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Date:   Tue, 28 Aug 2018 16:22:07 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Waiman Long <longman@redhat.com>,
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Subject: Re: [PATCH 2/2] fs/dcache: Make negative dentries easier to be
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On Tue, 28 Aug 2018 16:10:24 -0700 Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Tue, Aug 28, 2018 at 3:29 PM Waiman Long <longman@redhat.com> wrote:
> >
> > Yes, I can rewrite it. What is the problem with the abbreviated form?
> 
> Either gcc rewrites it for you, or you end up _actually_ using a
> function pointer and calling through it.
> 
> The latter would be absolutely horribly bad for something like
> "list_add()", which should expand to just a couple of instructions.
> 
> And the former would be ok, except for the "you wrote code the garbage
> way, and then depended on the compiler fixing it up". Which we
> generally try to avoid in the kernel.
> 
> (Don't get me wrong - we definitely depend on the compiler doing a
> good job at CSE and dead code elimination etc, but generally we try to
> avoid the whole "compiler has to rewrite code to be good" model).
> 

And the "abbreviated form" will surely explode if one or both of those
"functions" happens to be implemented (or later reimplemented) as a macro.
It's best not to unnecessarily make such assumptions.