libsanitizer merge from upstream r208536

[Ramana Radhakrishnan]

On Tue, May 27, 2014 at 11:00 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> 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

FTR, this call down to the library function should only be for legacy
architectures.

On ARM we have a 64 bit atomic compare and exchange which can be done
with the ldrexd / strexd instructions at the right architecture level
(v6k and above IIRC).

Ramana