On 4/23/2013 9:02 AM, Patrick Walton wrote: > On 4/23/13 7:48 AM, sw...@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. > > Patrick > > _______________________________________________ > Rust-dev mailing list > Rust-dev@mozilla.org > https://mail.mozilla.org/listinfo/rust-dev > Patrick, I would agree with you if it weren't for the cases where F-division performs *better*. Division by a constant power-of-two is pretty common, and is much faster in F-division.
I don't think correctness should be sacrificed for a slight performance improvement in one case on one supported architecture. I've had real bugs in my C code caused by incorrectly casting (or forgetting to cast) to unsigned before a % operation. T-division causes *real* bugs. I'm willing to spend some time implementing F-division, if I can get some buy-in that the patch would be accepted. Erik _______________________________________________ Rust-dev mailing list Rust-dev@mozilla.org https://mail.mozilla.org/listinfo/rust-dev
