[rust-dev] Division and modulo for signed numbers
lists at dhardy.name
Tue Apr 23 08:53:35 PDT 2013
On Tuesday 23 April 2013 08:02:22 Patrick Walton wrote:
> On 4/23/13 7:48 AM, swede at earthling.net wrote:
> > Performance should be about the same when using F-division:
> > * Performance will go up for division by constant powers of two.
> > * Performance will stay the same for division by compile-time constants,
> > since these are transformed by the compiler into multiplies. (I
> > actually think performance will go up slightly in this case, but it's
> > been a while since I looked at the algorithm.) * Performance on ARM
> > will stay the same for divides by variables (not known at
> > compile-time), since ARM does not have a hardware divider. *
> > Performance on x86/x64 for divides by variables will go down slightly,
> > since Intel's idiv instruction implements F-division.>
> > So one already very slow operation (x86 idiv) gets slightly slower, one
> > fast operation (divide by power-of-two) gets quite a bit faster. It
> > probably nets out near zero.
> I worry quite a bit about that last one. Rust language semantics strive
> to mirror the CPU instructions as closely as possible. This is why, for
> example, we were forced to make `<` respect NaN, unlike Haskell--if we
> didn't, we couldn't use the hardware floating point comparison instructions.
> I'm also nervous about C interoperability.
> Including F-division as a library function sounds fine to me
> though--macros or dot notation may be able to sweeten the syntax.
May I jump on the band-wagon?
I also worry slightly about C interop — not from using libraries but because
copy and paste subtly changes the meaning.
However, I agree with Erik — F-division should be the default. For any case in
which I've cared, it's what I've wanted, and I've had a couple of bugs because
it's not in C++ (in code like arr[(ind - offset) % size]).
I suspect (please correct me if I'm wrong) that if it wasn't for C and x86
compatibility then most people would fall into two categories: don't
know/don't care, and prefer F-division. It's one of those little things like
tau vs. pi which would have been less confusing if we'd started off on the
other foot to start with.
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