RFR: 8331311: C2: Big Endian Port of 8318446: optimize stores into primitive arrays by combining values into larger store [v6]

[Amit Kumar]

On Wed, 5 Jun 2024 20:07:17 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

>> This pr adds a few tweaks to [JDK-8318446](https://bugs.openjdk.org/browse/JDK-8318446) which allows enabling it also on big endian platforms (e.g. AIX, S390). JDK-8318446 introduced a C2 optimization to replace consecutive stores to a primitive array with just one store.
>> 
>> By example (from `TestMergeStores.java`):
>> 
>> 
>>     static Object[] test2a(byte[] a, int offset, long v) {
>>         if (IS_BIG_ENDIAN) {
>>             a[offset + 0] = (byte)(v >> 56);
>>             a[offset + 1] = (byte)(v >> 48);
>>             a[offset + 2] = (byte)(v >> 40);
>>             a[offset + 3] = (byte)(v >> 32);
>>             a[offset + 4] = (byte)(v >> 24);
>>             a[offset + 5] = (byte)(v >> 16);
>>             a[offset + 6] = (byte)(v >> 8);
>>             a[offset + 7] = (byte)(v >> 0);
>>         } else {
>>             a[offset + 0] = (byte)(v >> 0);
>>             a[offset + 1] = (byte)(v >> 8);
>>             a[offset + 2] = (byte)(v >> 16);
>>             a[offset + 3] = (byte)(v >> 24);
>>             a[offset + 4] = (byte)(v >> 32);
>>             a[offset + 5] = (byte)(v >> 40);
>>             a[offset + 6] = (byte)(v >> 48);
>>             a[offset + 7] = (byte)(v >> 56);
>>         }
>>         return new Object[]{ a };
>>     }
>> 
>> 
>> Depending on the endianess 8 bytes are stored into an array. The order of the stores is the same as the order of an 8-byte-store therefore 8 1-byte-stores can be replaced with just one 8-byte-store (if there aren't too many range checks).
>> 
>> Additionally I've fixed a few comments and a test bug.
>> 
>> The optimization seems to be a little bit more effective on big endian platforms.
>> 
>> Again by example:
>> 
>> 
>>     static Object[] test800a(byte[] a, int offset, long v) {
>>         if (IS_BIG_ENDIAN) {
>>             a[offset + 0] = (byte)(v >> 40); // Removed from candidate list
>>             a[offset + 1] = (byte)(v >> 32); // Removed from candidate list
>>             a[offset + 2] = (byte)(v >> 24); // Merged
>>             a[offset + 3] = (byte)(v >> 16); // Merged
>>             a[offset + 4] = (byte)(v >> 8);  // Merged
>>             a[offset + 5] = (byte)(v >> 0);  // Merged
>>         } else {
>>             a[offset + 0] = (byte)(v >> 0);  // Removed from candidate list
>>             a[offset + 1] = (byte)(v >> 8);  // Removed from candidate list
>>             a[offset + 2] = (byte)(v >> 16); // Not merged
>>             a[offset + 3] = (byte)(v >> 24); // Not merged
>>             a[offset + 4] = (byte)(v >> 32); // Not merge...
>
> Richard Reingruber 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 10 additional commits since the last revision:
> 
>  - Merge branch 'master' into 8331311_merge_stores_on_big_endian
>  - Feedback Emanuel
>  - Eliminate IS_BIG_ENDIAN and always execute both variants
>  - test2BE: big endian version of test2
>  - Improve make_merged_input_value based on Emanuel's feedback
>  - Improve comment
>  - Improve comment
>  - Add bug id
>  - Typo
>  - 8331311: C2: Big Endian Port of 8318446: optimize stores into primitive arrays by combining values into larger store

I did another round of testing on s390x. looks good.

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PR Comment: https://git.openjdk.org/jdk/pull/19218#issuecomment-2151550321