software pipelining

[Andrey Belevantsev]

Hi,

On 10.11.2010 12:32, roy rosen wrote:
> Hi,
>
> I was wondering if gcc has software pipelining.
> I saw options -fsel-sched-pipelining -fselective-scheduling
> -fselective-scheduling2 but I don't see any pipelining happening
> (tried with ia64).
> Is there a gcc VLIW port in which I can see it working?
You need to try -fmodulo-sched.  Selective scheduling works by default on 
ia64 with -O3, otherwise you need -fselective-scheduling2 
-fsel-sched-pipelining.  Note that selective scheduling disables autoinc 
generation for the pipelining to work, and modulo scheduling will likely 
refuse to pipeline a loop with autoincs.

Modulo scheduling implementation in GCC may be improved, but that's a 
different topic.

Andrey

>
> For an example function like
>
> int nor(char* __restrict__ c, char* __restrict__ d)
> {
>      int i, sum = 0;
>      for (i = 0; i<  256; i++)
>          d[i] = c[i]<<  3;
>      return sum;
> }
>
> with no pipelining a code like
>
> r1 = 0
> r2 = c
> r3 = d
> _startloop
> if r1 == 256 jmp _end
> r4 = [r2]+
> r4>>= r4
> [r3]+ = r4
> r1++
> jmp _startloop
> _end
>
> here inside the loop there is a data dependency between all 3 insns
> (only the r1++ is independent) which does not permit any parallelism
>
> with pipelining I expect a code like
>
> r1 = 2
> r2 = c
> r3 = d
> // peel first iteration
> r4 = [r2]+
> r4>>= r4
> r5 = [r2]+
> _startloop
> if r1 == 256 jmp _end
> [r3]+ = r4 ; r4>>= r5 ; r5 = [r2]+
> r1++
> jmp _startloop
> _end
>
> Now the data dependecy is broken and parlallism is possible.
> As I said I could not see that happening.
> Can someone please tell me on which port and with what options can I
> get such a result?
>
> Thanks, Roy.