On Tuesday 29 January 2013, 03:27:41, Artyom Kazak 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?
It's not necessarily performed twice.
func a b = case a `quotRem` b of
(q, r) -> q+r
produces
idivq 8(%rbp)
movq %rax,%rbx
movq $GHC.Int.I32#_con_info,-8(%r12)
movslq %edx,%rax
movslq %ebx,%rbx
addq %rax,%rbx
as the relevant part of the assembly, with only one idivq instruction.
I don't know whether you can rely on the code generator emitting only one
division instruction in all cases, but in simple cases, it does (on x86_64, at
least).
Cheers,
Daniel
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