Re: Syscall changes registers beyond %eax, on linux-i386

[J.A. Magallon]

On 2002.09.19 Richard B. Johnson wrote:
>On Thu, 19 Sep 2002, Richard Henderson wrote:
>
[...]
>> > It's really bad code because it could have done:
>> > 
>> > 	incl	$0x04(%esp)
>> > 	incl	$0x08(%esp)
>> > 	incl	$0x1c(%esp)
>> > 	jmp	bar
>> 
[...]
>
>It's a problem with a 'general purpose' compiler that wants to
>be "all things" to all people. If somebody made a gcc-compatible
>compiler, tuned to the ix86 characteristics, I think we could
>cut the extra instructions by at least 1/2, maybe more.
>

Curiosity killed the cat....
Just tried it with gcc-3.2.
C code:
extern void bar(int x, int y, int z);
void foo(const int a, const int b, const int c)
{
        bar(a+1, b+1, c+1);
}

- gcc -S -O0:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $8, %esp
        subl    $4, %esp
        movl    16(%ebp), %eax
        incl    %eax
        pushl   %eax
        movl    12(%ebp), %eax
        incl    %eax
        pushl   %eax
        movl    8(%ebp), %eax
        incl    %eax
        pushl   %eax
        call    bar
        addl    $16, %esp
        leave
        ret

- gcc -S -O1:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $12, %esp
        movl    16(%ebp), %eax
        incl    %eax
        pushl   %eax
        movl    12(%ebp), %eax
        incl    %eax
        pushl   %eax
        movl    8(%ebp), %eax
        incl    %eax
        pushl   %eax
        call    bar
        addl    $16, %esp
        movl    %ebp, %esp
        popl    %ebp
        ret

- gcc -S -O2:
        movl    12(%esp), %eax
        incl    %eax
        movl    %eax, 12(%esp)
        movl    8(%esp), %eax
        incl    %eax
        movl    %eax, 8(%esp)
        movl    4(%esp), %eax
        incl    %eax
        movl    %eax, 4(%esp)
        jmp     bar

- gcc -S -O2 -march=[i686,pentium2,pentium3]:
        incl    4(%esp)
        movl    8(%esp), %eax
        incl    %eax
        movl    %eax, 8(%esp)
        movl    12(%esp), %eax
        incl    %eax
        movl    %eax, 12(%esp)
        jmp     bar

- gcc -S -O2 -march=pentium4:
        movl    8(%esp), %eax
        addl    $1, 4(%esp)
        addl    $1, %eax
        movl    %eax, 8(%esp)
        movl    12(%esp), %eax
        addl    $1, %eax
        movl    %eax, 12(%esp)
        jmp     bar

-- 
J.A. Magallon <[e-mail]>      \                 Software is like sex:
werewolf.able.es                         \           It's better when it's free
Mandrake Linux release 9.0 (Cooker) for i586
Linux 2.4.20-pre7-jam0 (gcc 3.2 (Mandrake Linux 9.0 3.2-1mdk))