The __AVXVNNI__ branches of dot_i8i8/dot_i4i8 (quant.h) accumulated every vpdpbusd
into a single register, a serial dependency chain (vpdpbusd is latency-bound ~5c).
Use 4 independent accumulators, mirroring the NEON 4-accumulator path in this file.
On x86 the tiled SMMLA _mm path is ARM-only, so these two dots are THE int8/int4
IDOT matmul kernels on x86 (g_idot=1 default), prefill and decode.
Integer accumulation is associative -> bit-identical (test_idot passes bit-for-bit).
Microbench (tests/bench_idot, i7-12700H P-core, I=6144):
dot_i8i8 6.85 -> 19.26 GB/s (2.81x)
dot_i4i8 3.41 -> 7.25 GB/s (2.13x)
cdhdt
c4b2cf6db0