clm-0050 — claim — HALOBENCH

On gfx1151 at f16 KV, stock Vulkan beats stock ROCm in EVERY cell of a matched matrix (one binary commit 3653e6d, one model, depths 0 to 131,072): decode +19-21% at every depth, prefill +4% to +20% growing with depth to 65k. The r/LocalLLaMA claim that ROCm leads Vulkan 3.5x at 65k depth — measured on an RDNA2 V620 — inverts on this chip. The community fork (v0.6.1) over stock Vulkan is a prefill-only win at f16 KV: +13/+13/+1/+2.5/+16% by depth, decode unchanged (±1%), and no stride bug (41/41 layers on GPU, CPU utilisation identical to stock).

METHOD — three arms, one model, matched flags. A = stock llama.cpp 3653e6d ROCm, B = stock 3653e6d Vulkan, C = fork v0.6.1 commit 3be50ccc2 with its bundled RADV 26.3.0-devel. Qwen3.6-35B-A3B UD-Q4_K_XL, fa on, pp1024/tg256, llama-bench defaults otherwise (-b 2048 / -ub 512), median of 3 fresh-process reps with page cache dropped between reps; 72/72 reps rc=0 across both KV matrices. One benign outlier rep in the matrix (median absorbs it). f16 KV, pp/tg by depth: | depth | A ROCm | B Vulkan | C fork | |---|---|---|---| | 0 | 1068.1 / 51.1 | 1110.5 / 61.6 | 1251.5 / 61.8 | | 4,096 | 962.6 / 49.7 | 1020.0 / 59.3 | 1149.5 / 59.9 | | 32,768 | 595.8 / 42.3 | 697.7 / 50.4 | 705.4 / 51.0 | | 65,536 | 412.5 / 36.4 | 495.1 / 43.5 | 507.3 / 43.9 | | 131,072 | 253.6 / 28.5 | 278.3 / 34.2 | 321.6 / 34.5 | The counter-claim this answers: a r/LocalLLaMA report of ROCm leading 3.5x at 65k depth was measured on a V620 (RDNA2 — a different GPU family). At 65k on gfx1151 the same comparison reads Vulkan +20% prefill / +19.5% decode. Backend verdicts do not transfer across GPU families; this claim is scoped to gfx1151. FORK ATTRIBUTION CAVEAT: arm C differs from arm B in two ways at once — the fork's kernels AND the newer bundled Mesa (RADV 26.3.0-devel vs system RADV). The d0/d4096 prefill edge is plausibly part-Mesa; the cells are not single-variable the way clm-0022's cherry-pick was. Decode being flat (±1%) at f16 is expected per the fork docs for an hd256 MoE — hd128 models are documented to gain more, so these fork deltas are a FLOOR, not the fork's headline. Single model tested; medium confidence until a second model (ideally hd128 dense) fills the matrix. Stride-bug check: v0.6.1 shows no stride bug — 41/41 layers verified on GPU, CPU% identical to stock during runs. RELIABILITY COUNTERWEIGHT (2026-08-13): llama.cpp issue 25664 (open, field reports incl. Strix Halo) documents Vulkan/RADV DeviceLostError at ~80k context on this hardware class - our matrix ran clean to 131k, but a naive "switch to Vulkan" operational conclusion should carry this open issue until it resolves. CHALLENGER ON RECORD (2026-08-13): stew675's public rdna-boosts branch (kernel-fusion campaign, see clm-0044) claims to cut stock ROCm's decode deficit to ~3-5% and take prefill +36.5% past Vulkan with BF16 KV. Untested here; a rdna-boosts-vs-stock-Vulkan rerun of this matrix is queued.

Note — the record's own working