[Bug target/93089] New: Force mprefer-vector-width=512 in 'e' simd clones

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93089

            Bug ID: 93089
           Summary: Force mprefer-vector-width=512 in 'e' simd clones
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
  Target Milestone: ---

The 'e' mangled simd clones that use 512-bit arguments/return value won't be
really called if the caller isn't preferring 512-bit vectors, so it seems much
better not to emit terrible code in those functions by trying to use only
256-bit vectors in there, which isn't fully possible anyway, as it needs to
extract the arguments from 512-bit vectors and store the result there too.
Unfortunately, we don't even allow prefer-vector-width= in target attribute.