RFR: 8304948: [vectorapi] C2 crashes when expanding VectorBox [v3]

[Quan Anh Mai]

On Tue, 25 Apr 2023 15:12:12 GMT, Eric Liu <eliu at openjdk.org> wrote:

>> This patch fixes C2 failure with SIGSEGV due to endless recursion.
>> 
>> With test case VectorBoxExpandTest.java in this patch, C2 would generate IR graph like below:
>> 
>> 
>>                ------------
>>               /            \
>>       Region |  VectorBox   |
>>            \ | /            |
>>             Phi             |
>>              |              |
>>              |              |
>>       Region |  VectorBox   |
>>            \ | /            |
>>             Phi             |
>>              |              |
>>              |------------/
>>              |
>> 
>> 
>> 
>> This Phi will be optimized by merge_through_phi [1], which transforms `Phi (VectorBox VectorBox)` into `VectorBox (Phi Phi)` to pursue opportunity of combining VectorBox with VectorUnbox. In this process, either the pre type check [2] or the process cloning Phi nodes [3], the circle case is well considered to avoid falling into endless loop.
>> 
>> After merge_through_phi, each input Phi of new VectorBox has the same shape with original root Phi before merging (only VectorBox has been replaced). After several other optimizations, C2 would expand VectorBox [4] on a graph like below:
>> 
>> 
>>                ------------
>>               /            \
>>       Region |  Proj        |
>>            \ | /            |
>>             Phi             |
>>              |              |
>>              |              |
>>       Region |  Proj        |
>>            \ | /            |
>>             Phi             |
>>              |              |
>>              |------------/
>>              |
>>              |     Phi
>>              |     /
>>           VectorBox
>> 
>> 
>> which the circle case should be taken into consideration as well.
>> 
>> [TEST]
>> Full Jtreg passed without new failure.
>> 
>> [1] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/cfgnode.cpp#L2554
>> [2] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/cfgnode.cpp#L2571
>> [3] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/cfgnode.cpp#L2531
>> [4] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/vector.cpp#L311
>
> Eric Liu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   expand vector box in local
>   
>   Change-Id: Ie7bddc049b479aad4f953ec920d83b91e7de2152

src/hotspot/share/opto/vector.cpp line 345:

> 343:                                               box_type, vect_type, visited);
> 344:       if (!new_box->is_Phi()) {
> 345:         C->initial_gvn()->hash_delete(vbox);

May I ask why is this needed? Thanks.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/13489#discussion_r1182590704