Date: Wed, 2 May 2007 13:02:04 +0100
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Josh Triplett <josh@freedesktop.org>, linux-sparse@vger.kernel.org,
       linux-kernel@vger.kernel.org, Al Viro <viro@ftp.linux.org.uk>
Subject: Re: sparse -Wptr-subtraction-blows: still needed?
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On Tue, 1 May 2007 17:24:54 -0700 (PDT)
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> 
> 
> On Tue, 1 May 2007, Josh Triplett wrote:
> > 
> > Do you know whether the current version of GCC generates poor code for pointer
> > subtraction?
> 
> You _cannot_ generate good code.
> 
> When you subtract two pointers, the C definition means that you first 
> subtract the values (cheap), and then you *divide* the result by the size 
> of the object the pointer points to (expensive!).

Good compilers even in the 1990's would defer the divide and try and
propogate it out as a multiply the other side for constants, and they'll
also use shifts when possible.

Thus they'll turn

	(ptr.element - base.element) < NELEM

into
	(ptr.char - base.char) < (constant) [NELEM *sizeof(element) ]


at least for constant operations. Dunno if gcc is that clever

Alan
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