[ENGINE] CCCL kernel_scan.cuh dual-algorithm dispatch: v1 (online norm) vs v2 (deferred norm)
Source: cccl_upstream/cub/cub/device/dispatch/kernels/kernel_scan.cuh
Target: vllm/attention/ops/prefix_prefill.py
CCCL kernel_scan.cuh implements compile-time algorithm selection:
- lookback: AgentScan with delay_constructor_t (safe default)
- lookahead: warpspeed pipeline stages (SM90+, deferred reduction)
Applied to prefix_prefill Triton kernels:
- _fwd_kernel (v1) = lookback: online softmax norm per block
- _fwd_kernel_flash_attn_v2 = lookahead: deferred normalization
Saves (ctx_len / BLOCK_N) divisions per query row.
Before: v2 kernel NEVER called — dead code since initial commit.
After: v2 dispatched for standard Qwen3.6 path (no alibi, no sliding
window, power-of-2 head_dim, no FP8).
BI-V100: v2 saves 64 fdiv/row at ctx_len=4096, BLOCK_N=64.
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