RFR: 8347405: MergeStores with reverse bytes order value

[Emanuel Peter]

On Fri, 10 Jan 2025 10:56:59 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> This patch enhance MergeStores optimization to support merge value with reverse byte order.
>> 
>> Below is benchmark result before and after the patch:
>> 
>> On aliyun g8a (aarch64)
>> |name   | before        | score2        | ratio |
>> |---|---|---|---|
>> |MergeStoreBench.setCharBS      |5669.655000    |5669.566000    | 0.00 %|
>> |MergeStoreBench.setCharBV      |5516.911000    |5516.273000    | 0.01 %|
>> |MergeStoreBench.setCharC       |5578.644000    |5552.809000    | 0.47 %|
>> |MergeStoreBench.setCharLS      |5782.140000    |5779.264000    | 0.05 %|
>> |MergeStoreBench.setCharLV      |5496.403000    |5499.195000    | -0.05 %|
>> |MergeStoreBench.setIntB        |6087.703000    |2768.385000    | 119.90 %|
>> |MergeStoreBench.setIntBU       |6733.813000    |2950.240000    | 128.25 %|
>> |MergeStoreBench.setIntBV       |1362.233000    |1361.821000    | 0.03 %|
>> |MergeStoreBench.setIntL        |2834.785000    |2833.042000    | 0.06 %|
>> |MergeStoreBench.setIntLU       |2947.145000    |2946.874000    | 0.01 %|
>> |MergeStoreBench.setIntLV       |5506.791000    |5506.229000    | 0.01 %|
>> |MergeStoreBench.setIntRB       |7634.279000    |5611.058000    | 36.06 %|
>> |MergeStoreBench.setIntRBU      |7766.737000    |5551.281000    | 39.91 %|
>> |MergeStoreBench.setIntRL       |5689.793000    |5689.385000    | 0.01 %|
>> |MergeStoreBench.setIntRLU      |5628.287000    |5628.789000    | -0.01 %|
>> |MergeStoreBench.setIntRU       |5536.039000    |5534.910000    | 0.02 %|
>> |MergeStoreBench.setIntU        |5595.363000    |5567.810000    | 0.49 %|
>> |MergeStoreBench.setLongB       |13722.671000   |6811.098000    | 101.48 %|
>> |MergeStoreBench.setLongBU      |13728.844000   |4280.240000    | 220.75 %|
>> |MergeStoreBench.setLongBV      |2785.255000    |2785.949000    | -0.02 %|
>> |MergeStoreBench.setLongL       |5714.615000    |5710.402000    | 0.07 %|
>> |MergeStoreBench.setLongLU      |4128.746000    |4129.324000    | -0.01 %|
>> |MergeStoreBench.setLongLV      |2793.125000    |2794.438000    | -0.05 %|
>> |MergeStoreBench.setLongRB      |14465.223000   |7015.050000    | 106.20 %|
>> |MergeStoreBench.setLongRBU     |14546.954000   |6173.210000    | 135.65 %|
>> |MergeStoreBench.setLongRL      |6816.145000    |6813.348000    | 0.04 %|
>> |MergeStoreBench.setLongRLU     |4289.445000    |4284.239000    | 0.12 %|
>> |MergeStoreBench.setLongRU      |3132.471000    |3133.093000    | -0.02 %|
>> |MergeStoreBench.setLongU       |3086.779000    |3087.298000    | -0.02 %|
>> 
>> AMD EPYC 9T24
>> ...
>
> src/hotspot/share/opto/memnode.cpp line 3002:
> 
>> 3000:                Matcher::match_rule_supported(Op_ReverseBytesL)) {
>> 3001:       _value_order = DataOrder::Reverse;  // only support reverse bytes
>> 3002: #endif
> 
> Can you leave a comment why we only need this for little endian? It seems you are now generating `ReverseByte` nodes on any platform, right?

Ah, maybe that is because you have not `big-endian` machine to test on. We could leave this to someone who cares about big-endian - and they could also adjust the tests accordingly.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23030#discussion_r1910213753