The IQK quantized GEMM kernels (ggml/src/iqk/iqk_gemm_*.cpp) are gated
by HAVE_FANCY_SIMD in iqk_config.h, which requires five AVX-512 macros
to be defined: __AVX512F__, __AVX512VNNI__, __AVX512VL__, __AVX512BW__,
__AVX512DQ__. If they are not defined, the AVX-512 quantized matmul
path is skipped silently — no build warning, no runtime symptom, just
lower performance than the hardware can deliver. Surprises users on
Windows/MSVC where -march=native semantics are not propagated.
Adds a docs/build.md section that documents:
- Which macros gate which path (HAVE_FANCY_SIMD for quant GEMM,
__AVX512F__ alone for f16/f32, __AVX512BF16__ for bf16, __AVXVNNI__
for AVX2+VNNI-only CPUs).
- Linux/GCC: GGML_NATIVE=ON (default) handles this automatically on
Zen4 / Sapphire Rapids; just verify with objdump.
- Windows/MSVC and cross-compile: explicit GGML_ARCH_FLAGS with
-D__AVX512* defines is required.
- Note on Zen4 implementing AVX-512 as 256-bit double-pumped.
Documentation only — no code changes, no behavioural changes, no
new CMake options introduced.
418d60a909