Thanks David. Can I ask why? Is it because the first constructor is treated specially? (perhaps because it has zeroed tag bits)? Or is it just because there's always an if/else chain in order of the constructor definition regardless of the order of the case statement so the higher up the list the better?
On Wed, Feb 23, 2022 at 1:34 PM David Feuer <david.fe...@gmail.com> wrote: > I can answer one of your questions for sure: the order of your case > branches doesn't matter at all. However, the order of the data constructors > in the type declaration does matter. Put your most likely one first. > > On Tue, Feb 22, 2022, 9:09 PM Clinton Mead <clintonm...@gmail.com> wrote: > >> Hi All >> >> I'm developing an unbounded integer type, which I won't go into the >> details here but in some circumstances has better performance than the >> standard "Integer". >> >> Anyway, whilst there are complex cases, the most common case is a >> standard machine int multiplication. >> >> Hence I want the type to be optimised for that case. >> >> I'm going to have a few constructors, anyway, so I first considered >> something like this: >> >> `data MyInt = BasicZero | BasicPos Word | BasicNeg Word | ComplexPosA ... >> | ComplexNegA ... | ComplexPosB ... | ComplexNegB ...` >> >> I'd naturally make the "Word"s in "BasicPos" and "BasicNeg" >> strict/unpack, hopefully eliminating the indirection, or perhaps just >> making them primitive directly. >> >> This has 7 constructors, which quite nicely I believe fits into the three >> spare bits in a 64 bit pointer which GHC optimises I believe. >> >> However, this approach I thought of because I assumed that GHC would do a >> switch style statement on the constructors, so once I have more than one I >> might as well have as many as I want (well, up to 7, until I lose the >> optimisation). >> >> But if GHC compiles this to a "if ... else if..." chain, a better >> representation is the following: >> >> `data MyInt = BasicInt Int | ComplexPosA ... | ComplexNegA ... | >> ComplexPosB ... | ComplexNegB ...` >> >> That way I can match against BasicInt first, and knock that out of the >> way as my "hot" case. However, using "Int" instead of "Word" does make the >> logic a bit more complex, but it may be worth it if I'm reducing the number >> of patterns I have to check against for all arguments. >> >> I was just wondering if anyone could share some insight on what GHC does >> in these circumstances? For example, does the order I list my cases in a >> case statement matter if they're non-overlapping? Will GHC match them in >> the order I list, or will it just make them into a switch statement so it >> doesn't matter if I reorder them? >> >> I guess I could benchmark all this (and probably will) but some insights >> and general guidance would be good. >> >> Thanks, >> Clinton >> _______________________________________________ >> Glasgow-haskell-users mailing list >> Glasgow-haskell-users@haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users >> >
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