https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109072
rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Ever confirmed|0 |1
Last reconfirmed| |2023-03-09
CC| |rguenth at gcc dot gnu.org
--- Comment #1 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org>
---
(In reply to Tamar Christina from comment #0)
> The SLP costs went from:
>
> Vector cost: 2
> Scalar cost: 4
>
> to:
>
> Vector cost: 12
> Scalar cost: 4
>
> it looks like it's no longer costing it as a duplicate but instead 4 vec
> inserts.
We do cost it as a duplicate, but we only try to vectorize up to
the stores, rather than up to the load back. So we're costing
the difference between:
fmov s1, s0
stp s1, s1, [x0]
stp s1, s1, [x0, 8]
(no idea why we have an fmov, pretend we don't) and:
fmov s1, s0
dup v1.4s, v1.s[0]
str q1, [x0]
If we want the latter as a general principle, the PR is
easy to fix. But if we don't, we'd need to make the
vectoriser start at the load or (alternatively) fold
to a constructor independently of vectorisation.
