`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
