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

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Date:	Sun, 1 Mar 2009 22:44:41 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	David Miller <davem@...emloft.net>
Cc:	andi@...stfloor.org, 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

On Sat, Feb 28, 2009 at 07:08:45PM -0800, David Miller wrote:
> 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.

Yes old Alpha is the only one I know, although I don't claim to know every 
weirdness of every embedded CPU out there.

My point was merely that 16bit is not in any way better than 8 bit.
> 
> I don't think it's a relevant concern today.  Especially
> on x86 if that's why you are alluding to Andi.

x86 normally[1] doesn't care, but it also doesn't save anything normally
for a on stack (or in register) variable because there's typically
padding anyways.

[1] there are some issues on older Intel CPUs with partial register
stall in a few cases.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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