RFR: 8325252: C2 SuperWord: refactor the packset [v4]

[Emanuel Peter]

On Wed, 27 Mar 2024 08:32:58 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

>> Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   use left/right instead of s1/s2 in some obvious simple places
>
> src/hotspot/share/opto/superword.hpp line 69:
> 
>> 67:   // Doubly-linked pairs. If not linked: -1
>> 68:   GrowableArray<int> _left_to_right; // bb_idx -> bb_idx
>> 69:   GrowableArray<int> _right_to_left; // bb_idx -> bb_idx
> 
> I think it's a good solution but still found myself revisiting this several times while looking at the methods below how it works. Would it maybe help to give a visual example? For example:
> 
> 
> left_to_right:
>  index:    0   1    2    3
>  value: | -1 | 3 | -1 | -1 | ...
> 
> => Node with bb_idx 1 is in the left slot of a pair which has the node with bb_idx 3 in the right slot.
> => Nodes with bb_idx 0, 2, and 3 are not found in a left slot of any pair.
>       
> right_to_left:
>  index:    0    1    2   3
>  value: | -1 | -1 | -1 | 1 | ...
> 
> => Node with bb_idx 3 is in the right slot of a pair which has the node with bb_idx 1 in the left slot.
> => Nodes with bb_idx 0, 1, and 2 are not found in a right slot of any pair.
>  ```

Great idea, I made a slightly more complex example, inspired by yours!

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PR Review Comment: https://git.openjdk.org/jdk/pull/18276#discussion_r1541332549