RFR: 8300256: C2: vectorization is sometimes skipped on loops where it would succeed [v3]

[Roland Westrelin]

> 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).

Roland Westrelin has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - @bug in test
 - Merge branch 'master' into JDK-8300256
 - review
 - Merge branch 'master' into JDK-8300256
 - more
 - maybe more
 - more
 - vectorization not run

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/12116/files
  - new: https://git.openjdk.org/jdk/pull/12116/files/2140b251..c48ccd47

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12116&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12116&range=01-02

  Stats: 18570 lines in 785 files changed: 5451 ins; 3876 del; 9243 mod
  Patch: https://git.openjdk.org/jdk/pull/12116.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12116/head:pull/12116

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