linux-kernel - Re: [PATCH v2] clarify usage expectations for cnt32_to

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Date:	Mon, 10 Nov 2008 19:26:36 -0500 (EST)
From:	Nicolas Pitre <nico@....org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	mathieu.desnoyers@...ymtl.ca, torvalds@...ux-foundation.org,
	rmk+lkml@....linux.org.uk, dhowells@...hat.com, mingo@...e.hu,
	a.p.zijlstra@...llo.nl, linux-kernel@...r.kernel.org,
	ralf@...ux-mips.org, benh@...nel.crashing.org, paulus@...ba.org,
	davem@...emloft.net, mingo@...hat.com, tglx@...utronix.de,
	rostedt@...dmis.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH v2] clarify usage expectations for cnt32_to_63()

On Mon, 10 Nov 2008, Andrew Morton wrote:

> On Mon, 10 Nov 2008 18:15:32 -0500 (EST)
> Nicolas Pitre <nico@....org> wrote:
> 
> > On Mon, 10 Nov 2008, Andrew Morton wrote:
> > 
> > > This references its second argument twice, which can cause correctness
> > > or efficiency problems.
> > > 
> > > There is no reason that this had to be implemented in cpp. 
> > > Implementing it in C will fix the above problem.
> > 
> > No, it won't, for correctness and efficiency reasons.
> > 
> > And I've explained why already.
> 
> I'd be very surprised if you've really found a case where a macro is
> faster than an inlined function.  I don't think that has happened
> before.

That hasn't anything to do with "a macro is faster" at all.  It's all 
about the order used to evaluate provided arguments.  And the first one 
might be anything like a memory value, an IO operation, an expression, 
etc.  An inline function would work correctly with pointers only and 
therefore totally break apart on x86 for example.


Nicolas
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