[Bug middle-end/95014] New: gcc fails to merge two identical returns

[Bug middle-end/95014] New: gcc fails to merge two identical returns

[rafael at espindo dot la]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95014

            Bug ID: 95014
           Summary: gcc fails to merge two identical returns
           Product: gcc
           Version: 10.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rafael at espindo dot la
  Target Milestone: ---

Given

#include <new>
union any {
    any(any&& x) noexcept {
        if (x.st < 4) {
            st = x.st;
            x.st = 0;
        } else {
            ex = x.ex;
            x.ex = nullptr;
        }
    }
    long st;
    void* ex;
};
void f(any* a, any* b) {
    new (a) any(std::move(*b));
}

gcc produces

        movq    (%rsi), %rax
        movq    %rax, (%rdi)
        movq    $0, (%rsi)
        cmpq    $3, %rax
        jg      .L2
        ret
.L2:
        ret


clang produces

        movq    (%rsi), %rax
        cmpq    $3, %rax
        movq    %rax, (%rdi)
        movq    $0, (%rsi)
        retq

So clang has a redundant cmpq and gcc has the cmpq and two identical branch
destinations.

[Bug middle-end/95014] gcc fails to merge two identical returns

[vanyacpp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95014

Ivan Sorokin <vanyacpp at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vanyacpp at gmail dot com

--- Comment #1 from Ivan Sorokin <vanyacpp at gmail dot com> ---
I think this might be a duplicate of 82689.

[Bug middle-end/95014] gcc fails to merge two identical returns

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95014

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-08-01
           Severity|normal                      |enhancement

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

Note I have not looked into why but on aarch64-linux-gnu GCC can remove the
cmp/jump.

[Bug middle-end/95014] gcc fails to merge two identical returns

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95014

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note cross jumping on the RTL level is what is able to combine both basic
blocks.