[Bug rtl-optimization/64907] New: Suboptimal code (saving rbx on stack in order to save another reg in rbx)

[vda.linux at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64907

            Bug ID: 64907
           Summary: Suboptimal code (saving rbx on stack in order to save
                    another reg in rbx)
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vda.linux at googlemail dot com

void put_16bit(unsigned short v);
void put_32bit(unsigned v)
{
        put_16bit(v);
        put_16bit(v >> 16);
}

With gcc 4.7.2 the above compiles to the following assembly:

put_32bit:
        pushq   %rbx
        movl    %edi, %ebx
        andl    $65535, %edi
        call    put_16bit
        movl    %ebx, %edi
        popq    %rbx
        shrl    $16, %edi
        jmp     put_16bit

Code saves %rbx on stack only in order to save %edi to %ebx.
A simpler alternative is to just save %rdi on stack:

put_32bit:
        pushq   %rdi
        andl    $65535, %edi
        call    put_16bit
        popq    %rdi
        shrl    $16, %edi
        jmp     put_16bit