some quesion about assembly when calling fork

some quesion about assembly when calling fork

[loody]

Dear all:
i copy the assembly code as below, I use uclibc.

       if ((pid = fork()) < 0) {
    84b4:       ebffffb5        bl      8390 <.text-0x48>
    84b8:       e1a03000        mov     r3, r0
    84bc:       e50b3010        str     r3, [fp, #-16]    xx1
    84c0:       e51b3010        ldr     r3, [fp, #-16]    xx2
    84c4:       e3530000        cmp     r3, #0  ; 0x0
    84c8:       aa000002        bge     84d8 <main+0x3c>
                printf("fork error");
    84cc:       e59f0040        ldr     r0, [pc, #64]   ; 8514 <.text
+0x13c>
    84d0:       ebffffba        bl      83c0 <.text-0x18>
    84d4:       ea00000b        b       8508 <main+0x6c>

why we store r3 and ld it back at the same place in xx1 and xx2.
does that have any relationship with fork?
appreciate your help,
miloody

some quesion about assembly when calling fork

[Ben Dooks]

On Wed, Sep 16, 2009 at 10:23:40PM +0800, loody wrote:
> Dear all:
> i copy the assembly code as below, I use uclibc.
> 
>        if ((pid = fork()) < 0) {
>     84b4:       ebffffb5        bl      8390 <.text-0x48>
>     84b8:       e1a03000        mov     r3, r0
>     84bc:       e50b3010        str     r3, [fp, #-16]    xx1
>     84c0:       e51b3010        ldr     r3, [fp, #-16]    xx2
>     84c4:       e3530000        cmp     r3, #0  ; 0x0
>     84c8:       aa000002        bge     84d8 <main+0x3c>
>                 printf("fork error");
>     84cc:       e59f0040        ldr     r0, [pc, #64]   ; 8514 <.text
> +0x13c>
>     84d0:       ebffffba        bl      83c0 <.text-0x18>
>     84d4:       ea00000b        b       8508 <main+0x6c>
> 
> why we store r3 and ld it back at the same place in xx1 and xx2.
> does that have any relationship with fork?
> appreciate your help,

It is possible that the compiler is not being told to optimise enough
and that it is failing to spot that it can fold those together, or that
it think there is somewhere else in the code that could branch back
here.

-- 
Ben

Q:      What's a light-year?
A:      One-third less calories than a regular year.

some quesion about assembly when calling fork

[Alessandro Rubini]

>        if ((pid = fork()) < 0) {
>     84b4:       ebffffb5        bl      8390 <.text-0x48>
>     84b8:       e1a03000        mov     r3, r0
>     84bc:       e50b3010        str     r3, [fp, #-16]    xx1
>     84c0:       e51b3010        ldr     r3, [fp, #-16]    xx2
>     84c4:       e3530000        cmp     r3, #0  ; 0x0

You are not optimizing. So the compiler follows the statements
literally: first it stores the result of fork in pid. Then loads pid to
compare with zero.

If you optimize, I'm sure it won't even copy r0 to r3 before comparing.

/alessandro