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

[Fei Gao]

On Fri, 11 Nov 2022 08:38:34 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

>> 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
>
> test/hotspot/jtreg/compiler/loopopts/TestUnsupportedConditionalMove.java line 28:
> 
>> 26:  * @bug 8295407
>> 27:  * @summary C2 crash: Error: ShouldNotReachHere() in multiple vector tests with
>> 28:  *          -XX:-MonomorphicArrayCheck -XX:-UncommonNullCast
> 
> `MonomorphicArrayCheck` and `UncommonNullCast` are debug flags, the test will fail with a release build.

@TobiHartmann, thanks for your review! The options added here is commented as part of summary title, not as JVM options. I suppose it should be fine for a release build, right? :-)

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