[Bug target/79261] New: vec_xxpermdi appears to have endian issues

[wschmidt at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79261

            Bug ID: 79261
           Summary: vec_xxpermdi appears to have endian issues
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wschmidt at gcc dot gnu.org
  Target Milestone: ---

Use of the vec_xxpermdi built-in appears to work incorrectly on little endian;
it seems to be using big-endian semantics.  This may be working as designed
(direct access to the instruction), but if so it is a confusing implementation.
 Note that this is not an official API function as defined in the ELFv2 ABI
appendices, so the semantics are not well-defined.

This should either be fixed to use little-endian semantics, or documented
carefully in the GCC user manual.  Since it has not been previously
well-documented, we probably have the flexibility to change the implementation.