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Re: [PATCH v2 2/2] powerpc32: optimise csum_partial() loop

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On Thu, Aug 06, 2015 at 05:45:45PM -0500, Scott Wood wrote:
> > The original loop was already optimal, as the comment said.
> 
> The comment says that bdnz has zero overhead.  That doesn't mean the adde 
> won't stall waiting for the load result.

adde is execution serialising on those cores; it *always* stalls,
that is, it won't run until it is next to complete.

> > The new code adds extra instructions and a mispredicted branch.
> 
> Outside the main loop.

Sure, I never said it was super-bad or anything.

> >   You also might get less overlap between the loads and adde (I didn't check
> > if there is any originally): those instructions are no longer
> > interleaved.
> >
> > I think it is a stupid idea to optimise code for all 32-bit PowerPC
> > CPUs based on solely what is best for a particularly simple, slow
> > implementation; and that is what this patch is doing.
> 
> The simple and slow implementation is the one that needs optimizations the 
> most.

And, on the other hand, optimising for atypical (mostly) in-order
single-issue chips without branch folding, hurts performance on
other chips the most.  Well, dual-issue in-order might be worse :-P

> If this makes performance non-negligibly worse on other 32-bit chips, and is 
> an important improvement on 8xx, then we can use an ifdef since 8xx already 
> requires its own kernel build.  I'd prefer to see a benchmark showing that it 
> actually does make things worse on those chips, though.

And I'd like to see a benchmark that shows it *does not* hurt performance
on most chips, and does improve things on 8xx, and by how much.  But it
isn't *me* who has to show that, it is not my patch.

If these csum routines actually matter for performance that much, there
really *should* be chip-specific implementations.


Segher
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[PATCH v2 0/2] powerpc32: Optimise csum_partial() Christophe Leroy <christophe.leroy@c-s.fr> - 2015-08-05 15:30 +0200
  [PATCH v2 2/2] powerpc32: optimise csum_partial() loop Christophe Leroy <christophe.leroy@c-s.fr> - 2015-08-05 15:40 +0200
    Re: [PATCH v2 2/2] powerpc32: optimise csum_partial() loop Segher Boessenkool <segher@kernel.crashing.org> - 2015-08-06 02:40 +0200
      Re: [PATCH v2 2/2] powerpc32: optimise csum_partial() loop Scott Wood <scottwood@freescale.com> - 2015-08-06 04:40 +0200
        Re: [PATCH v2 2/2] powerpc32: optimise csum_partial() loop Segher Boessenkool <segher@kernel.crashing.org> - 2015-08-06 06:50 +0200
          Re: [PATCH v2 2/2] powerpc32: optimise csum_partial() loop Scott Wood <scottwood@freescale.com> - 2015-08-07 00:50 +0200
            Re: [PATCH v2 2/2] powerpc32: optimise csum_partial() loop Segher Boessenkool <segher@kernel.crashing.org> - 2015-08-07 01:30 +0200
              Re: [PATCH v2 2/2] powerpc32: optimise csum_partial() loop leroy christophe <christophe.leroy@c-s.fr> - 2015-08-17 13:00 +0200
                Re: [PATCH v2 2/2] powerpc32: optimise csum_partial() loop leroy christophe <christophe.leroy@c-s.fr> - 2015-08-17 13:10 +0200
                Re: [PATCH v2 2/2] powerpc32: optimise csum_partial() loop leroy christophe <christophe.leroy@c-s.fr> - 2015-08-17 15:10 +0200
  [PATCH v2 1/2] powerpc32: optimise a few instructions in csum_partial() Christophe Leroy <christophe.leroy@c-s.fr> - 2015-08-05 15:40 +0200

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