RFR(s): 8076185: Provide SafeFetchX implementation for zero

[Thomas Stüfe]

Hi all,

Please review this change which provides a real SafeFetch implementation
on zero.

webrev:
http://cr.openjdk.java.net/~stuefe/webrevs/8076185/webrev.01/webrev/
bug:     https://bugs.openjdk.java.net/browse/JDK-8076185

It works like this:

Before a load is attempted from a potentially unsafe address, we set up a
jump buffer with sigsetjmp(). In the signal handler, for SIGSEGV and SIGBUS,
we test whether there is a jump buffer set and if yes, take this as an
indication
that the crash was an attempted SafeFetch. In this case we jump back via
longjmp().

Coding is a bit more difficult because we need to be threadsafe. We keep
the jump
buffer on the thread stack - this is ok, because that stack never gets
unwinded
 - either we crash, in which case signal handler stack frames are built up
below
us, or we don't crash, in which case we never leave the SafeFetch function.
In
both cases, we never loose the jump buffer.

To communicate the jump buffer location to the signal handler, TLS is used.
I
use POSIX tls, because that always works and is not dependend on VM
infrastructure.

---

I built and tested this on Linux x64 zero (Ubuntu 14.4). It works and the
initialization tests for SafeFetch in the stub routine initialization now
work for
zero too.

Note that I do not have a BSD system right now, so I cannot check whether
this
change builds and works for BSD. But the change only requires POSIX Apis, so
BSD should probably be ok.

Someone from the zero team should definitly check and test this for other
zero
platforms, but the chances are good that this just works.

Thanks and Kind Regards,

Thomas Stuefe