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Message-ID: <20070502130204.2cc2fd1a@the-village.bc.nu> Date: Wed, 2 May 2007 13:02:04 +0100 From: Alan Cox <alan@...rguk.ukuu.org.uk> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Josh Triplett <josh@...edesktop.org>, linux-sparse@...r.kernel.org, linux-kernel@...r.kernel.org, Al Viro <viro@....linux.org.uk> Subject: Re: sparse -Wptr-subtraction-blows: still needed? On Tue, 1 May 2007 17:24:54 -0700 (PDT) Linus Torvalds <torvalds@...ux-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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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