netdev - Re: [PATCH net-next 17/17] tcp: get rid of two unnecessary u16s in TCP skb flags copying

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Date:	Sat, 28 Feb 2009 19:08:45 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	andi@...stfloor.org
Cc:	ilpo.jarvinen@...sinki.fi, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 17/17] tcp: get rid of two unnecessary u16s in
 TCP skb flags copying

From: Andi Kleen <andi@...stfloor.org>
Date: Sun, 01 Mar 2009 03:39:02 +0100

> "Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi> writes:
> 
> > From: Ilpo JÃ¤rvinen <ilpo.jarvinen@...sinki.fi>
> >
> > I guess these fields were one day 16-bit in the struct but
> > nowadays they're just using 8 bits anyway.
> >
> > This is just a precaution, didn't result any change in my
> > case but who knows what all those varying gcc versions &
> > options do. I've been told that 16-bit is not so nice with
> > some cpus.
> 
> Typically when 16bit is not nice, 8bit isn't nice either.
> For general flags the most save in terms of uniform performance
> is "int"

It's really only the truly ancient alpha cpus where this is
a measurable issue.  Those chips only have 32-bit loads
and stores, so ever sub-32-bit access involves a 32-bit
load/store and a bunch of byte extraction instructions.

I don't think it's a relevant concern today.  Especially
on x86 if that's why you are alluding to Andi.

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