[8u] backport of 8077504: Unsafe load can loose control dependency and cause crash

[Roland Westrelin]

8u backport request. 8077504 was pushed to jdk9 2 weeks ago and it hasn’t caused any new failures during nightly testing. The change doesn’t apply cleanly to 8u. One reason is that we haven’t backported:

8036851: volatile double accesses are not explicitly atomic in C2

which applies cleanly to 8u so I’d like to backport it as well. There are other changes that were not backported and are causing the change to not apply cleanly (https://bugs.openjdk.java.net/browse/JDK-8054033 is one) but it’s probably too late for a backport of those.

https://bugs.openjdk.java.net/browse/JDK-8036851
http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/446bb44e5367

https://bugs.openjdk.java.net/browse/JDK-8077504
http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/rev/ed9cc6871da2
webrev against 8u (only 8077504 contrary to the webrev comment):
http://cr.openjdk.java.net/~roland/8077504/webrev.03/

The 8u change went through jprt.

Roland.