected SDK versions:
EgorBo commentedon Sep 18, 2026
@tannergooding would you consider backporting the fix from #133589 to
release/11.0?It currently only exists in
main(12.0). The repro still fails on the shipped 11.0 RC2 SDK (11.0.100-rc.2.26467.112), while 8.0.425, 9.0.318 and 10.0.401 all behave correctly, so this is a regression introduced during the 11.0 cycle rather than a long-standing bug — 11.0 would ship with it.Note: this comment was produced by an AI agent; the repro was re-run against all four released SDKs to confirm the results above.
[release/11.0] Fix not-greater-than predicate when lowering SIMD sele…