Re: Software pipelining capabilities

[Vasanth <geekvasanth at gmail dot com>]

Hi Ayal,

Thanks for the inputs, I will try this on GCC 4.0, which sounds quite
interesting.

regards,
Vasanth

On 6/15/05, Ayal Zaks <ZAKS@il.ibm.com> wrote:
> 
> 
> 
> 
> > Vasanth <geekvasanth@gmail.com>
> 
> > I am using powerpc-eabi-gcc (3.4.1) and trying to retarget it for a
> > fully pipelined FPU. I have a DFA model for the FPU. I am looking at
> > the code produced for a simple FIR algorithm (a loop iterating over an
> > array, with a multiply-add operation per iteration). (I am not using
> > the fused-madd)
> >
> > for (i = 0; i < 64; i++)
> >  accum = z[i] * h[i];
> >
> > I have the FIR loop partially unrolled, yet am not seeing the multiply
> > from say, iteration i+1, overlapping with the multiply from iteration
> > i. From the scheduling dumps, I do see that the compiler knows that
> > each use of the multiply is incurring the full latency of the multiply
> > instead of having reduced latency by pipelining in software. The adds
> > are also completely linked by data flow and the compiler does not seem
> > to be using temporary registers to be able to exploit executing some
> > of the adds in parallel. Hence, each add is stalled on the previous
> > add.
> >
> > fadds   f5,f0,f8
> > fadds   f4,f5,f6
> > fadds   f2,f4,f11
> > fadds   f1,f2,f3
> > fadds   f11,f1,f13
> >
> > The register pressure is not very high. Registers f15-f31 are not used at
> all.
> 
> To break the linkage between the adds, try to keep the original loop
> (instead of partially unrolling it yourself) and use -funroll-loops
> -fvariable-expansion-in-unroller --param
> max-variable-expansions-in-unroller=8 (or some other number greater than 1
> but small enough to avoid spills).
> 
> (see http://gcc.gnu.org/ml/gcc/2004-09/msg01554.html)
> 
> This too was introduced in GCC 4.0.
> Ayal.
> 
> 
> >
> > My question is, am I expecting the wrong version of GCC to be doing
> > this. I saw the following thread about SMS.
> >
> > http://gcc.gnu.org/ml/gcc/2003-09/msg00954.html
> >
> > that seems relevant. Would GCC 4.x be a better version for my
> > requirement? If not, any ideas would be greatly appreciated.
> >
> > thanks in advance,
> > Vasanth
> 
>