Fix Avx.CompareScalar and Avx512F.Compare incorrectly folded with True/False comparison modes by Copilot · Pull Request #125179 · dotnet/runtime

Copilot · 2026-03-04T14:45:15Z

Description

Fixes two issues in JIT constant folding of comparison intrinsics with OrderedFalse*/UnorderedTrue* modes in gtFoldExprHWIntrinsic:

Avx.CompareScalar: CompareScalar preserves upper elements from the first operand, but the folding replaced the entire vector with zero or AllBitsSet, discarding those upper elements.

[MethodImpl(MethodImplOptions.NoInlining)]
static Vector128<float> CompareFalse(Vector128<float> x, Vector128<float> y)
    => Avx.CompareScalar(x, y, FloatComparisonMode.OrderedFalseSignaling);

// Expected: <0, 1, 1, 1>
// Actual:   <0, 0, 0, 0>
CompareFalse(Vector128<float>.One, Vector128<float>.Zero);

Avx512F.Compare (NI_AVX512_CompareMask): CompareMask returns TYP_MASK, but gtNewZeroConNode/gtNewAllBitsSetConNode don't handle mask types, causing an unreached assertion failure.

static Vector512<float> CompareFalse(Vector512<float> x, Vector512<float> y)
    => Avx512F.Compare(x, y, FloatComparisonMode.OrderedFalseSignaling);
// Assert failure: 'unreached'

Changes

src/coreclr/jit/gentree.cpp:
- Skip the True/False constant-folding when ni == NI_AVX_CompareScalar, since CompareScalar must preserve upper elements from op1.
- For NI_AVX512_CompareMask, use getSIMDTypeForSize(simdSize) instead of retType when creating zero/AllBitsSet constants, since retType is TYP_MASK. The existing varTypeIsMask conversion at the end of the function wraps the result with CvtVectorToMask.
Regression test (Runtime_125160): Covers all four True/False modes for both float and double scalar comparisons (asserting lowest element result and upper element preservation), and all four True/False modes for Avx512F.Compare with Vector512<float> and Vector512<double> (using Vector512<T>.AllBitsSet for UnorderedTrue assertions since all-bits-set produces 0xFFFFFFFF per element, not -1f).

Original prompt

This section details on the original issue you should resolve

<issue_title>JIT: Avx.CompareScalar incorrectly folded with True/False comparison modes</issue_title>
<issue_description>### Description

CompareScalar is supposed to return the upper elements of the first operand unchanged, but folding for True/False comparison modes returns a full Zero or AllBitsSet vector.

https://github.com/dotnet/runtime/blob/f62984ce62deecfb90a8ab991e972d1bc460b93e/src/coreclr/jit/gentree.cpp#L33828-L33842

Reproduction Steps
using System.Runtime.CompilerServices;
using System.Runtime.Intrinsics;
using System.Runtime.Intrinsics.X86;

Console.WriteLine(CompareFalse(Vector128<float>.One, Vector128<float>.Zero));

[MethodImpl(MethodImplOptions.NoInlining)]
static Vector128<float> CompareFalse(Vector128<float> x, Vector128<float> y)
	=> Avx.CompareScalar(x, y, FloatComparisonMode.OrderedFalseSignaling);
Expected behavior
> dotnet run -c release -f net9.0
<0, 1, 1, 1>

Actual behavior
> dotnet run -c release -f net10.0
<0, 0, 0, 0>

Regression?

Yes

Known Workarounds

No response

Configuration

No response

Other information

No response</issue_description>

<agent_instructions>Please fix #125160 and create a jit regression test for it</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes JIT: Avx.CompareScalar incorrectly folded with True/False comparison modes #125160

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dotnet-policy-service · 2026-03-04T14:47:33Z

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

…gression test CompareScalar preserves upper elements from the first operand, so folding to a full zero or AllBitsSet vector is incorrect. Skip folding for NI_AVX_CompareScalar with True/False modes so the hardware instruction produces the correct result. Fixes #125160 Co-authored-by: EgorBo <523221+EgorBo@users.noreply.github.com>

Copilot

Pull request overview

This PR fixes a JIT constant-folding regression introduced in net10.0 where Avx.CompareScalar with OrderedFalse* or UnorderedTrue* comparison modes was incorrectly folded to a full zero or AllBitsSet vector, discarding the upper elements that should be preserved from the first operand.

Changes:

Skip constant folding for NI_AVX_CompareScalar in the OrderedFalse* and UnorderedTrue* cases in gtFoldExprHWIntrinsic, since CompareScalar only affects element 0 and must preserve upper elements from op1
Add regression test covering all four True/False modes for both float and double, asserting correct scalar result in element 0 and upper element preservation

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
`src/coreclr/jit/gentree.cpp`	Adds an early `break` guard for `NI_AVX_CompareScalar` in both `OrderedFalse` and `UnorderedTrue` comparison mode cases to prevent incorrect whole-vector folding
`src/tests/JIT/Regression/JitBlue/Runtime_125160/Runtime_125160.cs`	New regression test covering all four True/False comparison modes for both float and double, verifying scalar result in element 0 and upper element preservation
`src/tests/JIT/Regression/Regression_ro_2.csproj`	Registers the new regression test in the merged test runner project

saucecontrol · 2026-03-04T18:31:35Z

@copilot there's also an issue with CompareMask in that same folding code, because CnsVec is not the correct return type.

static Vector512<float> CompareFalse(Vector512<float> x, Vector512<float> y)
    => Avx512F.Compare(x, y, FloatComparisonMode.OrderedFalseSignaling);

Assert failure(PID 49888 [0x0000c2e0], Thread: 21696[0x54c0]): Assertion failed 'unreached' in 'Program:<<Main>$>g__CompareFalse|0_1(System.Runtime.Intrinsics.Vector512`1[float],System.Runtime.Intrinsics.Vector512`1[float]):System.Runtime.Intrinsics.Vector512`1[float]' during 'Importation' (IL size 10; hash 0x21e0833f; FullOpts)

    File: C:\git\runtime\src\coreclr\jit\gentree.cpp:8229
    Image: c:\git\runtime\artifacts\bin\coreclr\windows.x64.Checked\CoreRun.exe

EgorBo · 2026-03-04T18:56:03Z

@copilot there's also an issue with CompareMask in that same folding code, because CnsVec is not the correct return type.

static Vector512<float> CompareFalse(Vector512<float> x, Vector512<float> y)
    => Avx512F.Compare(x, y, FloatComparisonMode.OrderedFalseSignaling);

Assert failure(PID 49888 [0x0000c2e0], Thread: 21696[0x54c0]): Assertion failed 'unreached' in 'Program:<<Main>$>g__CompareFalse|0_1(System.Runtime.Intrinsics.Vector512`1[float],System.Runtime.Intrinsics.Vector512`1[float]):System.Runtime.Intrinsics.Vector512`1[float]' during 'Importation' (IL size 10; hash 0x21e0833f; FullOpts)

    File: C:\git\runtime\src\coreclr\jit\gentree.cpp:8229
    Image: c:\git\runtime\artifacts\bin\coreclr\windows.x64.Checked\CoreRun.exe

…2 test For NI_AVX512_CompareMask, retType is TYP_MASK which gtNewZeroConNode and gtNewAllBitsSetConNode don't handle (hitting unreached()). Use the SIMD vector type instead; the existing varTypeIsMask conversion at the end of gtFoldExprHWIntrinsic will wrap the result with CvtVectorToMask. Co-authored-by: EgorBo <523221+EgorBo@users.noreply.github.com>

Copilot · 2026-03-04T19:25:36Z

@copilot there's also an issue with CompareMask in that same folding code, because CnsVec is not the correct return type.

...

Fixed in 01f88f3. gtNewZeroConNode/gtNewAllBitsSetConNode don't handle TYP_MASK, so now when retType is a mask type, we create the constant using getSIMDTypeForSize(simdSize) instead. The existing varTypeIsMask conversion at the end of gtFoldExprHWIntrinsic then wraps it with CvtVectorToMask. Also added Avx512F.Compare test coverage for all four True/False modes.

EgorBo · 2026-03-04T23:20:36Z

@copilot address the CI failures, e.g. Test global::Runtime_125160.TestCompareMask() has failed

…redTrue assertions Vector512.Create(-1f) has bit pattern 0xBF800000 per element, but UnorderedTrue comparison modes produce all-bits-set (0xFFFFFFFF per element, which is NaN). Use Vector512<T>.AllBitsSet to correctly match the expected result. Co-authored-by: EgorBo <523221+EgorBo@users.noreply.github.com>

Copilot · 2026-03-04T23:50:26Z

@copilot address the CI failures, e.g. Test global::Runtime_125160.TestCompareMask() has failed

Fixed in 1826aad. The TestCompareMask failure was due to incorrect expected values: Vector512.Create(-1f) has bit pattern 0xBF800000 per element, but UnorderedTrue comparison modes produce all-bits-set (0xFFFFFFFF per float element = NaN). Replaced with Vector512<float>.AllBitsSet / Vector512<double>.AllBitsSet.

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Copilot AI assigned Copilot and EgorBo Mar 4, 2026

Copilot started work on behalf of EgorBo March 4, 2026 14:45 View session

github-actions bot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Mar 4, 2026

Copilot AI changed the title ~~[WIP] Fix avx.CompareScalar folding for True/False comparison modes~~ Fix Avx.CompareScalar incorrectly folded with True/False comparison modes Mar 4, 2026

Copilot finished work on behalf of EgorBo March 4, 2026 15:26

EgorBo requested a review from Copilot March 4, 2026 16:18

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Copilot AI reviewed Mar 4, 2026

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Copilot started work on behalf of EgorBo March 4, 2026 18:56 View session

Copilot AI changed the title ~~Fix Avx.CompareScalar incorrectly folded with True/False comparison modes~~ Fix Avx.CompareScalar and Avx512F.Compare incorrectly folded with True/False comparison modes Mar 4, 2026

Copilot finished work on behalf of EgorBo March 4, 2026 19:27

Copilot started work on behalf of EgorBo March 4, 2026 23:20 View session

Copilot finished work on behalf of EgorBo March 4, 2026 23:51

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Fix Avx.CompareScalar and Avx512F.Compare incorrectly folded with True/False comparison modes#125179

Fix Avx.CompareScalar and Avx512F.Compare incorrectly folded with True/False comparison modes#125179
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Description

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Reproduction Steps

Expected behavior

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