RFR: 8295407: C2 crash: Error: ShouldNotReachHere() in multiple vector tests with -XX:-MonomorphicArrayCheck -XX:-UncommonNullCast [v2]

[Fei Gao]

> For unsupported `CMove` patterns, [JDK-8293833](https://bugs.openjdk.org/browse/JDK-8295407) helps remove unused `CMove` and related packs from superword candidate packset by the function `remove_cmove_and_related_packs()`, but it only works when `-XX:+UseVectorCmov` is enabled[1]. When the option is not enabled, these unsupported `CMove` packs are still kept in the superword packset, causing the same failure.
> 
> Actually, the function `filter_packs()` in superword is to filter out unsupported packs but it can't work as expected currently for these `CMove` cases. As we know, not all `CMove` packs can be vectorized. `merge_packs_to_cmovd()`[2] looks through all packs in the superword packset and generates a `CMove` candidate packset to collect all qualified `CMove` packs. Hence, only `CMove` packs in the `CMove` candidate packset are our target patterns and can be vectorized. But `filter_packs()` thinks, if the `CMove` pack is in a superword packset and its vector node is implemented in the current platform, then it can be vectorized. Therefore, the function doesn't remove these unsupported packs.
> 
> We can adjust the function `implemented()` in the stage of `filter_packs()` to check if the current `CMove` pack is in the `CMove` candidate packset. If not, `filter_packs()` considers it not to be vectorized and then remove it. After the fix, whether
> `-XX:+UseVectorCmov` is enabled or not, these unsupported packs can be removed by `filter_packs()`. In this way, we don't need the function`remove_cmove_and_related_packs()` anymore and thus the patch also cleans related code.
> 
> [1] https://github.com/openjdk/jdk/blob/9b9be88bcaa35c89b6915ff0c251e5a04b10b330/src/hotspot/share/opto/superword.cpp#L537
> [2] https://github.com/openjdk/jdk/blob/9b9be88bcaa35c89b6915ff0c251e5a04b10b330/src/hotspot/share/opto/superword.cpp#L1892

Fei Gao 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:

 - Clean up related code
 - Merge branch 'master' into fg8295407
 - 8295407: C2 crash: Error: ShouldNotReachHere() in multiple vector tests with -XX:-MonomorphicArrayCheck -XX:-UncommonNullCast
   
   For unsupported `CMove` patterns, JDK-8293833 helps remove unused
   `CMove` and related packs from superword candidate packset by the
   function `remove_cmove_and_related_packs()`, but it only works
   when `-XX:+UseVectorCmov` is enabled[1]. When the option is not
   enabled, these unsupported `CMove` packs are still kept in the
   superword packset, causing the same failure.
   
   Actually, the function `filter_packs()` in superword is to filter
   out unsupported packs but it can't work as expected currently for
   these `CMove` cases. As we know, not all `CMove` packs can be
   vectorized. `merge_packs_to_cmovd()`[2] looks through all packs
   in the superword packset and generates a `CMove` candidate
   packset to collect all qualified `CMove` packs. Hence, only
   `CMove` packs in the `CMove` candidate packset are our target
   patterns and can be vectorized. But `filter_packs()` thinks,
   if the `CMove` pack is in a superword packset and its vector
   node is implemented in the current platform, then it can
   be vectorized. Therefore, the function doesn't remove
   these unsupported packs.
   
   We can adjust the function `implemented()` in the stage of
   `filter_packs()` to check if the current `CMove` pack is in
   the `CMove` candidate packset. If not, `filter_packs()` considers
   it not to be vectorized and then remove it. After the fix,
   whether `-XX:+UseVectorCmov` is enabled or not, these
   unsupported packs can be removed by `filter_packs()`. In this
   way, we don't need the function`remove_cmove_and_related_packs()`
   anymore and thus the patch also cleans related code.
   
   [1] https://github.com/openjdk/jdk/blob/9b9be88bcaa35c89b6915ff0c251e5a04b10b330/src/hotspot/share/opto/superword.cpp#L537
   [2] https://github.com/openjdk/jdk/blob/9b9be88bcaa35c89b6915ff0c251e5a04b10b330/src/hotspot/share/opto/superword.cpp#L1892
   [3] https://github.com/openjdk/jdk/blob/9b9be88bcaa35c89b6915ff0c251e5a04b10b330/src/hotspot/share/opto/superword.cpp#L2701

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11034/files
  - new: https://git.openjdk.org/jdk/pull/11034/files/97a27264..bcf6a21e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11034&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11034&range=00-01

  Stats: 11431 lines in 460 files changed: 4358 ins; 5241 del; 1832 mod
  Patch: https://git.openjdk.org/jdk/pull/11034.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11034/head:pull/11034

PR: https://git.openjdk.org/jdk/pull/11034