123456789bitwise: 0 of 40 (pos0, ntok) cases differ PASSdevice Apple M5: GPU us per dispatch, median of 9 paired rounds of 32 (ref / this) pos0 512 ntok 1 attn 14.79 / 14.79 merge 3.71 / 2.83 pos0 512 ntok 4 attn 50.16 / 50.18 merge 8.29 / 6.55 pos0 2048 ntok 1 attn 50.46 / 50.53 merge 10.13 / 6.23 pos0 2048 ntok 4 attn 190.94 / 190.92 merge 45.72 / 43.38 pos0 8192 ntok 1 attn 50.71 / 51.02 merge 9.94 / 6.06 pos0 8192 ntok 4 attn 193.76 / 193.71 merge 45.88 / 43.66