netdev - Re: [PATCH net-next] ipv4: use IS_ENABLED() macro to cleanup code

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Date:	Wed, 26 Oct 2011 15:56:20 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] ipv4: use IS_ENABLED() macro to cleanup code

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 26 Oct 2011 08:34:34 +0200

> Le mardi 25 octobre 2011 à 19:30 -0400, David Miller a écrit :
> 
>> net/ipv4/ip_sockglue.c: In function ‘do_ip_setsockopt’:
>> net/ipv4/ip_sockglue.c:523:29: error: ‘LOOPBACK4_IPV6’ undeclared (first use in this function)
>> net/ipv4/ip_sockglue.c:523:29: note: each undeclared identifier is reported only once for each function it appears in
>> 
>> This fails because ip_sockglue.c guards the net/transp_v6.h header
>> inclusion with a real CPP guard.
> 
> Yep, it seems compiler was not able to perform optimisation of dead
> code.

It's not the issue of optimizing the code out, in the end the compiler
will optimize this stuff out.

Rather, the issue is that the code seen by the compiler must be parsable.