[Bug libfortran/78379] New: Processor-specific versions for matmul

[tkoenig at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78379

            Bug ID: 78379
           Summary: Processor-specific versions for matmul
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: libfortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tkoenig at gcc dot gnu.org
  Target Milestone: ---

Now that a patch for PR51119 is in, we can think about
inserting processor-specific versions.

target_clones looks to be a good idea for this, see

https://gcc.gnu.org/onlinedocs/gcc-6.2.0/gcc/Common-Function-Attributes.html

There are still a few issues to be resolved, for example which
architectures to chose. Also, selecting an architecture which does
not exist on the platform leads to errors, so we probably need
to guard with appropriate #ifdefs.

A wrapper function to call the actual matmul is probably a good idea,
because it is the caller who generates the code to select.