[Bug rtl-optimization/110479] New: Unnecessary register move

[Bug rtl-optimization/110479] New: Unnecessary register move

[tkoenig at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110479

            Bug ID: 110479
           Summary: Unnecessary register move
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tkoenig at gcc dot gnu.org
  Target Milestone: ---

May be related to / a dup of PR110240.

The function

unsigned int bar(unsigned int a)
{
  return 1u << (((a >> 10) & 3) + 3);
}

is compiled, with a relatively recent trunk and -O3, to

bar:
.LFB12:
        .cfi_startproc
        shrl    $10, %edi
        movl    $1, %eax
        movl    %edi, %ecx
        andl    $3, %ecx
        addl    $3, %ecx
        sall    %cl, %eax
        ret

where the register move seems unnecessary.

[Bug rtl-optimization/110479] Unnecessary register move

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110479

--- Comment #1 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Thomas Koenig from comment #0)

>         movl    %edi, %ecx

This one? It is needed because SAL wants its count argument in %cl and first
argument is passed in %edi (mandated by x86_64 ABI).

With -mbmi2, one gets:

        shrl    $10, %edi
        movl    $1, %eax
        andl    $3, %edi
        addl    $3, %edi
        shlx    %edi, %eax, %eax
        ret

[Bug rtl-optimization/110479] Unnecessary register move

[tkoenig at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110479

Thomas Koenig <tkoenig at gcc dot gnu.org> changed:

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

--- Comment #2 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
(In reply to Uroš Bizjak from comment #1)
> (In reply to Thomas Koenig from comment #0)
> 
> >         movl    %edi, %ecx
> 
> This one? It is needed because SAL wants its count argument in %cl and first
> argument is passed in %edi (mandated by x86_64 ABI).
> 
> With -mbmi2, one gets:
> 
>         shrl    $10, %edi
>         movl    $1, %eax
>         andl    $3, %edi
>         addl    $3, %edi
>         shlx    %edi, %eax, %eax
>         ret

Hm, you're right.  The intricacies of x86...

Closing.