netdev - Re: [PATCH 3/9] [TCP]: Remove unnecessary local variables

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Date:	Mon, 31 Dec 2007 04:47:59 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	ilpo.jarvinen@...sinki.fi
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH 3/9] [TCP]: Remove unnecessary local variables

From: "Ilpo_Järvinen" <ilpo.jarvinen@...sinki.fi>
Date: Mon, 31 Dec 2007 12:47:51 +0200

> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
 ...
>  
> -	in_flight = tcp_packets_in_flight(tp);
> -	cwnd = tp->snd_cwnd;
> -	if (in_flight < cwnd)
> -		return (cwnd - in_flight);
> +	if (tcp_packets_in_flight(tp) < tp->snd_cwnd)
> +		return tp->snd_cwnd - tcp_packets_in_flight(tp);
>  

I don't know about this one.

Although tcp_packets_in_flight() is inline and the compiler
should CSE the first call into a local register and not
redo the calculation:

1) That isn't something to rely upon.  The compiler might look
   at a function or set of functions in this file and decide
   to not inline tcp_packets_in_flight() or not see the CSE
   opportunity and that the second call is redundant.

2) If we stop inlining tcp_packets_in_flight() then nobody, and I
   mean nobody, is going to remember to come back here and
   add back the code to put the result in a local variable.

So best to keep the local vars here.

I also think the code is clearer that way too.

Thanks.
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