Modernizing atomics

[Norbert Thiebaud]

On Mon, Oct 26, 2015 at 2:56 PM, Ashod Nakashian <ashnakash at gmail.com> wrote:
> On Mon, Oct 26, 2015 at 2:21 PM, Norbert Thiebaud <nthiebaud at gmail.com>
> wrote:
>>
>> On Mon, Oct 26, 2015 at 1:00 PM, Ashod Nakashian <ashnakash at gmail.com>
>> wrote:
>> > On Mon, Oct 26, 2015 at 1:35 PM, Norbert Thiebaud <nthiebaud at gmail.com>
>> > wrote:
>> >>
>> >> On Mon, Oct 26, 2015 at 12:14 PM, Ashod Nakashian <ashnakash at gmail.com>
>> >> wrote:
>> >> > OSL provides atomic helpers (osl_atomic_xxx) in the form of a GNU
>> >> > builtin
>> >> > (where available) or a platform-specific implementation.
>> >> >
>> >> > Any reason for not using modern std::atomic (besides possible lack of
>> >> > volunteers) ?
>> >> >
>> >> >
>> >> > As a transitional phase, we can maintain the same interface but with
>> >> > std:atomic as the implementation.
>> >> >
>> >> > Thoughts?
>> >>
>> >> osl atomic are c interface, used in c-source...
>> >>
>> > Thanks. Is there equivalent used in C++ ? (osl atomics only work for
>> > sal_Int32 values, which is another potential issue for 64-bit
>> > portability.)
>>
>> the c++ code use these too.
>
>
> Would there be support for using std::atomic in C++ code?
>
> There is a case to be made in terms of performance if nothing else (in some
> scenarios they are hotspots, according to my profiler).

I seriously doubt that std:: will improve the performance over
__sync_add_and_fetch((p), 1)
and
__sync_sub_and_fetch((p), 1)

and fro windows, the only real gain would be to move the
implementation in include/osl
with
#define osl_atomic_increment(p) InterlockedIncrement(pCount)
and
#define osl_atomic_decrement(p) InterlockedDecrement(pCount)

that will give you most it not all of the gain.

(Note: I did not mess with Windows back then when I did that for gcc,
as I was not in a position to test it properly,
nor did I have the inclination to mess with Windows in general, as
long as I can avoid it.
but really that should be fairly easy to do)

>
>>
>>
>> relying on atomic on 64 bits is going to be a problem as long as we
>> support 32 bits OS.
>
>
> I believe most modern hardware support atomic operations on wide words (i.e.
> 64-bit even when running in 32-bit mode).

yes, but bear in mode that we had to rollback patches that required
SSE2 on windows...
The hardware baseline is quite old...
In any case see below, osl only provide atomic increment/decrement for
sal_uInt32 explicitly.
If there is a need for atomic over another type of data, that will
involve something else than sal/osl

Note also that the osl API is a published API... that is why, although
internally on gcc/clang platfrom we use
the built-in directly via macro, the entry point in osl is maintained
for API compatibility purpose.


>
>>
>> and mostly these atomic are used to ref-count... and there is really
>> no reasonable need to have 64 bits ref-count is there ?
>
>
> True for ref-counting. Not so for compare-exchange obviously (but I don't
> know if these are used and how much).

osl does not implement/expose any compare-and-swap api AFAIK.
And honestly considering the horror of your locking 'model', having
such CAS api would be pretty silly.

Norbert