[Haskell-cafe] quotRem and divMod

[Shachaf Ben-Kiki]

On Mon, Jan 28, 2013 at 4:27 PM, Artyom Kazak <artyom.kazak at gmail.com> wrote:
> Hi!
>
> I’ve always thought that `quotRem` is faster than `quot` + `rem`, since both
> `quot` and `rem` are just "wrappers" that compute both the quotient and the
> remainder and then just throw one out. However, today I looked into the
> implementation of `quotRem` for `Int32` and found out that it’s not true:
>
>     quotRem x@(I32# x#) y@(I32# y#)
>         | y == 0                     = divZeroError
>         | x == minBound && y == (-1) = overflowError
>         | otherwise                  = (I32# (narrow32Int# (x# `quotInt#`
> y#)),
>                                         I32# (narrow32Int# (x# `remInt#`
> y#)))
>
> Why? The `DIV` instruction computes both, doesn’t it? And yet it’s being
> performed twice here. Couldn’t one of the experts clarify this bit?
>

That code is from base 4.5. Here's base 4.6:

    quotRem x@(I32# x#) y@(I32# y#)
        | y == 0                     = divZeroError
          -- Note [Order of tests]
        | y == (-1) && x == minBound = (overflowError, 0)
        | otherwise                  = case x# `quotRemInt#` y# of
                                       (# q, r #) ->
                                           (I32# (narrow32Int# q),
                                            I32# (narrow32Int# r))

So it looks like it was improved in GHC 7.6. In particular, by this
commit: http://www.haskell.org/pipermail/cvs-libraries/2012-February/014880.html

    Shachaf