Re: 3.0 Changes in ASM

[Erik Mouw <J dot A dot K dot Mouw at ITS dot TUDelft dot NL>]

On Sun, Feb 18, 2001 at 03:28:52AM -0600, Bill Wendling wrote:
> I was wondering about the changes in the assembly language for the
> following C code:
> 
> #include <stdio.h>
> 
> int main(void)
> {
>     printf("hello world\n");
>     return 0;
> }
> 
> GCC 2.95.2 outputs the following asm code:
> 
> main:
>         pushl %ebp
>         movl %esp,%ebp
>         subl $8,%esp
>         addl $-12,%esp
>         pushl $.LC0
>         call printf
>         addl $16,%esp
>         xorl %eax,%eax
>         jmp .L2
>         .p2align 4,,7
> .L2:
>         movl %ebp,%esp
>         popl %ebp
>         ret
> 
> while 3.0 outputs:
> 
> main:
>         pushl   %ebp
>         movl    %esp, %ebp
>         subl    $8, %esp
>         subl    $12, %esp
>         pushl   $.LC0
>         call    printf
>         addl    $16, %esp
>         movl    $0, %eax
>         movl    %ebp, %esp
>         popl    %ebp
>         ret
> 
> The question is, why the "movl $0, %eax" when an "xor" is typically
> faster (or so I'm lead to believe)?

Try compiling with -O2:

main:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $20, %esp
        pushl   $.LC0
        call    puts
        xorl    %eax, %eax
        movl    %ebp, %esp
        popl    %ebp
        ret

Apparently the faster code is only generated with -O2 or better.

(Good to see that gcc-3.0 combines the "subl $8,%esp; addl $-12,%esp"
pair from gcc-2.95.2 into a single "subl $20, %esp".)


Erik

-- 
J.A.K. (Erik) Mouw, Information and Communication Theory Group, Department
of Electrical Engineering, Faculty of Information Technology and Systems,
Delft University of Technology, PO BOX 5031,  2600 GA Delft, The Netherlands
Phone: +31-15-2783635  Fax: +31-15-2781843  Email: J.A.K.Mouw@its.tudelft.nl
WWW: http://www-ict.its.tudelft.nl/~erik/