Hi,
> > 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.
currently, there is no reliable way how to pass this information to RTL. The
best
I can come up with (without significant amount of changes to other parts of the
compiler)
would be to insert a code like
if (n > 5)
special_abort ();
before the loop in the vectorizer if you know for sure that the loop will
iterate at most
5 times, use these hints to bound the number of iterations in the unroller (we
do not do this
at the moment, but it should be easy), and remove the calls to special_abort
and the
corresponding branches after the unroller.
Zdenek
