What if his code used fx+ and friends instead of + and friends? Robby
On Sat, Feb 18, 2012 at 10:04 PM, Vincent St-Amour <[email protected]> wrote: > At Sat, 18 Feb 2012 21:07:05 -0500, > SF wrote: >> I'm trying to optimize some code that processes a bunch of data, >> basically image pixels, which currently takes on the order of several >> seconds and seems like it should be much faster (important because >> it's an interactive application). Coming from C++ I have a good idea >> in my mind of how I'd write it with low-level operations. I read up on >> Racket optimization. It is still somewhat unclear to me how Racket >> deals with fixnums, flonums, unsafe operations, Typed Racket types, >> and how it all fits together. > > Racket's unsafe operations do not perform any checks and guide the > compiler's unboxing optimizations. Using them can lead to significant > speedups. > > Typed Racket performs type-driven optimization when it can guarantee > that doing so is safe. Most of its optimizations replace generic > arithmetic with specialized unsafe operations. The performance > benefits are equivalent to using unsafe operations manually. The > advantage of using TR is that it won't introduce bugs by optimizing > when it's not safe to do so. > > In practice, TR will optimize pretty much any floating-point > computation. In the case of fixnum operations, it optimizes only when > it can guarantee that no overflow will happen. This means that you may > need to help it a bit if you want to get fixnum optimizations. > > To see how TR optimizes your code (for example, to make sure that you > get all the optimizations you want), check out the "performance > report" DrRacket tool. > >> I chose to optimize my program (partly >> to learn how) incrementally, trying all the options, timing at every >> step with debugging info turned off. This worked until I wanted to >> convert Integers to Fixnums in Typed Racket (which has been giving me >> much faster results than anything in untyped Racket). > > You don't need to convert between integers and fixnums. Fixnums are > simply integers that are small enough to be represented as tagged > machine integers. If an operation on fixnums would cause an overflow, > the result is transparently promoted to a bignum instead. > > To use fixnums in TR, you can annotate your variables as being of > `Fixnum' type. However, due to the promotion semantics, fixnums have > pretty bad closure properties, which means that the result of, say, > adding two fixnums is not guaranteed to be a fixnum itself and has to > be given the `Integer' type. > > To help with this, TR provides an `Index' type which is for integers > bounded by the length of the longest possible vector (smaller than a > fixnum). Adding two of these is guaranteed to produce a `Fixnum'. > > The bottom line is that it's possible to get fixnum optimizations in > TR, but since TR has to be conservative in order to be safe, sometimes > you may have to use unsafe fixnum operations manually to get the > performance you want. > >> The problem is that I can't find a way to convert a Float to a Fixnum, >> only to an Integer. None of fl->fx, unsafe-fl->fx, or >> fl->exact-integer work, as Type Checker tells me they're untyped >> identifiers. > > That should work. I'll look into that. > > Vincent > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
