ggml-hrx: fence global F32 output before Qwen Q8 packing - #174

Overview

Fence global F32 stores before the fused Qwen MoE normalization kernel reloads them for Q8 packing. Producer and packer workitems own different channel ranges; the existing workgroup-memory barrier does not order their global-memory accesses. This follows the same fix used for decode attention in #154.

The change replaces one barrier memory domain and documents why. It preserves the existing arithmetic and 32-byte LDS allocation.

Validation

On RX 7900 XTX (gfx1100), HRX 82214df, upstream base 930e43fe9, flash attention and F16 KV:

• The retained model-trace checker fails on original captures and passes on six fresh fence-only captures. All 80 Q8 groups match canonical packing and repeat identically. The model-specific harness is retained as investigation evidence, outside this PR.
• Qwen3-Coder 30B-A3B Q4_K_M full WikiText-2: 146 chunks, 149,358 scored tokens, 49/49 GPU layers. HRX PPL 8.1441 ± 0.06075; Vulkan 8.1470 ± 0.06078. All 146 printed HRX cumulative rows equal the previously validated shared-memory candidate.
• Same-window five-repeat median pp256: fence 3589.39, shared-memory candidate 3589.78 tok/s (-0.011%). Median tg128: 223.994 vs 223.642 tok/s (+0.157%).
• Qwen3-30B shared-consumer smoke: two-token prompt exercised the Q8 wrapper, twelve-token prompt exercised the F32 wrapper; each produced identical greedy output for two HRX runs and Vulkan. A separate one-token decode path did not select the changed wrapper and produced differing text; that result is excluded from the wrapper validation claim and remains unresolved.
• Fence LDS 32 B, versus 8,224 B for the shared-memory candidate; SGPR24/VGPR54 for both.

Requirements

• Contributing guidelines have been read by the agent; human review and acceptance remain pending for this draft.
• AI usage disclosure: YES — Codex assisted with implementation, review, validation, and this PR description. Submitted as a draft for human review.