serve: decode-split flash attention back on for --device hrx

[bong-water-water-bong]

Reverts the #148 default now that #176 fixed the cause (#123/#140).

Post-fix decode tok/s (llama-bench, interleaved runs on strixhalo):

model	ctx 0 on / off	ctx 512 on / off	ctx 2100 on / off
Qwen3-0.6B	286 / 296–306	254 / 220–223	140–158 / 118–122
Qwen3-Coder-30B-A3B	78–85 / 78–81	81 / 65–69	42–62 / 47–49
ZAYA1-8B	39–41 / 39–42	40–43 / 39–40	38–42 / 34–36

The old docs table (split slower on 0.6B) was measured while the race was live.

Checks: serve_e2e (vulkan, hrx, vulkan+hrx prefill) and registry tests pass. With the default, the launched llama-server has no GGML_HRX_DISABLE_DISPATCH; with ONEBIT_HRX_DECODE_SPLIT=0 it gets decode_split. Correctness with split on, under forced queue evictions on the #176 build: 0.6B 0/59 divergent, Coder-30B 8/8 with no NaN.

Behaviour change: ONEBIT_HRX_DECODE_SPLIT=0 is now the opt-out (it used to be =1 to opt in).

🤖 Generated with Claude Code

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Commit e6a9eb8

[github-actions]

PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

🎫 Ticket compliance analysis 🔶 148 - Partially compliant Compliant requirements: 1bit serve --device hrx now starts llama-server without GGML_HRX_DISABLE_DISPATCH=decode_split by default User-set values for GGML_HRX_DISABLE_DISPATCH still win ONEBIT_HRX_DECODE_SPLIT=0 turns the kernel off as intended --prefill-device hrx remains unchanged Non-compliant requirements: None Requires further human verification: None 176 - Partially compliant Compliant requirements: Moves third_party/llama.cpp to 00adc2b Includes the root-cause fix for HRX: decode-split multipass path intermittently faults (HSA_STATUS_ERROR_MEMORY_FAULT) above capacity 2048 #123 and HRX: decode_split_next_q8 gives wrong, nondeterministic attention at short context (up to 3.66 nats between identical requests) #140 All tests pass and performance improvements are confirmed Correctness verified under forced queue evictions Non-compliant requirements: None Requires further human verification: None 123 - Partially compliant Compliant requirements: The fix for the intermittent faults is implemented in the llama.cpp update The fix involves changing kernel.barrier<workgroup> to kernel.barrier<global> at 5 sites The fix also adds missing global barrier to undispatched standalone reduce_f32 kernels Non-compliant requirements: None Requires further human verification: None

⏱️ Estimated effort to review: 3 🔵🔵🔵⚪⚪

🧪 PR contains tests

🔒 No security concerns identified

⚡ Recommended focus areas for review Inverted Logic for Decode-Split Control The logic for controlling the decode_split flash attention kernel has been inverted. Previously, ONEBIT_HRX_DECODE_SPLIT=1 was used to enable the kernel, but now ONEBIT_HRX_DECODE_SPLIT=0 is used to disable it. This change in behavior should be clearly documented and tested to ensure backward compatibility and correct operation. if (device == "hrx" && keep_split && std::string(keep_split) == "0") env.push_back("GGML_HRX_DISABLE_DISPATCH=decode_split"); Documentation Update Required The documentation in docs/hrx.md needs to be updated to reflect the new default behavior where decode-split flash attention is enabled by default. The previous documentation indicated that it was disabled by default, which is no longer accurate. - **Decode-split flash attention is on.** `flash_attention_decode_split_next_q8` used to give nondeterministic attention on HRX0 ([#140](https://github.com/1bit-MONSTER/engine/issues/140)) and to fault Qwen3-Coder-30B-A3B, so `1bit serve --device hrx` turned it off (#148). Since llama.cpp `00adc2b` (see the #123 section below) it is correct, and it is as fast or faster: | model (Q4_K_M), tg tok/s | ctx 0 | ctx 512 | ctx 2100 | |---|---|---|---| | Qwen3-0.6B, decode-split on | 286 | 254 | 140–158 | | Qwen3-0.6B, off | 296–306 | 220–223 | 118–122 | | Qwen3-Coder-30B-A3B, on | 78–85 | 81 | 42–62 | | Qwen3-Coder-30B-A3B, off | 78–81 | 65–69 | 47–49 | | ZAYA1-8B, on | 39–41 | 40–43 | 38–42 | | ZAYA1-8B, off | 39–42 | 39–40 | 34–36 | (llama-bench `-p 0 -n 32/64`, runs interleaved on a shared strixhalo, 2026-09-27.) `ONEBIT_HRX_DECODE_SPLIT=0` turns it off; a `GGML_HRX_DISABLE_DISPATCH` you set yourself wins.