Re: GCC libatomic ABI specification draft

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

On 20/12/16 13:26, Ulrich Weigand wrote:
> Torvald Riegel wrote:
>> On Fri, 2016-12-02 at 12:13 +0100, Gabriel Paubert wrote:
>>> On Thu, Dec 01, 2016 at 11:13:37AM -0800, Bin Fan at Work wrote:
>>>> Thanks for the comment. Yes, the ABI requires libatomic must query the hardware. This is 
>>>> necessary if we want the compiler to generate inlined code for 16-byte atomics. Note that 
>>>> this particular issue only affects x86. 
>>>
>>> Why? Power (at least recent ones) has 128 bit atomic instructions
>>> (lqarx/stqcx.) and Z has 128 bit compare and swap. 
>>
>> That's not the only factor affecting whether cmpxchg16b or such is used
>> for atomics.  If the HW just offers a wide CAS but no wide atomic load,
>> then even an atomic load is not truly just a load, which breaks (1)
>> atomic loads on read-only mapped memory and (2) volatile atomic loads
>> (unless we claim that an idempotent store is like a load, which is quite
>> a stretch for volatile I think).
> 
> 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.