PowerPC lwsync Instruction

[Joel Sherrill]

Hi,

I ran into something tracking down a test
failure on psim and now thing there is a
target specific issue that needs addressing.

libstdc++-v3/config/cpu/powerpc/atomic_word.h
uses the lwsync instruction if __NO_LWSYNC__
is not defined. 

psim does not implement the lwsync instruction.
I checked my 601, 603, and 403 manuals and did
not find this instruction in any of them.

I think the powerpc port is enabling this instruction
on variants that don't have it.  Which PowerPC
cores have this instruction?

It should be fairly straightforward to address since
it is just a matter of setting the __NO_LWSYNC__
macro correctly for each CPU variant.

Thanks.

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