[losewayy]

Summary

On CUDA, INT8 linear layers with bias used cuBLAS to write an INT32 intermediate, then launched a separate kernel to dequantize it and add bias.

Add the per-output bias in the existing CUTLASS epilogue after both scales are applied. Layers with and without bias share the same kernel templates. When the bias pointer is null, the bias broadcast uses a default of negative zero to preserve signed-zero outputs. Unsupported shapes and unaligned bias still use the existing fallback.

Related Issue / Discussion

The Sol VAE benchmark was run on the #2317 branch with this change applied.

Additional Information

Tested on an RTX 5070 Ti Laptop GPU (sm_120) with CUDA 13.4.

• Correctness: All 10 CUDA fixtures matched the previous implementation bit for bit (3,565,604 outputs). Separate tests also matched for both signs of zero in the regular and Stream-K kernels.
• Wan2.2-S2V-14B INT8 ConvRot: At 320×320 and 33 frames with disk offload, sampling steps 2–4 averaged 12.80 s per step versus 13.49 s on the baseline rerun (5.1% less time per step). The generated videos were byte-identical. End-to-end CLI time did not consistently improve.
• Sol VAE on #2317: Mean decode time fell from 2.655 s to 2.352 s (11.4%). CUPTI recorded 936 fewer standalone dequantization launches, and the decoded output was bit-identical.
• Bias-free Qwen Image 2.1 linear shapes: Across four projection replays, mean times were within 0.21% of baseline, with no consistent slowdown. This measured the linear layers, not full image generation.
• Build: The number of CUDA GEMM kernel instantiations stayed at 28. Compiling i8_gemm.cu with the same NVCC command took 60.0 s before and 64.9 s after. The CUDA Release build, clang-format, and git diff --check passed.

Checklist

• I have read and confirmed this PR follows the contribution guidelines.

Summary by CodeRabbit

• New Features
  • Eligible CUDA INT8 linear layers can now apply a 16-byte-aligned bias during computation. Other supported shapes continue using their existing scale-restoration paths.
• Documentation
  • Clarified how eligible INT8 linear layers handle scales and bias.

[coderabbitai]

Walkthrough

CUDA INT8 CUTLASS GEMM now supports aligned bias for eligible MAT operations. The GEMM epilogue adds bias after scale restoration. SwiGLU and packed-W4 plans reject bias, and split execution passes no bias. The CUDA documentation describes this behavior.

Changes: CUDA INT8 GEMM Bias

Layer / File(s)	Summary
GEMM bias epilogue and execution src/core/cuda/i8_gemm.h, src/core/cuda/i8_gemm.cu	The GEMM execution API accepts optional bias. Supported epilogues add bias after scaling; the launch checks bias alignment. SwiGLU and packed-W4 plans reject bias, and split execution passes no bias.
CUTLASS eligibility and integration src/core/cuda/int8.cu, docs/int8_convrot.md	CUTLASS permits bias for MAT operations when it is 16-byte aligned, passes the bias pointer to GEMM, and documents the bias behavior.

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Refactor

Sequence Diagram(s)

sequenceDiagram
  participant int8_cu as int8.cu
  participant cutlass_compatible
  participant sd_cuda_i8_gemm_execute
  participant GEMM_epilogue
  int8_cu->>cutlass_compatible: Check bias alignment
  cutlass_compatible-->>int8_cu: Return compatibility result
  int8_cu->>sd_cuda_i8_gemm_execute: Pass operation bias
  sd_cuda_i8_gemm_execute->>GEMM_epilogue: Launch biased GEMM
  GEMM_epilogue->>GEMM_epilogue: Restore scale, then add bias

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Possibly related PRs

• leejet/stable-diffusion.cpp#2290: Adds split SwiGLU execution to the CUDA INT8 GEMM implementation, which this change updates to pass no bias.
• leejet/stable-diffusion.cpp#2278: Adds packed-W4 mode, which this change keeps outside the bias-enabled GEMM configuration.
• leejet/stable-diffusion.cpp#2253: Introduces the CUDA INT8 extension and CUTLASS GEMM path updated here.

Suggested reviewers: leejet

Merge Risk: Minimal · up to 02afc

The reviewed CUDA bias path preserves the expected per-output bias behavior, with no identified issue requiring a fix before merge.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentat…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: fusing bias into INT8 CUTLASS epilogues.
Description check		The description includes every required section, explains the change, provides related discussion and verification details, and completes the checklist.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 8 functions across 3 files. (1 skipped: 1 unsupported.)

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

Finishing Touches

Generate unit tests (beta)

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Failed pre-merge checks:
- Docstring Coverage:
Write docstrings for the functions missing them to satisfy the coverage
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Commits

Reviewing files that changed from the base of the PR and between a3f46b6 and 02afc83.

Files selected for processing: 4

• docs/int8_convrot.md
• src/core/cuda/i8_gemm.cu
• src/core/cuda/i8_gemm.h
• src/core/cuda/int8.cu

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