On Tue, 1 May 2007 17:24:54 -0700 (PDT)
Linus Torvalds <[email protected]> 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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