[Bug middle-end/51766] [4.7 regression] sync_fetch_and_xxx atomicity

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51766

--- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-01-10 09:43:16 UTC ---
(In reply to comment #3)
> > It says above them "In most cases, these
> > builtins are considered a full barrier." and only __sync_lock_test_and_set and
> > __sync_lock_release specify different barrier semantics.
> 
> The next sentence is:
> 
> "That is, no memory operand will be moved across the operation, either forward
> or
> backward."
> 
> Note that this refers to memory operands, not memory operations -- memory
> stores and memory loads referenced in documentation of the other sync builtins.
>  In other words, one could interpret "full memory barrier" as:
> 
> asm volatile ("" : : : "memory");
> 
> that refers to a GCC scheduling barrier.
> 
> The GCC documentation references Intel processors, which do not have have a
> distinction between instructions for RELEASE, ACQ_REL and SEQ_CST semantics.
> 
> The basic problem is that the GCC builtins and atomic instruction semantics
> were designed for Intel processors that do not provide the level of granularity
> implemented in POWER processors.  The POWER port implemented lighter weight
> ACQ_REL semantics. Retrofitting the original builtins on the new C++11 memory
> model semantics and imposing SEQ_CST interpretation has changed the behavior
> and performance on POWER, but not on other targets.

But for more precise semantics we now have the __atomic_* builtins, right?
And the __sync_* ones are deprecated.  I don't see how we can preserve
old behavior for the __sync_* ones without adding a new target hook.
The documentation would need to be adjusted, of course, I'm not sure
that different atomic semantics are "useful" for these "portable"
synchronization primitives?

Thus, fixing the libstdc++ side seems worthwhile, but I'm not sure
with respect to the deprecated __sync builtins?