RFR: 8275330: C2: assert(n->is_Root() || n->is_Region() || n->is_Phi() || n->is_MachMerge() || def_block->dominates(block)) failed: uses must be dominated by definitions [v2]

[Roland Westrelin]

> This is similar to previous bugs where:
> 
> - a cast/conv node captures a narrow type in a loop body because of a
>   range check,
> 
> - the range check is optimized out of the loop, pre/main/post loop are
>   created
> 
> - overunrolling causes the main loop to become unreachable (the range
>   check, if still in the main loop, would fail), the cast transforms to
>   top but c2 can't optimize the loop out
> 
> This was fixed by adding predicates above the main loop. With this
> particular bug, the cast node is in the post loop. The fix I propose
> is to also add predicates above the post loop. There are a few
> locations in the code that cause a post loop to be added: either the
> initial post loop or some other post loops for vectorization
> support. I think the new predicates are needed in a all cases. To be
> able to add predicates at these different points in the optimization
> process, the new predicates are copied from the main loop predicates.
> 
> I also delayed folding of Opaque4 nodes to macro expansion rather than
> post loop opts igvn. The reason for that is that I believe there's a
> risk that an Opaque4 is removed (that is replaced by its input 2)
> before its input 1 has a chance to constant fold. That wouldn't happen
> with a debug build because we leave the tests in (that is replace the
> Opaque4 node by its input 1) so that corner case is not properly
> tested currently. The reason for leaving the tests in was to sanity
> check that the tests are indeed correct.

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 three additional commits since the last revision:

 - Merge branch 'master' into JDK-8275330
 - reviews
 - fix

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6429/files
  - new: https://git.openjdk.java.net/jdk/pull/6429/files/50c8c797..b194700e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6429&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6429&range=00-01

  Stats: 57869 lines in 931 files changed: 38122 ins; 10933 del; 8814 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6429.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6429/head:pull/6429

PR: https://git.openjdk.java.net/jdk/pull/6429