[Bug c/100301] New: sum of __int128 - regression since 8.2

[Bug c/100301] New: sum of __int128 - regression since 8.2

[dushistov at mail dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100301

            Bug ID: 100301
           Summary: sum of __int128 - regression since 8.2
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dushistov at mail dot ru
  Target Milestone: ---

For such simple function:

__int128 add1(__int128 a, __int128 b) {
    return a + b;
}

gcc 8.2 generates for a + b:

        mov     r9, rdi
        mov     r10, rsi
        add     r9, rdx
        adc     r10, rcx
        mov     rax, r9
        mov     rdx, r10
        ret
and for b + a:

add1(__int128, __int128):
        mov     rax, rdx
        mov     rdx, rcx
        add     rax, rdi
        adc     rdx, rsi
        ret

but gcc 11.1 generates for both cases:
add1(__int128, __int128):
        mov     r9, rdi
        mov     rax, rdx
        mov     r8, rsi
        mov     rdx, rcx
        add     rax, r9
        adc     rdx, r8
        ret

4 moves instead of 2.

Recent versions of clang and icc generates the same number and type of
instruction of both cases,
and only 2 moves and 2 additions:

icc:
add1(__int128, __int128):
        add       rdi, rdx
        mov       rax, rdi
        adc       rsi, rcx
        mov       rdx, rsi
        ret        

add1(__int128, __int128):
        add       rdx, rdi
        mov       rax, rdx
        adc       rcx, rsi
        mov       rdx, rcx
        ret                                       

clang:

add1(__int128, __int128):
        mov     rax, rdi
        add     rax, rdx
        adc     rsi, rcx
        mov     rdx, rsi
        ret

add1(__int128, __int128): 
        mov     rax, rdi
        add     rax, rdx
        adc     rsi, rcx
        mov     rdx, rsi
        ret

Not sure is this target specific issue,
I get this example from this article: https://habr.com/ru/post/554760/ ,
where risc-v gcc 8.2.0 has similar problem that "a + b" and "b + a" uses
different number of instructions.

[Bug target/100301] sum of __int128 - regression since 8.2

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100301

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

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
GCC has always had issues with double register integer types and sometimes the
register allocator choses worse and sometimes better.

*** This bug has been marked as a duplicate of bug 97756 ***