Re: [Xen-devel] [PATCH 2/2] xen: arm: update arm32 assembly primitives to Linux v3.16-rc6

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2014-07-25 at 16:48 +0100, Julien Grall wrote:
> On 07/25/2014 04:48 PM, Ian Campbell wrote:
> > On Fri, 2014-07-25 at 16:42 +0100, Julien Grall wrote:
> >> Hi Ian,
> >>
> >> On 07/25/2014 04:22 PM, Ian Campbell wrote:
> >>> bitops, cmpxchg, atomics: Import:
> >>>   c32ffce ARM: 7984/1: prefetch: add prefetchw invocations for barriered 
> >>> atomics
> >>
> >> Compare to Linux we don't have specific prefetch* helpers. We directly
> >> use the compiler builtin ones. Shouldn't we import the ARM specific
> >> helpers to gain in performance?
> > 
> > My binaries are full of pld instructions where I think I would expect
> > them, so it seems like the compiler builtin ones are sufficient.
> > 
> > I suspect the Linux define is there to cope with older compilers or
> > something.
> 
> If so:

The compiled output is very different if I use the arch specific
explicit variants. The explicit variant generates (lots) more pldw and
(somewhat) fewer pld. I've no idea what this means...

Note that the builtins presumably let gcc reason about whether preloads
are needed, whereas the explicit variants do not. I'm not sure how that
results in fewer pld with the explicit variant though! (unless it's
doing some sort of peephole optimisation and throwing them away?)

I've no idea what the right answer is.

How about we take the updates for now and revisit the question of
builtin vs explicit prefetches some other time?


> Acked-by: Julien Grall <julien.grall@xxxxxxxxxx>
> 
> Regards,
> 



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