Re: [Xen-devel] [PATCHv2 4/6] x86: provide xadd()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.04.15 at 14:55, <ian.campbell@xxxxxxxxxx> wrote:
> On Thu, 2015-04-16 at 13:49 +0100, Tim Deegan wrote:
>> At 12:49 +0100 on 16 Apr (1429188559), Jan Beulich wrote:
>> > >>> On 16.04.15 at 13:25, <tim@xxxxxxx> wrote:
>> > > But also: AFAICS the GCC builtin __sync_fetch_and_add() does almost
>> > > exactly this (the difference being that those are also compiler
>> > > barriers where this is only a CPU barrier).  Should we be using it
>> > > instead?
>> > 
>> > I'm afraid that's useful only from gcc 4.5.x onwards; earlier versions
>> > (on x86 at least) simply generate a function call relying on a library to
>> > implement it.
>> 
>> Darn. :(
> 
> Might it still be a better generic fallback than the C/cmpxchg one?
> 
> On x86 you'd get the asm version, and on ARM you need 4.5.x onwards

Is that the case even for ARM32?

> anyway, plus in general the ARM folks seem to recommend using the
> builtins more anyway.

I assume you checked that 4.5.x actually expands said builtin for
ARM (as I only checked x86)?

If the answers to both are yes, then I certainly agree.

Jan


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