[leejet]

Summary

• Add NV cooperative matrix kernels for W4A4 and W4A8 while keeping weights packed.
• Use direct W4A4 decoding with separate full-tile and tail specializations, plus double-buffered tiles for other shapes.
• Reuse W4A8 activation fragments and cache FP8 scales while preserving per-group scale restoration.
• Select kernels by device capabilities, alignment, and shared-memory limits. Retain the packed dot-product fallback and add GGML_VK_W4_FORCE_DP4A.

Related Issue / Discussion

N/A

Additional Information

Hardware: NVIDIA GeForce RTX 4090 24 GB, NVIDIA driver 596.36.

Build: Windows, Vulkan SDK 1.4.328.1, Release, NV cooperative matrix 2 enabled.

DiT files

Model	Format	DiT file
Krea2 Turbo	W4A4	krea2_turbo_int4_tensorwise_mixed.safetensors
Krea2 Turbo	INT8	krea2_turbo_int8_convrot.safetensors
P-H3 Image	W4A8	p-h3-image_w4a8.safetensors
P-H3 Image	INT8	p-h3-image_int8_convrot.safetensors

Sampling throughput

Sampling throughput in it/s; higher is better. Before/after uses the same INT4 weights:

Model	Resolution	sd.cpp before	sd.cpp after	Improvement
Krea2 Turbo W4A4	512×512	2.113	7.855	+271.69%
P-H3 Image W4A8	2912×2912	0.119	0.351	+194.62%

INT8 reference

INT8 reference using the same inference settings:

Model	Optimized INT4	INT8
Krea2 Turbo	7.855	9.224
P-H3 Image	0.351	0.571

Configuration and measurement

• Vulkan backend, batch 1, CFG 1, Euler, seed 42, CPU offloading enabled.
• Automatic flash attention enabled on Vulkan0 (NVIDIA RTX).
• Eight steps per run; all eight execute. Throughput is 6 divided by the total logged duration of steps 3–8, excluding steps 1–2.
• Krea2 uses the discrete scheduler, Qwen3-VL-4B-Instruct-Q4_K_M.gguf, and wan_2.1_vae.safetensors.
• P-H3 uses --sigmas p-h3-image --rng cpu --max-vram 20, qwen3vl_4b_bf16.safetensors, p-h3-image_text_connector.safetensors, and p-h3-image_vae_bf16.safetensors.
• P-H3 before, after, and INT8 reference runs all use --max-vram 20.

Checklist

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

Summary by CodeRabbit

• Performance
  • Added optimized Vulkan processing for supported packed INT4 workloads, with device-specific fallbacks for other configurations.
  • Added cooperative-matrix paths for INT4 weights with INT4 or INT8 activations.
• Documentation
  • Updated Vulkan INT4 documentation with supported processing paths, fallback behavior, and notes on possible differences in floating-point accumulation.

[coderabbitai]

Walkthrough

The Vulkan INT8 backend adds cooperative-matrix shaders for packed W4A4 and W4A8 operations. Device capability checks and runtime conditions determine when these shaders run; other cases retain the DP4A path.

Changes: Vulkan W4 cooperative-matrix paths

Layer / File(s)	Summary
W4A4 cooperative-matrix kernels src/core/vulkan/shaders/mul_mat_w4a4_convrot_cm2.comp, src/core/vulkan/shaders/mul_mat_w4a4_convrot_decode_cm2.comp	The two W4A4 shaders unpack packed values, accumulate cooperative-matrix products, apply scales and optional bias, and store results. The decode shader uses additional shape and alignment conditions for its specialized path.
W4A8 cooperative-matrix kernel src/core/vulkan/shaders/mul_mat_w4a8_convrot_cm2.comp	The shader stages packed weights and activations, computes grouped integer products, applies group scales during floating-point accumulation, and stores dequantized results.
Shader registration and runtime selection src/core/vulkan/CMakeLists.txt, src/core/vulkan/int8.cpp, docs/int8_convrot.md	CMake registers the three CM2 shader variants. Device checks and multiply_w4 select eligible paths, with GGML_VK_W4_FORCE_DP4A disabling CM2 selection. The documentation describes supported paths, conditions, and fallbacks.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant VulkanDevice as Vulkan device properties
  participant DeviceInfo as device_info
  participant MultiplyW4 as multiply_w4
  participant CM2Shader as W4 CM2 shader
  VulkanDevice->>DeviceInfo: provide cooperative-matrix properties
  DeviceInfo->>MultiplyW4: set per-device CM2 support flags
  MultiplyW4->>CM2Shader: dispatch when alignment and shape conditions pass

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Possibly related PRs

• leejet/stable-diffusion.cpp#2276: Adds the packed INT4 ConvRot operation and baseline kernels that these cooperative-matrix paths accelerate.
• leejet/stable-diffusion.cpp#2278: Implements related packed W4A4 and W4A8 matrix multiplication paths for CUDA.
• leejet/stable-diffusion.cpp#2253: Moves Vulkan INT8 registration and shader generation into the backend-extension framework used by this change.

Merge Risk: Minimal · up to 5c8ae

The change adds optional cooperative-matrix fast paths for packed INT4 on supported Vulkan devices. Other devices keep the dot-product fallback, and no concrete merge-blocking defect was identified.

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 describes the packed INT4 cooperative-matrix acceleration.
Description check		The description includes all template sections, implementation details, and benchmark information.
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 2 functions across 1 files. (5 skipped: 5 unsupported.)

Resolution

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

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Failed pre-merge checks:
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Commits

Reviewing files that changed from the base of the PR and between f5df5f8 and 5c8ae89.

Files selected for processing: 6

• docs/int8_convrot.md
• src/core/vulkan/CMakeLists.txt
• src/core/vulkan/int8.cpp
• src/core/vulkan/shaders/mul_mat_w4a4_convrot_cm2.comp
• src/core/vulkan/shaders/mul_mat_w4a4_convrot_decode_cm2.comp
• src/core/vulkan/shaders/mul_mat_w4a8_convrot_cm2.comp

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