linux-kernel - sparse -Wptr-subtraction-blows: still needed?

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Date:	Tue, 01 May 2007 14:08:13 -0700
From:	Josh Triplett <josh@...edesktop.org>
To:	linux-sparse@...r.kernel.org, linux-kernel@...r.kernel.org
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@....linux.org.uk>
Subject: sparse -Wptr-subtraction-blows: still needed?


Sparse has a warning -Wptr-subtraction-blows (off by default) which generates
a warning for any pointer subtractions.  This warning relates to GCC
shortcomings observed in 2005; the original log message:

> commit 6889bd0f84939675c743229d6fe623513b95e057
> Author: Linus Torvalds <torvalds@...970.osdl.org>
> Date:   Fri Jan 7 15:06:24 2005 -0700
>
>     Add option "-Wptr-subtraction-blows" to warn about expensive
>     pointer subtractions.
>     
>     Not only does it generate bad code (that can often be rewritten
>     to not do that), it also causes gcc to go into horrible contortions,
>     and Al Viro reports that it can make a factor of 2.5 difference in
>     kernel build times to have just a few of these in common header
>     file inline functions.

Does this still apply?  Do current versions of GCC still have this problem?
If not, can the option and warning go away?

- Josh Triplett


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