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Re: GCC libatomic ABI specification draft


Szabolcs Nagy wrote:
> On 20/12/16 13:26, Ulrich Weigand wrote:
> > I may have missed the context of the discussion, but just on the
> > specific ISA question here: both Power and z not only have the
> > 16-byte CAS (or load-and-reserve/store-conditional), but they also both
> > have specific 16-byte atomic load and store instructions (lpq/stpq
> > on z, lq/stq on Power).
> > 
> > Those are available on any system supporting z/Architecture (z900 and up),
> > and on any Power system supporting the V2.07 ISA (POWER8 and up).  GCC
> > does in fact use those instructions to implement atomic operations on
> > 16-byte data types on those machines.
> 
> that's a bug.
> 
> at least i don't see how gcc makes sure the libatomic
> calls can interoperate with inlined atomics.

Hmm, interesting.  On z, there is no issue with ISA levels, since *all*
64-bit platforms support the 16-byte atomics (and on non-64-bit platforms,
16-byte data types are not supported at all).

However, there still seems to be a problem, but this time related to
alignment issues.  We do have the 16-byte atomic instructions, but they
only work on 16-byte aligned data.  This is a problem in particular
since the default alignment of 16-byte data types is still 8 bytes
on our platform (since the ABI only guarantees 8-byte stack alignment).

That's why the libatomic configure check thinks it cannot use the
atomic instructions when building on z, and generates code that uses
the separate lock.  However, *if* a particular object can be proven
by the compiler to be 16-byte aligned, it will emit the inline
atomic instruction.  This means there is indeed a bug if that same
object is also operated on via the library routine.

Andreas suggested that the best way to fix this would be to add a
runtime alignment check to the libatomic routines and also use the
atomic instructions in the library whenever the object actually
happens to be correctly aligned.  It seems that this should indeed
fix the problem (and also use the most efficient way in all cases).


Not sure about Power -- adding David and Segher on CC ...


Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  GNU/Linux compilers and toolchain
  Ulrich.Weigand@de.ibm.com


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