On 14/01/13 16:04, Tim Prince wrote:
> It's a Frequently Encountered Problem. What did
> -ftree-vectorizer-verbose=3 produce?
Nothing. At 5 it gave:
27: versioning for alias required: can't determine dependence between
*D.1967_20 and *D.1988_49
27: mark for run-time aliasing test between *D.1967_20 and *D.1988_49
[...]
27: disable versioning for alias - max number of generated checks
exceeded.
which implies that "restrict" is being clobbered.
> Part of the problem is that the OpenMP chunks won't have the
> alignments you set carefully for the start of the array, unless the
> loop count happens to be a multiple of number of threads times
> unrolling factor times vector register width, thus unknown at compile
> time. It remains to be seen how much OpenMP 4.0 proposals for pragmas
> to deal with this may help. Until then, OpenMP tends to work better
> with at least 2 levels of loops, where the outer is parallelizable
> and the inner vectorizable.
Okay. Can anyone suggest a good blocking methodology such that given
for (int i = 0; i < n; ++i)
// Code which uses parameters ...
where we require the parameters ... have an alignment of X 'items' (so
for 256-bit AVX registers and float types X = 32/4 = 8) yields:
for (outer)
for (inner)
// Code
such that the outer loop can be hit with OpenMP and the inner loop with
auto-vectorization.
Regards, Freddie.
