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

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Other	JDK 6	JDK 7	Other
1.4.2_31,hs17Fixed	6u21-revFixed	7Fixed	hs17Fixed

BOT bulk-updates use memset(). However on Niagara memset() uses BIS
for speed and this exposes concurrent readers to out-of-thin-air zeroes,
which are fatal for concurrent BOT walks. Such concurrent reads of
BOT entries subject to concurrent update are possible with non-contiguous
spaces such as CMS (but not any other current GC in HotSpot).

For this fix to work we need to properly detect the actual CPU version. For the new SPARC-M series this detection was broken until "7188227: VM should recognize M-series SPARC" (https://bugs.openjdk.java.net/browse/JDK-7188227) was fixed.
14-10-2013
EVALUATION http://hg.openjdk.java.net/hsx/hsx17/baseline/rev/47d6ab4dec3b
22-04-2011
EVALUATION http://hg.openjdk.java.net/jdk7/hotspot-gc/hotspot/rev/3bfae429e2cf
03-05-2010
EVALUATION As far as i can tell, a day-one bug that was in the JVM waiting for the use of BIS by memset() to happen.
03-05-2010
EVALUATION As in description and suggested fix.
29-04-2010
WORK AROUND A big-hammer workaround that doesn't use the optimized libc_psr memset() is:- export LD_NOAUXFLTR=1 This may reduce performance of the JVM in general and of native code using memset() in particular. CAVEAT: if your application uses native code that depends on auxiliary filters on certrain platforms for correctness, such aplications may even break when auxiliary filters are turned off above.
29-04-2010
SUGGESTED FIX Preliminary webrev, under perf and test:- http://analemma.sfbay.sun.com/net/neeraja/export/ysr/memset/webrev/ When using CMS on sparc4v, do not use sparc4v-specific optimized memset() for BOT updates.
29-04-2010

Relates :	JDK-6872049 - CMS: Failure in CompactibleFreeListSpace::block_size from ::block_start_unsafe()
Relates :	JDK-6955836 - BVF for the bug 6872049
Relates :	JDK-7188227 - VM should recognize M-series SPARC
Relates :	JDK-7192128 - G1: Extend fix for 6948537 to G1's BOT
Relates :	JDK-8039042 - G1: Phantom zeros in cardtable