[Haskell-cafe] How to write fast for loops

Niklas Hambüchen mail at nh2.me
Mon Apr 28 02:23:54 UTC 2014

I've just uploaded my benchmarks to https://github.com/nh2/loop.

Please take a look. There are some interesting results.

The first thing I don't understand at all is:


See how w32/loop and w32/unsafeLoop are equally fast. Then look at


Here I run the same thing over the whole of Word32.
See how `loop` is faster here than `unsafeLoop`? How does that make sense?

Next thing:

It seems that V.enumFromTo and V.fromList are actually the same:


However in my benchmark, at least for `Int`, the performance is
different - am I overlooking something?

On 28/04/14 01:34, John Lato wrote:
> It can make a difference in that, with unboxed vectors, the compiler can
> statically determine that it is able to use unboxed values, and
> therefore is more likely to do so.  Having finally broken down and run
> some tests, I can report that on my system using V.enumFromTo with
> unboxed vectors results in the same performance as the hand-written loop.

I cannot see a difference between Vector.enumFromTo and
Vector.Unboxed.enumFromTo in my benchmark.

Vector.enumFromTo is as fast as the hand-written loop, but only for
`Int`. For `Word32`, it is 5 times slower. Any idea why?

>     > I would expect it's because you never force the argument.  With
>     > `enumFromTo` the argument is forced because it needs to be checked for
>     > termination, but `enumFromN` is probably building up a big chain of
>     > thunks.  I guess for this case `enumFromN` has no benefit over
>     > `enumFromTo` because the intention is to create a single loop
>     instead of
>     > actually allocating the vector, so the warning in the documentation
>     > doesn't necessarily apply.
>     Also haven't checked that yet, but I suspect that instead of something
>     thunk-related, the thing plainly allocates the vector.
>     Just to clarify: `V.enumFromTo` works much better than `V.enumFromN`
>     because in contrast to the latter it doesn't actually try to create the
>     fully sized vector.
> I believe that if you check this with ghc-7.6.3 and -O2, you will
> discover that my analysis is correct :)

Ok, I think I understand now what you mean.

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