https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120893
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2025-06-30
Status|UNCONFIRMED |NEW
Blocks| |53947
Ever confirmed|0 |1
Keywords| |missed-optimization
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
vect_build_slp_tree_1 handles references to existing vector elements and
rejects combining from two such. vect_build_slp_tree_2 handles a single vector
source
as extern unpermuted vector + VEC_PERM. Extending this to exactly _two_
vector sources (re-use the two_operands flag maybe?) should be trivial
given we already emit a VEC_PERM.
Note this interacts with permuting operands upthread which might be able
to permute the graph to a shape where all existing vectors appear on
their own, like { a[0] + b[0], b[1] + a[1] } vs. { a[0] + b[0], a[1] + b[1] }
where we obviously prefer to vectorize as a + b rather than
perm (a, b, { 0, 2 }) + perm (a, b { 3, 1 }).
While we have a permute optimization phase that does not have the power
to "undo" such permute.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations
