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

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Thu, 2015-04-16 at 14:19 +0100, Jan Beulich wrote:
> >>> 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?

Erm, I thought so when I wrote it but now I think again I'm not actually
so sure.

> > 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)?

No, I hadn't, I should though, you are right.

Ian.


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