Re: __sync_lock_test_and_set on ARM

[Andrew Haley <aph-gcc at littlepinkcloud dot COM>]

Phil Endecott writes:
 > David Daney wrote:
 > > Andrew Haley wrote:
 > >> Phil Endecott writes:
 > >>  > Andrew Haley wrote:
 > >>  > > this stuff really should be done by the compiler. 
 > >>  > 
 > >>  > Yes.  I've filed a bug asking for a __sync_lock_test_and_set builtin:
 > >>  > 
 > >>  > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33413
 > >> 
 > >> Surer, but the problem is that for most of the things we want to do
 > >> (lightweight locks, for example) __sync_lock_test_and_set() doesn't
 > >> really do what we need: we need compare_and_swap().  That's why the
 > >> kernel helper is so useful, because it's robust even if we are on
 > >> pre-ARMv6 hardware.
 > >
 > > Probably the enhancement request should be expanded to include *all* the 
 > > __sync_* atomic memory primitives  which includes compare_and_swap.
 > 
 > No, because none of the others can be implemented without kernel 
 > support or other magic.
 
Yeah, I think that's right.  Even in the case of post-v6 -- where it
actually is possible to do this safely in userland -- if it's
necessary to execute a bunch of instructions you might as well call a
subroutine.

Andrew.