The grouped-int4 (fmt=4) loader hard-coded gs=128 at all three detection
sites (resident weights + the two expert-load paths: mmap and pread-slab).
A checkpoint converted with --group-size 64 (or any non-128 size) wrote a
valid file that the engine then misdetected as plain per-row int4 (fmt=2)
and read the wrong number of scales -> garbage output, silently.
The conversion path (convert_fp8_to_int4.py --group-size) already emits
arbitrary group sizes, and the compute kernel (matmul_i4_grouped) is
fully gs-generic (only constraint: gs multiple of 16, the AVX2 width).
The loader was the sole gap.
Replace the three hardcoded checks with one shared helper, detect_group_size(),
that derives gs from the scale-array byte count by probing candidate sizes
{16,32,48,64,96,128,192,256} finest-first. Data-driven: any listed size
just works; per-row int4 (ns == O*4) correctly returns gs=0 -> stays fmt=2.
Verified against real GLM-5.2 expert dims (gate/up O=2048,I=6144 and down
O=6144,I=2048): g64/g128/g256 all detect correctly, and per-row is never
misdetected as grouped. g128 (the only previously-supported size) is
unchanged -> no regression for existing checkpoints.
This unblocks the g64 lane that ZacharyZcR's #225 ablation identified as
beating shipped per-row int4 (-7.5pp vs -9.3pp) at ~19% fewer bits: the
converter can produce it, and now the engine can load it.
Refs #225
woolcoxm
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