[rfc][ARM] Support for safepoint check based on memory protect rather than polling

[Andrew Dinn]

The Hotspot safepoint check, performed when a backward branch is taken
inside compiled Java code, is implemented as a read to the safepoint
page, a special page which in the normal case has PROT_READ protection.
The safepoint page can be mprotected with PROT_NONE when the VM thread
wants Java threads to move to a safepoint and suspend. Any resulting
SEGV is caught and after being validated as belonging to an injected
safepoint page read, leads to a branch of control to the safepoint
blocking routine. This allows a backward branch to proceed with almost
zero overhead in the normal case.

Currently the ARM safepoint check reads a shared flag. This patch
modifies the ARM code to use safepoint page protection in place of polling.

The code includes an extra optimization. By default the normal path
skips round the safepoint check code in order to reach the branch
test/backward branch instruction compiled by the caller. In the case
where the branch is unconditional the normal path branches back
immediately without the skip and a repeat branch is encoded at the end
of the safepoint check code.

regards,


Andrew Dinn
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