RFR: 8300256: C2: vectorization is sometimes skipped on loops where it would succeed

[Vladimir Kozlov]

On Fri, 20 Jan 2023 15:15:40 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> Vectorization for a counted loop cl only proceeds if
> cl->range_checks_present() returns true. The result of that method is
> computed lazily and its result cached in the CountedLoopNode and never
> re-computed. If PhaseIdealLoop::do_range_check() returns 0 then the
> result of that computation is overwritten (no range checks
> present). PhaseIdealLoop::do_range_check() counts the number of tests
> present in the loop body (which is really what range_checks_present()
> is about) and decrements that count for every check it eliminates
> except if it's not a comparison with a LoadRange (for a reason that I
> don't understand). In the case of the test (a pattern from a
> ByteBuffer benchmark), not all tests are with a LoadRange. As a
> result, PhaseIdealLoop::do_range_check() returns non zero even though
> it eliminates all tests. As a result, vectorization is never
> attempted.
> 
> There doesn't seem to be a value in caching the result of
> range_checks_present() in CountedLoopNode. It's not that expensive to
> compute, it's only used during loop opts and it's really hard to keep
> in sync with whether the loop has still tests: several different
> transformations could remove a test. What I propose instead is to keep
> roughly the same approach (compute the result lazily and cache it so
> it doesn't have to be re-computed) but to store it on the
> IdealLoopTree instead (so it's recomputed on every loop opts pass and
> there's no risk that it becomes out of sync).

Just one comment.

src/hotspot/share/opto/loopnode.cpp line 3993:

> 3991:     if (cl->is_reduction_loop()) tty->print(" reduction");
> 3992:     if (cl->is_vectorized_loop()) tty->print(" vector");
> 3993:     if (cl->range_checks_present()) tty->print(" rc ");

Can you use new `range_checks_present()` here? The information is still useful.

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PR: https://git.openjdk.org/jdk/pull/12116