[FFmpeg-cvslog] r24926 - trunk/libavcodec/x86/vp56dsp.asm

[Måns Rullgård]

Reimar D?ffinger <Reimar.Doeffinger at gmx.de> writes:

> On Thu, Aug 26, 2010 at 10:01:23PM +0100, M?ns Rullg?rd wrote:
>> Reimar D?ffinger <Reimar.Doeffinger at gmx.de> writes:
>> > On Thu, Aug 26, 2010 at 09:27:55PM +0100, M?ns Rullg?rd wrote:
>> >> Reimar D?ffinger <Reimar.Doeffinger at gmx.de> writes:
>> >> > But in case I was unclear, I actually managed to construct
>> >> > a case (of course very artificial) that shows the issue
>> >> > _in principle_ even with non-broken/ancient compilers.
>> >> >
>> >> > extern long j;
>> >> > extern char *array;
>> >> > int test(void)
>> >> > {
>> >> >   int i;
>> >> >   int s;
>> >> >   for (i = 0; i < j; i++)
>> >> >       s += array[i];
>> >> >   return s;
>> >> > }
>> >> > int test2(void)
>> >> > {
>> >> >   unsigned i;
>> >> >   int s;
>> >> >   for (i = 0; i < j; i++)
>> >> >       s += array[i];
>> >> >   return s;
>> >> > }
>> >> > int test3(void)
>> >> > {
>> >> >   unsigned long i;
>> >> >   int s;
>> >> >   for (i = 0; i < j; i++)
>> >> >       s += array[i];
>> >> >   return s;
>> >> > }
>> >> >
>> >> > Compiled on PowerPC 64 the three loops are:
>> >> > .L3:
>> >> >         lbzx 0,9,11
>> >> >         addi 11,11,1
>> >> >         add 0,0,3
>> >> >         extsw 3,0
>> >> >         bdnz .L3
>> >> >
>> >> > .L9:
>> >> >         lbzx 9,10,11
>> >> >         addi 0,11,1
>> >> >         rldicl 11,0,0,32
>> >> >         add 9,9,3
>> >> >         cmpd 7,11,8
>> >> >         extsw 3,9
>> >> >         blt 7,.L9
>> >> >
>> >> > .L14:
>> >> >         lbzx 0,9,11
>> >> >         addi 11,11,1
>> >> >         add 0,0,3
>> >> >         extsw 3,0
>> >> >         bdnz .L14
>> >> 
>> >> Yes, the compiler did badly with the 32-bit unsigned counter.  What
>> >> is that supposed to prove?
>> >
>> > That it seems like PPC64 also does not have a 32x32->32 addition
>> 
>> It doesn't have _unsigned_ 32-bit addition.
>
> Does it have a signed one? I assumed it just relied on signed
> overflow being undefined or something like that for the int case...

If the values are already sign-extended, there is no difference
between doing a 64-bit addition or a 32-bit addition followed by
sign-extend.

-- 
M?ns Rullg?rd
mans at mansr.com