On Mon, Jan 28, 2013 at 4:27 PM, Artyom Kazak
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