Array addition and array sum Panama benchmarks

[John Rose]

This fits into a loop optimization technique that Roland worked on, called loop predication. You speculate that the loop invariant inputs are somehow favorable and test. If all is well you run the loop transformed to exploit the speculated favorable condition. In this case it is disjointness of reads and writes. If the speculation fails you might recompile, or use a defined fallback loop.

If the loop is worth vectorizing the cost of checking disjointness is comparatively small, commensurate with other predicates we now use.

Certainly a disjointness test is cheaper than the very subtle range analysis we do routinely for range check elimination, of any loop containing array accesses linear in the loop trip count.

So, it’s merely one of those “small matters of programming”.

On Mar 20, 2024, at 11:51 AM, Maurizio Cimadamore <maurizio.cimadamore at oracle.com> wrote:



Thanks for the analysis Roland. If I understand correctly, even if this analysis is not supported today, in principle it could be done, right?

After all, we know ia/oa (e.g. their addresses) and the ranges we're going to access these at (otherwise we could not hoist bound checks outside the loop).

So, we might be able, in principle, to check that - perhaps the issue is that (as in this case) all the addresses are completely dynamic, so you really need a disjointness runtime check (outside the loop), which might be more expensive than the incremental benefit added by vectorization?

Maurizio


On 20/03/2024 16:26, Roland Westrelin wrote:

is that the compiler can't prove it's legal to vectorize. Doubles are
read from ia and oa and then added and written back to oa. There's no
way for the compiler to tell that the off heap areas pointed to by ia
and oa don't overlap. So possibly, the value written to:

oa + 8*i

is going to be read back at the next iteration with:

ia + 8*i

(ia could be oa+8)

The autovectorizer would need to insert a runtime check that the 2 areas
don't overlap but there's no support for that at this point. I suppose
the same issue exists with the MemorySegment API when memory is off
heap.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/panama-dev/attachments/20240320/6ee4dbd5/attachment-0001.htm>