[FFmpeg-devel] r9017 breaks WMA decoding on Intel Macs

[Matthieu CASTET]

Hi,

Trent Piepho <xyzzy <at> speakeasy.org> writes:

> I wouldn't consider it a workaround, but a better way to write the code to
> begin with.
> 
> The use of the asm("n+%0" :  "m"(foo)) syntax has a bug.  The asm block has an
> undeclared input or output.  The fact that the existing code works is simply
> luck that gcc's optimizer isn't able to do something to break it.
> 
> Consider something like this:
> int main(void)
> {
>     int x[2] = {1, 1};
>     asm("movl $42, %0 ; movl $42, 4+%0" : "=m"(x[0]));
>     printf("%d\n", x[1]);
> }
> 
You can declare the memory you are accessing with something like :

int main(void)
{
    int x[2] = {1, 1};
    struct { int a[2]; } *p = (void *)x;
    asm("movl $42, %0 \n\t movl $42, 4+%0" : 
            "+m" (*p)
       );
    printf("%d\n", x[1]);
}


Matthieu

PS : gcc manual [1] give a (ugly) syntax to make the struct cast in asm
input/ouput declaration, but I didn't manage to make it work for output.

[1]
If you know how large the accessed memory is, you can add it as input or output
but if this is not known, you should add `memory'. As an example, if you access
ten bytes of a string, you can use a memory input like:

     {"m"( ({ struct { char x[10]; } *p = (void *)ptr ; *p; }) )}.