Richard Guenther wrote:
> 2009/8/19 Albert Cohen <albert.co...@inria.fr>:
>> When debugging graphite, we ran into code bloat issues due to
>> pass_complete_unrolli being called very early in the non-ipa
>> optimization sequence. Much later, the full-blown pass_complete_unroll
>> is scheduled, and this one does not do any harm.
>>
>> Strangely, this early unrolling pass (tuned to only unroll inner loops)
>> is only enabled at -O3, independently of the -funroll-loops flag.
>>
>> Does anyone remember why it is there, for which platform it is useful,
>> and what are the perf regressions if we remove it?
>
> The early loop unrolling pass is very important to remove abstraction
> penalty for C++ programs that chose not to implement manual
> unrolling by relying on the inliner and template metaprogramming.
>
> In tramp3d you for example see (very much simplified, intermediate
> state after some inlining):
>
> foo (int i, int j, int k)
> {
> double a[][][];
> int index[3];
> const int dX[3] = { 1, 0, 0 };
> ...
> for (m=0; m<3; ++m)
> index[m] = 0;
> index[0] = i;
> index[1] = j;
> index[2] = k;
> ... a[index[0]][index[1]][index[2]];
> for (m=0; m<3; ++m)
> index[m] += dx[m];
> ... a[index[0]][index[1]][index[2]];
>
> etc. to access a[i][j][k] and a[i+1][j][k].
>
> There is an absoulte need to unroll these simple loops before
> CSE otherwise loop optimizations have no chance on optimizing
> anything here.
>
> Another benchmark that degrades considerably without early
> unrolling is 454.calculix (in fact that one was the reason to
> add this pass).
>
>> My guess is that it may only harm... disabling or damaging the
>> effectivenesss of the (loop-level) vectorizer and increasing compilation
>> time.
>
> No it definitely does not. But it has one small issue in that it sometimes
> also unrolls an outermost loop IIRC, that could be fixed.
Thanks a lot for the quick and detailed response.
It is more difficult than I thought, then :-( We'll think more, and
maybe come up with yet another pass ordering proposal, but definitely
this tramp3d code deserves to be processed by graphite AFTER
unrolling+cse has done its specialization trick.
Albert
