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Re: libsanitizer merge from upstream r208536

On Tue, May 27, 2014 at 11:50:33PM +0200, Jakub Jelinek wrote:
> On Tue, May 27, 2014 at 04:02:08PM -0500, Peter Bergner wrote:
> > It's being called form basically two files:
> > 
> > [bergner@makalu-lp1 gcc-fsf-mainline-asan-debug]$ find . -name '*.o' | xargs nm -AC | grep sync_fetch_and_add_8
> > ./powerpc64-linux/32/libsanitizer/sanitizer_common/.libs/sanitizer_allocator.o:         U __sync_fetch_and_add_8
> > ./powerpc64-linux/32/libsanitizer/sanitizer_common/sanitizer_allocator.o:         U __sync_fetch_and_add_8
> > ./powerpc64-linux/32/libsanitizer/asan/.libs/asan_allocator2.o:         U __sync_fetch_and_add_8
> > ./powerpc64-linux/32/libsanitizer/asan/asan_allocator2.o:         U __sync_fetch_and_add_8
> Does ppc32 have any atomic 64-bit loads/stores (in the sense that the aligned
> 64 bits are written as one memory transaction, not each 32-bit word
> separately)?
> In any case, atomic_uint64_t there seems to be used only for some statistic
> counter and not really atomic anyway, as additions aren't performed using
> atomic instructions, but just atomic load, followed by normal arithmetics,
> followed by atomic store.
> Can't 32-bit counters be used instead on 32-bit arches?
> I see there is another spot with atomic_uint64_t in sanitizer_lfstack.h,
> but for some reason it isn't used now at all (there it would want to use
> 64-bit compare and exchange).

On ARM, while it supposedly links, because __sync_compare_and_exchange_8
is defined in libgcc.a, it will only work with post-2011 kernels and is
going to be very slow (because you do a separate compare and exchange
to load and separate compare and exchange to store, i.e. twice as much
effort to actually not achieve atomicity, sometimes even 2 syscalls).


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