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Re: use of LIKELY() and UNLIKELY() branch predictors

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From:
Tim Bunce
Date:
January 29, 2013 15:36
Subject:
Re: use of LIKELY() and UNLIKELY() branch predictors
Message ID:
20130129153619.GB3369@timac.local
On Tue, Jan 29, 2013 at 11:39:50AM +0000, David Cantrell wrote:
> On Mon, Jan 28, 2013 at 04:24:38PM +0000, Dave Mitchell wrote:
> 
> > Perl defines the LIKELY() and UNLIKELY() macros, which (under gcc)
> > tell the compiler whether a particular expression is likely to be true or
> > not...
> > 
> >     if (UNLIKELY(rare_condition)) {
> > 	.. do something rare, e.g. warn ...
> >     }
> > 
> > which will hopefully allow the compiler to do clever things.
> > 
> > Now, my thoughts are:
> > 
> > 1) Has anyone looked at this recently to know whether gcc does anything
> > useful with it?
> > 
> > 2) does anyone know whether other compilers provide similar facilities,
> > and so want to contribute suitable macro defs?
> > 
> > 3) if it is useful, should we be using it in lots of places?
> 
> 4) is the cost in making the perl source code even more impenetrable due
> to being a twisty maze of macros worth it?

We won't know unless someone tries, and then we can judge before
applying the patch.

And, as Dave said "It seems more like work that nearly anyone could do".

Any volunteers?

Tim.

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