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?
Date: Wed, 2 May 2007 13:02:04 +0100 [thread overview]
Message-ID: <20070502130204.2cc2fd1a@the-village.bc.nu> (raw)
In-Reply-To: <alpine.LFD.0.98.0705011719520.3808@woody.linux-foundation.org>
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
next prev parent reply other threads:[~2007-05-02 11:59 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-05-01 21:08 Josh Triplett
2007-05-01 21:43 ` Linus Torvalds
2007-05-01 23:59 ` Josh Triplett
2007-05-02 0:24 ` Linus Torvalds
2007-05-02 0:35 ` Al Viro
2007-05-02 12:02 ` Alan Cox [this message]
2007-05-02 13:19 ` Andi Kleen
2007-05-02 0:26 ` Al Viro
2007-05-02 0:02 ` Al Viro
2007-05-02 2:42 ` Dave Jones
2007-05-02 13:03 ` Andi Kleen
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