[9] RFR(S): 8153527: break_tty_lock_for_safepoint causes "assert(false) failed: bad tag in log" and broken compile log

[Nils Eliasson]

Hi,

Please review this fix of print opto_assembly.

Summary:
The compilelog can get corrupted and the VM may assert on "failed: bad 
tag in log".

When printing assembly in output.cpp we first take the ttylock, print 
the head and then the method metadata. However the metadata printing 
makes a vm entry and may block for a safepoint and will then release the 
lock (break_tty_lock_for_safepoint). After that some of the other 
compiler thread that haven't safepointed will take the lock and the 
broken log will be a fact when the safepoint is over and the first 
thread starts logging again.

Solution:
Print the method metadata to a temporary buffer, then take the tty lock.

Testing:
Repro from bug stops failing.
Running :hotspot_all 
(http://jdash.se.oracle.com/rbt/rbt-nils.eliasson-compiler_control-20160415-1508-10854)

Bug: https://bugs.openjdk.java.net/browse/JDK-8153527
Webrev: http://cr.openjdk.java.net/~neliasso/8153527/webrev.01/

Regards,
Nils Eliasson
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