Mesh output write fixes by SteveUrquhart · Pull Request #9034 · microsoft/DirectXShaderCompiler

Steve Urquhart (SteveUrquhart) · 2026-10-15T20:04:06Z

Addresses 3 SPIRV mesh shader write bugs as outlined in #9033.

This PR conflicts with #8057, so I will need to update it after 8057 is accepted.

I'm also interested in #8982. I trimmed a lesser solution from this PR, but I'd need 8982 before proceeding with several more mesh shader PR's that I'd like to open. They depend on these 3, so for now I'll create issues. Diego Novillo (@dnovillo)

azure-pipelines · 2026-10-15T20:04:17Z

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🟡 Changes recommended

Output-argument handling still has incorrect copy-back behavior and an incomplete read diagnostic.

3 open findings

What changed in this PR

Addresses three SPIR-V mesh-output write bugs from #9033; inherited-struct support remains covered by #8982.

Changes:

Adds mesh-output copy-back for out arguments and rejects unreadable inout arguments.
Fixes position-component Y inversion and boolean-vector storage conversion.
Adds regression tests and upcoming-release notes.

File	Description
tools/clang/test/CodeGenSPIRV/meshshading.write-outputs.out-arg.mesh.hlsl	Tests output-argument writes and diagnostics.
tools/clang/test/CodeGenSPIRV/meshshading.write-outputs.invert-y-component.mesh.hlsl	Tests component-wise Y inversion.
tools/clang/test/CodeGenSPIRV/meshshading.write-outputs.bool-vector.mesh.hlsl	Tests boolean-vector output conversion.
tools/clang/lib/SPIRV/SpirvEmitter.cpp	Implements mesh-output write fixes.
docs/ReleaseNotes.md	Documents the three fixes.

🧠 Review effort: Balanced

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Copilot

🟡 Changes recommended

Output-argument handling still mishandles one-element vector swizzles and static data members.

2 open findings

Stop output traversal at static data members · New
Clear accessors for one-component vector bases · New

3 resolved since last review

🧠 Review effort: Balanced

Copilot

🟡 Changes recommended

Mesh-qualified helper calls can overwrite valid outputs, and matrix-element output arguments still silently lose writes.

2 open findings

2 resolved since last review

🧠 Review effort: Balanced

Copilot

🔵 Needs a closer look

Shared call-lowering changes, unresolved correctness issues, and the stated conflict with #8057 require human integration review.

1 open finding

Null matrix-write diagnostics cause nested assignment crashes · New

2 resolved since last review

Previously missed (2)

In code that hasn't changed since last review

Scalar swizzle normalization rejects valid chained swizzles

tools/clang/lib/SPIRV/SpirvEmitter.cpp:3408

setFloat(vertices[0].position.y.x) is a valid scalar swizzle, but the new argument handling rejects it. Condensing .x stops at the scalar-to-float1 splat; removing that splat exposes .y, which collectMSOutDest cannot collect. Repeat swizzle normalization after removing scalar splats so the inner component is preserved, and add this call to the regression test.

Parenthesized mesh output arguments are incorrectly rejected

tools/clang/lib/SPIRV/SpirvEmitter.cpp:3427

setFloat((vertices[0]).value) is rejected as an unsupported mesh output argument, although it is equivalent to the supported unparenthesized call. isMSOutputAccess unwraps parentheses, but collectArrayStructIndices stops at the ParenExpr beneath the member access and never identifies the output array. Normalize expr with IgnoreParens() at the start of each recursive collection step, and add this spelling to the out-argument regression test.

🧠 Review effort: Balanced

Copilot

🔵 Needs a closer look

Shared call and assignment lowering changes need human validation across EXT/NV paths and integration with the acknowledged #8057 conflict.

0 open findings

1 resolved since last review

Null matrix-write diagnostics cause nested assignment crashes

🧠 Review effort: Balanced

Steve Urquhart (SteveUrquhart) added 3 commits October 15, 2026 15:21

[SPIRV] Fix mesh shader out arguments

708f07d

[SPIRV] mesh shaders correctly invert only Y when -fvk-invert-y

1db90c2

[SPIRV] Fix bool vector mesh shader output writes

65e2e93

Copilot AI balanced review requested due to automatic review settings October 15, 2026 20:04

github-project-automation Bot added this to HLSL Roadmap Oct 15, 2026

github-project-automation Bot moved this to New in HLSL Roadmap Oct 15, 2026

Copilot started reviewing on behalf of Steve Urquhart (SteveUrquhart) October 15, 2026 20:04

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Comment thread tools/clang/lib/SPIRV/SpirvEmitter.cpp Outdated

Comment thread tools/clang/lib/SPIRV/SpirvEmitter.cpp Outdated

Comment thread tools/clang/lib/SPIRV/SpirvEmitter.cpp

[SPIRV] Respond to review comments

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Copilot AI balanced review requested due to automatic review settings October 15, 2026 22:28

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Comment thread tools/clang/lib/SPIRV/SpirvEmitter.cpp Outdated

Comment thread tools/clang/lib/SPIRV/SpirvEmitter.cpp

[SPIRV] Respond to review comments 2

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Copilot AI balanced review requested due to automatic review settings October 15, 2026 22:45

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Comment thread tools/clang/lib/SPIRV/SpirvEmitter.cpp Outdated

Comment thread tools/clang/lib/SPIRV/SpirvEmitter.cpp Outdated

[SPIRV] Respond to review comments 3

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Copilot AI balanced review requested due to automatic review settings October 15, 2026 23:04

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Comment thread tools/clang/lib/SPIRV/SpirvEmitter.cpp

[SPIRV] Respond to review comments 4

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Copilot AI balanced review requested due to automatic review settings October 16, 2026 16:57

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Mesh output write fixes - #9034

Mesh output write fixes#9034
Steve Urquhart (SteveUrquhart) wants to merge 7 commits into
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SteveUrquhart:mesh-output-write-fixes

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