[Bug c++/100171] New: autovectorizer

["g.peterhoff@t-online.de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100171

            Bug ID: 100171
           Summary: autovectorizer
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: g.peterhoff@t-online.de
  Target Milestone: ---

Hello gcc team,
I once wrote a small test case to show the problems with the autovectorizer
https://godbolt.org/z/xs35P45MM . In particular, the += operator is not
vectorized. The + operator works in the same context. I do not understand that.
If you decrement the arraysize in foo from 2 to 1 it doesn't work at all
anymore - scalar operations are always generated for ARR_2x.
In general, I made the experience that the autovectorizer starts much too late.
It should always do this from 2 values, even if these are much smaller than a
simd register. This also saves a lot of memory accesses - especially when the
data is linear in the memory (as in the example). Usually, however,
vectorization is only carried out when the data is at least as large as a simd
register, but often only when it is twice or even four times as large.
I think you should urgently update/optimize the autovectorizer.

thx & regards
Gero