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</o:shapelayout></xml><![endif]--></head><body lang=EN-GB link=blue vlink=purple><div class=WordSection1><p class=MsoNormal>Hi Arnold,<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Attached are two patches. The first ups the maximum unroll factor on AArch64 from 2 to 4, for C-A57 only at the moment as that’s all I’ve got data for. This gives us significant wins – ~14% on 462.libquantum at least.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>However it also causes some regressions. The second patch makes the loop vectorizer a bit more conservative with its unroll factor. The problem is purely for reductions within loops. The regressions I’ve seen are small (but runtime-known) trip count loops within a loop nest. A loop unroll factor of 2 is fine, but above 2 the reduction variable fixup logic after the loop increases the critical path length and resource usage. For most loops this isn’t a problem, but small loops in a larger loop nest will execute this fixup code many times.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>The heuristic is: if this is a (scalar) reduction, and the loop is nested, clamp the UF to a maximum of 2. With 2, we still get wins but we only add one fadd/fmul to the critical path.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Please take a look.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Cheers,<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>James <o:p></o:p></p></div></body></html>