Are constructors matched against using "switch" or chained if-else

[Clinton Mead]

David,

A random google search has revealed this StackOverflow answer
<https://stackoverflow.com/a/27324088/525980>, presumably by yourself or
your evil twin, which mentions a binary search being performed. However the
particular case you mention is a case match on "Ints", which are far from a
dense set, whereas presumably you could do something nicer on the tag bits
of constructors, which are a dense set?


On Wed, Feb 23, 2022 at 1:59 PM David Feuer <david.feuer at gmail.com> wrote:

> You can ask, but someone else will have to answer. Sorry.
>
> On Tue, Feb 22, 2022 at 9:52 PM Clinton Mead <clintonmead at gmail.com>
> wrote:
> >
> > 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.feuer at 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 <clintonmead at 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 at haskell.org
> >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
>
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