[WIP] gemm block quantization for llm decoder style

[nihui]

• move int468 dequant code to x86/arm/... create_pipeline
• how to encode block_size and storage type ?
• try fp4 e2m1/e3 type ?
• comp table ?
• expand to more general gemm ?
• port union hack to platform-independent style
• gemm test++
• doc++

./ncnnllm2int468 qwen3_decoder.ncnn.param qwen3_decoder.ncnn.bin qwen3_decoder-int6.ncnn.param qwen3_decoder-int6.ncnn.bin

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 7.52688% with 86 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.88%. Comparing base (37336e7) to head (58cc1f3).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/layer/gemm.cpp	7.52%	86 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6439      +/-   ##
==========================================
+ Coverage   95.62%   95.88%   +0.26%     
==========================================
  Files         844      844              
  Lines      266761   266834      +73     
==========================================
+ Hits       255080   255859     +779     
+ Misses      11681    10975     -706     

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[github-actions]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	15316400	15324592	+8192 ⚠️
armhf	6229892	6234020	+4128 ⚠️
aarch64	9527616	9527536	-80 😘

[Copilot]

Pull request overview

This WIP pull request implements block quantization for GEMM layers to support 4-bit, 6-bit, and 8-bit quantization for LLM decoder-style models. The changes introduce a new quantization tool and corresponding dequantization logic in the GEMM layer implementation.

Key changes:

• New ncnnllm2int468 tool for quantizing GEMM weight matrices with configurable block sizes and bit widths
• Block-based quantization scheme using per-block scaling factors stored in B_data_quantize_scales
• Dequantization logic in gemm.cpp that converts quantized weights back to fp32 during model loading

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 15 comments.

Show a summary per file

File	Description
tools/quantize/ncnnllm2int468.cpp	New quantization tool implementing 4/6/8-bit block quantization with custom bit-packed storage formats
tools/quantize/CMakeLists.txt	Build configuration adding the new ncnnllm2int468 executable
tools/modelwriter.h	Extended serialization to save block quantization scales for int8_scale_term values 4/5/6
src/layer/gemm.h	Added B_data_quantize_scales member to store per-block scaling factors
src/layer/gemm.cpp	Implemented loading and dequantization logic for 4/6/8-bit block-quantized weights

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[Copilot]

The new ncnnllm2int468 executable is not added to the virtual project group or installed via ncnn_install_tool(), unlike ncnn2int8 above. This creates inconsistency in how tools are organized and installed.

[Copilot]

The i6x4_t union definition is duplicated in both the quantization tool and the dequantization code in gemm.cpp (lines 193-203). Consider moving this to a shared header to maintain consistency and avoid duplication.

[Copilot]

The __attribute__((packed)) attribute is GCC-specific and not portable. This will fail on MSVC. Consider using #pragma pack for cross-platform compatibility or conditionally compile based on compiler.

[Copilot]

The i4x2_t union definition is duplicated in both the quantization tool and the dequantization code in gemm.cpp (lines 264-272). Consider moving this to a shared header to maintain consistency and avoid duplication.

[Copilot]

The block_size value of 64 is hardcoded here and also duplicated in gemm.cpp (lines 143, 183, 254). Consider making it a named constant or passing it as a configurable parameter to avoid inconsistencies if this value needs to change.