[committed] hppa: Revise atomic support to use sync barrier

[John David Anglin <dave dot anglin at bell dot net>]

It recently came to my attention that PA 2.0 supports out-of-order 
execution for loads and
stores.  Loads and stores are strongly ordered on PA 1.x.  This has 
caused no end of confusion.

This out-of-order execution is discussed in the following article:
http://web.archive.org/web/20040214092531/http://www.cpus.hp.com/technical_references/advperf.shtml

This patch defines a new barrier pattern to enforce ordering of memory 
accesses.

The existing atomic patterns have been removed from pa.md except for 
atomic_loaddi and atomic_storedi
which use floating point double instructions.  The defaults are okay 
when combined with the new barrier.

Since we want PA 1.x code to be PA 2.0 compatible, it gets the barrier 
as well.

The linux atomic support is modified to use the new barrier in the lock 
release code.  This should improve
performance.

Tested on hppa-unknown-linux-gnu.  Committed to trunk.  Probably, I will 
backport the change once the
change receives more testing.

Dave

--
John David Anglin  dave.anglin@bell.net

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