[Bug c++/110793] New: regression in optimizing unused string

[Bug c++/110793] New: regression in optimizing unused string

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110793

            Bug ID: 110793
           Summary: regression in optimizing unused string
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Consider this reduced, silly-looking example:

#include <string>

inline int zero(std::string const& = {}) { return 0; }

int main() {
    return zero();
}

In several versions of gcc (12.2, 11.4, 10.5), this simply compiles to:

main:
        xor     eax, eax
        ret

But in 13.1 (C++17, but not C++20), we get:

main:
        sub     rsp, 40
        lea     rax, [rsp+16]
        mov     rdi, rsp
        mov     BYTE PTR [rsp+16], 0
        mov     QWORD PTR [rsp], rax
        mov     QWORD PTR [rsp+8], 0
        call    std::__cxx11::basic_string<char, std::char_traits<char>,
std::allocator<char> >::_M_dispose()
        xor     eax, eax
        add     rsp, 40
        ret

Notably, 13.1 C++20 and C++23 also compiles to just xor. 

I'm not sure if this is a language or library issue, so tagging it language for
now.

[Bug c++/110793] regression in optimizing unused string

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110793

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Barry Revzin from comment #0)
> Notably, 13.1 C++20 and C++23 also compiles to just xor. 
> 

Probably because we don't use the extern template declarations for C++20.

[Bug ipa/110793] regression in optimizing unused string

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110793

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |marxin at gcc dot gnu.org
          Component|c++                         |ipa

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Actually this is more likely inlining heuristics that know that main is only
called once ever and does not inline as much into it.

Try renaming main to foo and try again b

[Bug ipa/110793] regression in optimizing unused string

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110793

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #2)
> Actually this is more likely inlining heuristics that know that main is only
> called once ever and does not inline as much into it.
> 
> Try renaming main to foo and try again b

Yes it is this heurstic.
Renaming main to foo produces:
foo():
.LFB1395:
        .cfi_startproc
        xorl    %eax, %eax
        ret

For all optimization levels.

[Bug ipa/110793] regression in optimizing unused string

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110793

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #3)
> For all optimization levels.

I mean for all C++ standards.