linux-kernel - Re: [PATCH 01/18] flag parameters: helper function

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Date:	Mon, 5 May 2008 19:09:32 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Ulrich Drepper <drepper@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	netdev@...r.kernel.org, mtk.manpages@...il.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 01/18] flag parameters: helper function

On Mon, 5 May 2008, Andrew Morton wrote:

> > +int flags_remap(const struct flags_rmap *m, int n,
> > +		int f, int *rf)
> > +{
> > +	int i;
> > +	for (i = 0, *rf = 0; f && i < n; i++, m++)
> > +		if (f & m->f) {
> > +			*rf |= m->of;
> > +			f &= ~m->f;
> > +		}
> > +	return f;
> > +}
> 
> hm, that looks expensive.  The compiler will need to generate a deref of m
> and rf multiple times around the loop.  Copying them into locals does
> improve that a lot.

That's not a fast path. The extra cycles in the ldr/str get lost in the 
overall syscall cost. But I guess Uli (or myself afterward) can change it 
if you fill picky about it ;)



> I'm only on [1/18] so I don't know how often this code gets executed.  If
> it's "on each open" then ouch, perhaps it might even be worth investigating a
> table-based implementation.
> 
> Also: sorry, but ugh-at-the-naming.  We don't *gain* anything from having
> idenitifers called f, of, m, n and rf.  And we lose quite a lot in
> readability and understandability.  It would be much nicer to invest a
> little bit more typing-time here, IMO.

Why? Don't you like nibble-sized variable names? :)
Uli took my email-code as is, so either he changes it, or I'll post 
patches over it later.




- Davide


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