On Tue, 6 Mar 2007, H. Peter Anvin wrote:
>
> That's true for *any* sprintf(), though. sprintf() converts all its arguments
> to 64 bits.
Well, it very much uses "do_div()", so that it can do a
64 / 32 -> (div64,mod32)
divide, which is often faster than a full 64:64 divide.
> However, this could be optimized. I think right now sprintf() uses a generic
> divide-by-base, but a divide by 8 and 16 can of course be handled with a
> shift, and divide by 10 can be replaced with a multiplication by
> 0x1999999999999999ULL on most architectures.
Nope. You need both the result of the division *and* the remainder, and
you can't do that with a single multiply.
Also, with modern hardware, divides are usually fairly cheap, with early
exit logic, so that the common case of small integers is fairly cheap.
Yeah, generating a full 64-bit number printout is still expensive, of
course (both because you need to do many divides *and* because only the
last few divides will be able to do any appreciable early exit logic.
Anyway, I think a full integer divide on Core 2 is something like 22
cycles. Yes, the multiply is much fasster (at 4 cycles), but I think that
22 cycles is actually worst-case.
Somebody who has a benchmark could try.
Linus
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