RFR: 8296507: GCM using more memory than necessary with in-place operations [v2]

[Valerie Peng]

On Mon, 21 Nov 2022 18:20:09 GMT, Anthony Scarpino <ascarpino at openjdk.org> wrote:

>> I would like a review of an update to the GCM code.  A recent report showed that GCM memory usage for TLS was very large.  This was a result of in-place buffers, which TLS uses, and how the code handled the combined intrinsic method during decryption.  A temporary buffer was used because the combined intrinsic does gctr before ghash which results in a bad tag.  The fix is to not use the combined intrinsic during in-place decryption and depend on the individual GHASH and CounterMode intrinsics.  Direct ByteBuffers are not affected as they are not used by the intrinsics directly.
>> 
>> The reduction in the memory usage boosted performance back to where it was before despite using slower intrinsics (gctr & ghash individually).  The extra memory allocation for the temporary buffer out-weighted the faster intrinsic.
>> 
>> 
>>     JDK 17:   122913.554 ops/sec
>>     JDK 19:    94885.008 ops/sec
>>     Post fix: 122735.804 ops/sec 
>> 
>> There is no regression test because this is a memory change and test coverage already existing.
>
> Anthony Scarpino has updated the pull request incrementally with one additional commit since the last revision:
> 
>   comment cleanup & finesse ByteBuffer implGCMCrypt better

src/java.base/share/classes/com/sun/crypto/provider/GaloisCounterMode.java line 778:

> 776:                             gctr, ghash);
> 777:                     } else {
> 778:                         int ofs = src.arrayOffset() + src.position();

Isn't this also used on line 774? Why not move this up and directly refer to it for both places?

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PR: https://git.openjdk.org/jdk/pull/11121