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6948537: CMS: BOT walkers observe out-of-thin-air zeros on sun4v sparc/CMT

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On sun4v/CMT avoid use of memset() in BOT updates so as to prevent concurrent BOT readers from seeing the phantom zeros arising from memset()'s use of BIS.

Reviewed-by: jmasa, johnc, minqi, poonam, tonyp
This commit is contained in:
Y. Srinivas Ramakrishna 2010-05-03 10:24:51 -07:00
parent 99196ff9ca
commit 762f60d343
4 changed files with 44 additions and 2 deletions
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6
hotspot/src/cpu/sparc/vm/vm_version_sparc.cpp
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@ -104,6 +104,12 @@ void VM_Version::initialize() {
if (FLAG_IS_DEFAULT(OptoLoopAlignment)) {
FLAG_SET_DEFAULT(OptoLoopAlignment, 4);
}
// When using CMS, we cannot use memset() in BOT updates because
// the sun4v/CMT version in libc_psr uses BIS which exposes
// "phantom zeros" to concurrent readers. See 6948537.
if (FLAG_IS_DEFAULT(UseMemSetInBOT) && UseConcMarkSweepGC) {
FLAG_SET_DEFAULT(UseMemSetInBOT, false);
}
}
// Use hardware population count instruction if available.