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Re: [Xen-devel] [PATCH v2 3/3] x86/mm: re-order a few conditionals


  • To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Mon, 6 Jan 2020 16:36:29 +0000
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  • Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Mon, 06 Jan 2020 16:36:35 +0000
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On 06/01/2020 15:35, Jan Beulich wrote:
> is_{hvm,pv}_*() can be expensive now, so where possible evaluate cheaper
> conditions first.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

> ---
> v2: New.
> ---
> I couldn't really decide whether to drop the two involved unlikely().

Personally, I don't think we should have any likely/unlikley annotations
at all.  They are difficult for humans to reason about (especially when
you're in a nested clause of annotated condition) - several of them
are(/were) wrong, and plenty are dubious.

People who actually care should be using PGO.  This is yet another
toolchain feature I'm hoping that we will get "for free" from Anthony's
work to switch to using kbuild.

If we were to delete all likely/unlikley annotations, and someone could
then measure a performance improvement from reinserting some of them,
then perhaps it would be ok to keep a few around, but my gut feeling is
that the compiler can do a better job in general than humans can.

~Andrew

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