On Thu, Dec 17, 2009 at 1:07 PM, Revital1 Eres <e...@il.ibm.com> wrote:
>
> Hello,
>
> Is there a way to pass to the unroller the maximum number of iterations
> of the loop such that it can decide to avoid unrolling if
> the maximum number is small.
>
> To be more specific, I am referring to the following case:
> After the vectorizer decides to peel for alignment
> it creates three loops:
> [1] scalar loop - the prologue to align
> memory access.
> [2] the vecorized loop
> [3] scalar loop - the remaining scalar computations.
>
> If the unroller does not know the number of iterations at compile time
> it unrolls loops with run-time checks in the following way
> (taken from loop-unroll.c):
>
> for (i = 0; i < n; i++)
> body;
>
> ==>
>
> i = 0;
> mod = n % 4;
>
> switch (mod)
> {
> case 3:
> body; i++;
> case 2:
> body; i++;
> case 1:
> body; i++;
> case 0: ;
> }
>
> while (i < n)
> {
> body; i++;
> body; i++;
> body; i++;
> body; i++;
> }
>
>
> The vectorizer knowns at compile time the maximum number of iterations
> that will be needed for the prologue and the epilogue. In some cases
> seems there is no need to unroll and create redundant loops.
You can set niter_max in the niter_desc of simple loops. There is
also nb_iter_bound for all loops. Of course the
issue is that loop information is destroyed sometimes. It also looks
like that RTL loop analysis may not re-use this information.
Maybe Zdenek knows a better answer.
Richard.
> Thanks,
> Revital
>
>
