linux-kernel - Re: [PATCH 10/25] Unionfs: add un/likely conditionals on copyup ops

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Date:	Wed, 26 Sep 2007 20:34:49 +0200
From:	Adrian Bunk <bunk@...nel.org>
To:	Erez Zadok <ezk@...sunysb.edu>
Cc:	"Kok, Auke" <auke-jan.h.kok@...el.com>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	viro@....linux.org.uk, hch@...radead.org
Subject: Re: [PATCH 10/25] Unionfs: add un/likely conditionals on copyup ops

On Wed, Sep 26, 2007 at 09:40:20AM -0400, Erez Zadok wrote:
>...
> Also, Auke, if indeed compilers are [sic] likely to do better than
> programmers adding un/likely wrappers, then why do we still support that in
> the kernel?  (Working for a company tat produces high-quality compilers, you
> may know the answer better.)
> 
> Personally I'm not too fond of what those wrappers do the code: they make it
> a bit harder to read the code (yet another extra set of parentheses); and
> they cause the code to be indented further to the right, which you sometimes
> have to split to multiple lines to avoid going over 80 chars.

There are some places in generic code where it makes sense, e.g.:
  #define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while(0)

If you run into a BUG() it's anyway game over.

And there are some rare hotpaths in the kernel where it might make 
sense, and many other places where the likely/unlikely usage that might 
be present doesn't make sense.

Unless you know you need it you simply shouldn't use likely/unlikely.

> Cheers,
> Erez.

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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