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GFX1100-TG200: T5a vectorizes shared Q8_K quant superblock body #2821

Description

@ghazni101

Row: GFX1100-TG200

T5a vectorizes the QuantQ8KSBlock body in rocm_act_quant.h with 16-byte
vector loads, replacing the scalar per-element loop with a vectorized path.
This is the root-cause fix for lever-C's norm-quant fusion regression: the
scalar quant body was ~50us/launch, and T5a reduces it to <2us.

Always-on (not env-gated). Byte-identical output — the vectorization only
changes the instruction sequence, not the computed values.

A/B benchmark (interleaved, 5 pairs, Qwen3.5-4B Q4_K_M, 256 tokens)

Arm Median tok/s Delta ±stdev
T25 parent (all flags) 45.2
T5a feature (all flags) 51.4 +13.6% 0.18%

+13.6% over T25 parent. The largest single-lever improvement in the
campaign. Brings the full stack from -6.7% below baseline to +6.2% above
baseline
(51.4 vs 48.4 tok/s).

Validated by test_rocm_quant_dot (12/12 cases, 797 assertions).

FOLLOWING_AGENTS_PROTOCOL

Following-Agents-Protocol: true
AI-Assisted: true
Assisted-by: AGENT:GLM-5-2 [OMP]

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