Fix spurious StorageImageReadWithoutFormat capability for write-only textures

[jhelferty-nv]

Fixes #9997

WTexture* types with unknown format were emitting both StorageImageReadWithoutFormat and StorageImageWriteWithoutFormat SPIR-V capabilities, even though write-only textures cannot be read. This caused pipeline rejection on hardware that doesn't support unformatted storage image reads (e.g. pre-Xe Intel GPUs).

Check the texture access mode when deciding which capabilities to emit: write-only textures now only emit StorageImageWriteWithoutFormat.

Add tests for both write-only (WTexture2D) and read-write (RWTexture2D) capability emission to prevent regressions.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 78c5a92e-dca1-44f6-9261-cc53a073f5af

📥 Commits

Reviewing files that changed from the base of the PR and between ca1b13d and 0b952ee.

📒 Files selected for processing (1)

• source/slang/slang-emit-spirv.cpp

🚧 Files skipped from review as they are similar to previous changes (1)

• source/slang/slang-emit-spirv.cpp

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📝 Walkthrough

Walkthrough

Adjusts SPIR‑V capability emission for storage images with unknown format: require StorageImageReadWithoutFormat only when the texture access permits reads; write-only textures require only StorageImageWriteWithoutFormat. Adds tests for RWTexture2D (both capabilities) and WTexture2D (write-only capability, NonReadable decoration).

Changes

Cohort / File(s)	Summary
SPIR-V capability logic source/slang/slang-emit-spirv.cpp	Modified conditional so StorageImageReadWithoutFormat is required only when texture access includes reads; StorageImageWriteWithoutFormat remains required for writes. Updated explanatory comments.
Tests: read/write image capability tests/spirv/read-write-image-capability.slang	Added test asserting an RWTexture2D without format emits both StorageImageReadWithoutFormat and StorageImageWriteWithoutFormat.
Tests: write-only image capability tests/spirv/write-only-image-capability.slang	Added test asserting a WTexture2D without format emits StorageImageWriteWithoutFormat, omits StorageImageReadWithoutFormat, and includes NonReadable decoration where applicable.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Poem

"I nibble bits of SPIR‑V code by night,
Moved a check so reads only come to light,
Write‑only carrots now taste just right,
No extra hops, the pipeline's tight,
— Rabbit 🥕"

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main fix: preventing spurious StorageImageReadWithoutFormat capability emission for write-only textures, which is the primary change in the PR.
Description check	✅ Passed	The description clearly explains the bug fix, the root cause, the solution approach, and references the related issue #9997 with details about added tests.
Linked Issues check	✅ Passed	The PR fully addresses issue #9997 by fixing the spurious capability emission for write-only textures and adding comprehensive tests for both write-only and read-write texture scenarios.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the StorageImageReadWithoutFormat capability issue: the core logic fix in slang-emit-spirv.cpp and two targeted test files for capability validation.

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• Create PR with unit tests
• Post copyable unit tests in a comment

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[Copilot]

Pull request overview

This PR fixes a bug where write-only textures (WTexture2D) were incorrectly emitting the StorageImageReadWithoutFormat SPIR-V capability, even though they only support write operations. This caused pipeline rejection on hardware that doesn't support unformatted storage image reads (e.g., pre-Xe Intel GPUs). The fix adds a conditional check to only emit the read capability for non-write-only textures.

Changes:

• Modified SPIR-V emission logic to check texture access mode before emitting StorageImageReadWithoutFormat capability
• Added regression test for write-only textures (WTexture2D) to ensure only write capability is emitted
• Added companion test for read-write textures (RWTexture2D) to ensure both capabilities are still emitted correctly

Reviewed changes

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

File	Description
source/slang/slang-emit-spirv.cpp	Added access mode check to conditionally emit StorageImageReadWithoutFormat capability only for non-write-only textures
tests/spirv/write-only-image-capability.slang	Test verifying WTexture2D only emits StorageImageWriteWithoutFormat and NonReadable decoration
tests/spirv/read-write-image-capability.slang	Test verifying RWTexture2D emits both read and write capabilities

[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@tests/spirv/write-only-image-capability.slang`:
- Line 13: The test declares a write-only texture without an explicit type
parameter; update the declaration from WTexture2D to WTexture2D<float4> so it
matches the companion read-write test and the Store call writing float4; locate
the WTexture2D declaration in write-only-image-capability.slang and add the
<float4> type parameter to the WTexture2D symbol.

[github-actions]

Code Review: Fix spurious StorageImageReadWithoutFormat capability for write-only textures

Summary

Clean, well-targeted fix that resolves #9997 — WTexture* types with unknown format were incorrectly requesting the StorageImageReadWithoutFormat SPIR-V capability, which caused pipeline rejection on hardware lacking that capability (e.g., pre-Xe Intel GPUs).

Correctness ✅

The fix is correct and directly addresses the existing TODO comment in the code:

1. Access mode routing: At line ~2962, SLANG_RESOURCE_ACCESS_WRITE, SLANG_RESOURCE_ACCESS_READ_WRITE, and SLANG_RESOURCE_ACCESS_RASTER_ORDERED all set sampled = readWriteImage, which causes all three to enter the capability block at line 3074. The new inst->getAccess() != SLANG_RESOURCE_ACCESS_WRITE guard correctly distinguishes write-only from the other two modes.

2. SPIR-V spec alignment: Per the SPIR-V spec, StorageImageReadWithoutFormat is required only when reading an image with Unknown format. Write-only textures (decorated with NonReadable) will never issue OpImageRead, so they should not declare this capability.

3. Consistency with existing code: The fix is consistent with maybeEmitWriteOnlyImageDecoration() (line ~7190) which already emits SpvDecorationNonReadable for write-only textures, and with the GLSL emitter (line ~1889) which emits writeonly for SLANG_RESOURCE_ACCESS_WRITE. The capability emission now matches these existing access-mode checks.

4. No other code paths affected: The only other place that emits StorageImageReadWithoutFormat is for subpass inputs (line ~2913), which is a completely separate path.

Test Coverage ✅

Good regression tests:

• write-only-image-capability.slang: Verifies WTexture2D only emits StorageImageWriteWithoutFormat (not StorageImageReadWithoutFormat), and confirms NonReadable decoration. The CHECK-NOT/CHECK/CHECK-NOT sandwich pattern correctly ensures absence throughout the output.
• read-write-image-capability.slang: Verifies RWTexture2D still emits both capabilities. Uses CHECK-DAG appropriately since capability order is not guaranteed.

Nit (non-blocking)

A test for RasterizerOrderedTexture2D<float4> with unknown format would fully cover the third access mode that enters this code path, but it is not strictly necessary since it follows the same != WRITE branch as RWTexture2D.

LGTM — minimal, well-scoped fix with good test coverage.