[Ffmpeg-devel] [PATCH] wrong use of ff_get_fourcc in mpjg.c

[Måns Rullgård]

Reimar D?ffinger said:
> Hello,
> On Wed, Feb 07, 2007 at 01:45:25PM -0000, M?ns Rullg?rd wrote:
>> Reimar Doeffinger said:
> [...]
>> > AV_RB32 etc. in general when there are optimized variants maybe not, but a
>> macro
>> > that expands to
>> > ("ABCD")[0] | (("ABCD")[1] << 8) ...
>> > I'd expect to resolve to a constant...
>>
>> Well, it doesn't.  Believe me, I've tried it.
>
> Maybe your info is outdated?
> It does for me with gcc 4.1.1 and starting from -O1.
> program:
> int main(int argc, char *argv[]) {
> #define TAG(s) ((s)[0] | ((s)[1] << 8) | ((s)[2] << 16) | ((s)[3] <<
> 24))
>   int t = TAG(argv[0]);
>   return t == TAG("test");
> }
>
> generated asm (64 bit mode, -O1):
>   400498:       48 8b 0e                mov    (%rsi),%rcx
>   40049b:       0f be 31                movsbl (%rcx),%esi
>   40049e:       0f be 41 01             movsbl 0x1(%rcx),%eax
>   4004a2:       c1 e0 08                shl    $0x8,%eax
>   4004a5:       09 c6                   or     %eax,%esi
>   4004a7:       0f be 51 02             movsbl 0x2(%rcx),%edx
>   4004ab:       c1 e2 10                shl    $0x10,%edx
>   4004ae:       0f be 41 03             movsbl 0x3(%rcx),%eax
>   4004b2:       c1 e0 18                shl    $0x18,%eax
>   4004b5:       09 c2                   or     %eax,%edx
>   4004b7:       09 d6                   or     %edx,%esi
>   4004b9:       81 fe 74 65 73 74       cmp    $0x74736574,%esi
>   4004bf:       0f 94 c0                sete   %al
>   4004c2:       0f b6 c0                movzbl %al,%eax

That proves nothing.  Here's a proper test with gcc 4.1.1:

$ cat tag.c
#define TAG(s) ((s)[0] | ((s)[1] << 8) | ((s)[2] << 16) | ((s)[3] << 24))
int test = TAG("test");

int foo(int f)
{
    switch(f){
    case TAG("test"):
        return 0;
    }

    return 1;
}
$ gcc -O2 -c tag.c
tag.c:2: error: initializer element is not constant
tag.c: In function 'foo':
tag.c:7: error: case label does not reduce to an integer constant

The optimizer may well reduce it to a constant, but it's not a constant
expression at the C language level.

-- 
M?ns Rullg?rd
mru at inprovide.com