[Bug target/108371] New: gcc for x86_64 may sign/zero extent arguments unnecessarily

[dhowells at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108371

            Bug ID: 108371
           Summary: gcc for x86_64 may sign/zero extent arguments
                    unnecessarily
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dhowells at redhat dot com
  Target Milestone: ---

When compiling for x86_64, bool, char and short arguments that are passed
directly to an argument of exactly the same type on another function with no
modification, e.g.:

   void __bio_release_pages(char mark_dirty);
   void bio_release_pages(char mark_dirty)
   {
        __bio_release_pages(mark_dirty);
   }

get sign/zero-extended unnecessarily.  In the case of the above code, it
compiles to:

   0:   40 0f be ff             movsbl %dil,%edi
   4:   e9 00 00 00 00          jmp    9 <bio_release_pages+0x9>

with "gcc -Os -c test.c".  Can the extension be optimised away?  Granted, the
upper bits bits of RDI could contain rubbish on entry to bio_release_pages(),
so sanitisation is not unreasonable - but on the other hand,
__bio_release_pages() would surely have to assume the same and do the same
sanitisation?

The toolchain used is the Fedora 37 system compiler:

gcc-12.2.1-4.fc37.x86_64
binutils-2.38-25.fc37.x86_64