<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body dir="auto">
<div dir="ltr"></div>
<div dir="ltr">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. </div>
<div dir="ltr"><br>
</div>
<div dir="ltr">If the loop is worth vectorizing the cost of checking disjointness is comparatively small, commensurate with other predicates we now use.</div>
<div dir="ltr"><br>
</div>
<div dir="ltr">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. </div>
<div dir="ltr"><br>
</div>
<div dir="ltr">So, it’s merely one of those “small matters of programming”.</div>
<div dir="ltr"><br>
<blockquote type="cite">On Mar 20, 2024, at 11:51 AM, Maurizio Cimadamore <maurizio.cimadamore@oracle.com> wrote:<br>
<br>
</blockquote>
</div>
<blockquote type="cite">
<div dir="ltr">
<p>Thanks for the analysis Roland. If I understand correctly, even if this analysis is not supported today, in principle it could be done, right?</p>
<p>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).</p>
<p>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?</p>
<p>Maurizio<br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 20/03/2024 16:26, Roland Westrelin wrote:<br>
</div>
<blockquote type="cite" cite="mid:8734sklvhu.fsf@redhat.com">
<pre>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.
</pre>
</blockquote>
</div>
</blockquote>
</body>
</html>