[Bug tree-optimization/110459] New: Trivial on stack variable was not optimized away

[Bug tree-optimization/110459] New: Trivial on stack variable was not optimized away

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110459

            Bug ID: 110459
           Summary: Trivial on stack variable was not optimized away
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

Consider the example:

struct array {
    char data[4];
};

auto sample2(char c) {
  array buffer = {c, 0, 0, 0};
  return buffer;
}


With GCC-14 and -O2 it produces the following assembly:

sample2(char):
        xor     eax, eax
        mov     BYTE PTR [rsp-22], 0
        mov     WORD PTR [rsp-24], ax
        mov     eax, DWORD PTR [rsp-24]
        sal     eax, 8
        mov     al, dil
        ret


It could be further optimized to just:

sample2(char):
        movzx   eax, dil
        ret


Godbolt playground: https://godbolt.org/z/nxKhvo3ns

[Bug middle-end/110459] Trivial on stack variable was not optimized away

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110459

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
   Last reconfirmed|                            |2023-06-28
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed, there might be a dup of this linked to PR 101926 .

[Bug middle-end/110459] Trivial on stack variable was not optimized away

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110459

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |x86_64-*-*

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Also on x86_64 I see DECL_ALIGN getting in the way of NRV as we seem to up
alignment on locals aggressively but DECL_RETURN stays at what the ABI
mandates.  Of course in this case !aggregate_value_p (result,
current_function_decl) anyway, still this extra copy could be elided IMHO.

Usually this is the job of SRA.

We also perform some of this coalescing in the frontend/gimplifier IIRC,
not sure what prevents it here.