RFR: 8276096: Simplify Unsafe.{load|store}Fence fallbacks by delegating to fullFence

[Aleksey Shipilev]

`Unsafe.{load|store}Fence` falls back to `unsafe.cpp` for `OrderAccess::{acquire|release}Fence()`. It seems too heavy-handed (useless?) to call to runtime for a single memory barrier. We can simplify the native `Unsafe` interface by falling back to `fullFence` when `{load|store}Fence` intrinsics are not available. This would be similar to what `Unsafe.{loadLoad|storeStore}Fences` do. 

This is the behavior of these intrinsics now, on x86_64, using benchmarks from JDK-8276054:


Benchmark          Mode  Cnt  Score   Error  Units

# Default
Single.acquire     avgt    3   0.407 ± 0.060  ns/op
Single.full        avgt    3   4.693 ± 0.005  ns/op
Single.loadLoad    avgt    3   0.415 ± 0.095  ns/op
Single.plain       avgt    3   0.406 ± 0.002  ns/op
Single.release     avgt    3   0.408 ± 0.047  ns/op
Single.storeStore  avgt    3   0.408 ± 0.043  ns/op

# -XX:DisableIntrinsic=_storeFence
Single.acquire     avgt    3   0.408 ± 0.016  ns/op
Single.full        avgt    3   4.694 ± 0.002  ns/op
Single.loadLoad    avgt    3   0.406 ± 0.002  ns/op
Single.plain       avgt    3   0.406 ± 0.001  ns/op
Single.release     avgt    3   4.694 ± 0.003  ns/op <--- upgraded to full
Single.storeStore  avgt    3   4.690 ± 0.005  ns/op <--- upgraded to full

# -XX:DisableIntrinsic=_loadFence
Single.acquire     avgt    3   4.691 ± 0.001  ns/op <--- upgraded to full
Single.full        avgt    3   4.693 ± 0.009  ns/op
Single.loadLoad    avgt    3   4.693 ± 0.013  ns/op <--- upgraded to full
Single.plain       avgt    3   0.408 ± 0.072  ns/op
Single.release     avgt    3   0.415 ± 0.016  ns/op
Single.storeStore  avgt    3   0.416 ± 0.041  ns/op

# -XX:DisableIntrinsic=_fullFence
Single.acquire     avgt    3   0.406 ± 0.014  ns/op
Single.full        avgt    3  15.836 ± 0.151  ns/op <--- calls runtime
Single.loadLoad    avgt    3   0.406 ± 0.001  ns/op
Single.plain       avgt    3   0.426 ± 0.361  ns/op
Single.release     avgt    3   0.407 ± 0.021  ns/op
Single.storeStore  avgt    3   0.410 ± 0.061  ns/op

# -XX:DisableIntrinsic=_fullFence,_loadFence
Single.acquire     avgt    3  15.822 ± 0.282  ns/op <--- upgraded, calls runtime
Single.full        avgt    3  15.851 ± 0.127  ns/op <--- calls runtime
Single.loadLoad    avgt    3  15.829 ± 0.045  ns/op <--- upgraded, calls runtime
Single.plain       avgt    3   0.406 ± 0.001  ns/op
Single.release     avgt    3   0.414 ± 0.156  ns/op
Single.storeStore  avgt    3   0.422 ± 0.452  ns/op

# -XX:DisableIntrinsic=_fullFence,_storeFence
Single.acquire     avgt    3   0.407 ± 0.016  ns/op
Single.full        avgt    3  15.347 ± 6.783  ns/op <--- calls runtime
Single.loadLoad    avgt    3   0.406 ± 0.001  ns/op
Single.plain       avgt    3   0.406 ± 0.002  ns/op 
Single.release     avgt    3  15.828 ± 0.019  ns/op <--- upgraded, calls runtime
Single.storeStore  avgt    3  15.834 ± 0.045  ns/op <--- upgraded, calls runtime

# -XX:DisableIntrinsic=_fullFence,_loadFence,_storeFence
Single.acquire     avgt    3  15.838 ± 0.030  ns/op <--- upgraded, calls runtime
Single.full        avgt    3  15.854 ± 0.277  ns/op <--- calls runtime
Single.loadLoad    avgt    3  15.826 ± 0.160  ns/op <--- upgraded, calls runtime
Single.plain       avgt    3   0.406 ± 0.003  ns/op
Single.release     avgt    3  15.838 ± 0.019  ns/op <--- upgraded, calls runtime
Single.storeStore  avgt    3  15.844 ± 0.104  ns/op <--- upgraded, calls runtime


Additional testing:
 - [x] Linux x86_64 fastdebug `tier1`

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

Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/6149/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6149&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276096
  Stats: 22 lines in 3 files changed: 6 ins; 11 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6149.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6149/head:pull/6149

PR: https://git.openjdk.java.net/jdk/pull/6149