RFR: 8256523: Streamline Java SHA2 implementation

[Aleksey Shipilev]

Current `sun/security/provider/SHA2` implementation is written with lots of small method invocations on the fastpath in `implCompress0`. Normally it does not matter much, because compilers are able to inline through it, and then some compilers even intrinsify the entire `implCompress0`.

But it comes as major downside for platforms that do not have SHA2 intrinsics, or those VM configs that blindly interpret the bytecode. Zero, for example, keeps re-entering the small methods when computing SHA2 digests during jmod/jlink generation during the build, and spends significant time there. Linux x86_64 Zero fastdebug builds **improve from 18.5 minutes to 11.0 minutes** with this change, which is a major win for development experience.

Note that SHA1 is already written in similar streamlined style. SHA5 is written with helper functions. This patch moves SHA2 to be closer to SHA1 style-wise.

Performance improvement in interpreted modes is substantial, about 4x..5x, while compiler modes are not affected. As measured by: `make test TEST="micro:MessageDigests" MICRO="FORK=1;ITER=5;WARMUP_ITER=5;WARMUP_TIME=1s;TIME=1s;OPTIONS=-p digesterName=sha256 -p provider=SUN"`

Before:

Benchmark              (digesterName)  (length)  (provider)   Mode  Cnt     Score   Error    Units

# Server, Default
MessageDigests.digest          sha256        64         SUN  thrpt    5  8585.532 ± 219.896  ops/ms
MessageDigests.digest          sha256      1024         SUN  thrpt    5  1545.994 ±  73.325  ops/ms
MessageDigests.digest          sha256     16384         SUN  thrpt    5   110.550 ±   1.576  ops/ms

# Server, -XX:+UnlockDiagnosticVMOptions -XX:-UseSHA256Intrinsics
MessageDigests.digest          sha256        64         SUN  thrpt    5  1426.117 ±  48.320  ops/ms
MessageDigests.digest          sha256      1024         SUN  thrpt    5   188.779 ±   0.097  ops/ms
MessageDigests.digest          sha256     16384         SUN  thrpt    5    12.512 ±   1.371  ops/ms

# Server, -Xint
MessageDigests.digest          sha256        64         SUN  thrpt    5     6.143 ±   0.126  ops/ms
MessageDigests.digest          sha256      1024         SUN  thrpt    5     0.769 ±   0.008  ops/ms
MessageDigests.digest          sha256     16384         SUN  thrpt    5     0.051 ±   0.001  ops/ms

# Zero
MessageDigests.digest          sha256        64         SUN  thrpt    5     5.358 ±   0.664  ops/ms
MessageDigests.digest          sha256      1024         SUN  thrpt    5     0.686 ±   0.003  ops/ms
MessageDigests.digest          sha256     16384         SUN  thrpt    5     0.046 ±   0.001  ops/ms

After:

Benchmark              (digesterName)  (length)  (provider)   Mode  Cnt     Score   Error     Units

# Server, Default
MessageDigests.digest          sha256        64         SUN  thrpt    5  8564.689 ± 1459.552  ops/ms
MessageDigests.digest          sha256      1024         SUN  thrpt    5  1548.582 ±   78.888  ops/ms
MessageDigests.digest          sha256     16384         SUN  thrpt    5   110.800 ±    0.057  ops/ms

# Server, -XX:+UnlockDiagnosticVMOptions -XX:-UseSHA256Intrinsics
MessageDigests.digest          sha256        64         SUN  thrpt    5  1471.399 ±   66.622  ops/ms
MessageDigests.digest          sha256      1024         SUN  thrpt    5   186.297 ±    0.127  ops/ms
MessageDigests.digest          sha256     16384         SUN  thrpt    5    12.448 ±    0.099  ops/ms

# Server, -Xint
MessageDigests.digest          sha256        64         SUN  thrpt    5    27.046 ±    0.304  ops/ms
MessageDigests.digest          sha256      1024         SUN  thrpt    5     3.538 ±    0.060  ops/ms
MessageDigests.digest          sha256     16384         SUN  thrpt    5     0.238 ±    0.002  ops/ms

# Zero
MessageDigests.digest          sha256        64         SUN  thrpt    5    26.696 ±    0.968  ops/ms
MessageDigests.digest          sha256      1024         SUN  thrpt    5     3.655 ±    0.024  ops/ms
MessageDigests.digest          sha256     16384         SUN  thrpt    5     0.241 ±    0.002  ops/ms

Addtional testing:
 - [x] `jdk/security` with Linux x86_64 server fastdebug
 - [x] `jdk/security` with Linux x86_64 zero fastdebug

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

Commit messages:
 - 8256523: Streamline Java SHA2 implementation

Changes: https://git.openjdk.java.net/jdk/pull/1283/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1283&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256523
  Stats: 123 lines in 1 file changed: 41 ins; 78 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1283.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1283/head:pull/1283

PR: https://git.openjdk.java.net/jdk/pull/1283