RFR: 8296507: GCM using more memory than necessary with in-place operations [v3]

[Anthony Scarpino]

> 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 update

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11121/files
  - new: https://git.openjdk.org/jdk/pull/11121/files/f595dc24..340ab22f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11121&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11121&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11121.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11121/head:pull/11121

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