[Haskell-cafe] Analyzing slow performance of a Haskell program

[Reiner Pope]

On 9 August 2011 10:06, Bryan O'Sullivan <bos at serpentine.com> wrote:

> On Mon, Aug 8, 2011 at 9:24 AM, Chris Yuen <kizzx2+haskell at gmail.com>wrote:
>
>>
>> For reference I have asked the same question on StackOverflow. One person
>> suggested that the reason might be that Int64 on Windows is broken (
>> http://stackoverflow.com/questions/6970904/analyzing-slow-performance-of-a-haskell-program/6976448#6976448
>> ).
>>
>
> No, they're barking up the wrong tree.
>
> I've put an idiomatic Haskell translation of your C++ algorithm at
> https://gist.github.com/1133048#file_wordy.hs
>
> (I've also included a copy of your original C++, with a bug fixed, in the
> same gist.)
>
> As you can see, the two are almost identical. Not surprisingly, each one
> spends the bulk of its time computing word lengths.
>
> GHC simply doesn't do a great job of compiling fairly tight code like this.
> gcc generates about 100 lines of assembly that's mostly easy to follow
> (except for some bit-twiddling tricks to avoid div instructions). Although
> the Core it generates looks fine, GHC spends quite a bit of time in its
> generated assembly on what looks to me like STG housekeeping (it spends only
> 0.3% of its time in the garbage collector, because it doesn't allocate
> memory). The overall result is that the Haskell code runs about 5x more
> slowly than the C++ code.
>
>
GHC generating bad assembly suggests trying the llvm codegen (see
http://donsbot.wordpress.com/2010/02/21/smoking-fast-haskell-code-using-ghcs-new-llvm-codegen/).
Compiling Bryan's code with

$ ghc -O2 -fllvm Wordy.hs

it now runs only 2x slower than the C++ code.

Reiner
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