On Tue, 1 May 2007 17:24:54 -0700 (PDT) Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> 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-sparse" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
