[PATCH][LoopVectorizer] Restrict the unroll factor of reductions in loops

[James Molloy]

Hi Arnold,

 

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.

 

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.

 

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.

 

Please take a look.

 

Cheers,

 

James 
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