Optimization of IORefs and STRefs - comparison to g++

Akio Takano tkn.akio at gmail.com
Fri Dec 11 10:26:06 UTC 2015

Hi Mateusz,

IORef and STRef are boxed references. That is, they are a mutable cell
that contains a pointer to some immutable Haskell value. When you
increment a (STRef Int), you first dereference the pointer, allocate a
new immutable heap object to represent the new integer value, then
mutate the reference to point to the new object. This costs much more
than updating a plain mutable integer.

As far as I know there is no particularly popular library that
provides mutable references like this. As a workaround, you can create
a 1-sized unboxed mutable vector using the vector package, and use it
like a reference.

On 3 December 2015 at 01:10, Mateusz Kłoczko
<mateusz.p.kloczko at gmail.com> wrote:
> Hello!
> I've performed a few simple tests using Haskell and C++ on primitives.
> I've compilled all Haskell programs with -O2 optimizations, and C++ ones
> with -O3
> ghc version - 7.10.2, gcc version : 5.1.1
> I'm sending the codes in the zip file.
> Problem1 -  100 000 000  iterations.
> Time of execution (in seconds):
> Int  pure tail recursion: 6.911011299962411e-2
> Int# pure tail recursion : 4.587398100011342e-2
> IORef for loop 1.1533970820000832
> IORef 'tail' recursion 1.0696569040001123
> STRef for loop 1.1545546840006864
> STRef tail recursion 1.1110019479992843
> C++ : 2.7e-07

On this one, g++ manages to eliminate the loop entirely, but GHC doesn't.

> The llvm version could be as fast as C++ one in this problem.
> Buuut... then there's problem 2 (using if or case) - 100 000 000 iterations:
> Int# tail recursion 1.315346227000191
> IORef for loop: 2.6442542390004746
> STRef for loop: 2.669217500999366
> C++: 0.158056
> Here haskell is about 9 times slower than C++.

The main difference on this comes from the fact that GHC does not
optimize (n `remInt#` 2#) into (n `andI#` 1#). There is a ticket [0]
that contains some discussion of this issue.

[0]: https://ghc.haskell.org/trac/ghc/ticket/5615

Hope this helps,
Takano Akio

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