# Loop optimisation with identical counters

Christian Hoener zu Siederdissen choener at tbi.univie.ac.at
Wed Nov 3 06:45:45 EDT 2010

```Thanks, I'll do some measurements on this with ghc7.

Gruss,
Christian

On 11/02/2010 01:23 PM, Simon Marlow wrote:
> On 02/11/2010 08:17, Christian Höner zu Siederdissen wrote:
>> Hi,
>>
>> is the following problem a job for ghc or the code generation backend
>> (llvm)?
>>
>> We are given this program:
>>
>> {-# LANGUAGE BangPatterns #-}
>>
>> module Main where
>>
>> f :: Int ->  Int ->  Int ->  Int ->  Int
>> f !i !j !s !m
>>    | i == 0    = s+m
>>    | otherwise = f (i-1) (j-1) (s + i+1) (m + j*5)
>>
>> g :: Int ->  Int
>> g !k = f k k 0 0
>>
>>
>> ff :: Int ->  Int ->  Int ->  Int
>> ff !i !s !m
>>    | i == 0    = s+m
>>    | otherwise = ff (i-1) (s + i+1) (m + i*5)
>>
>> gg :: Int ->  Int
>> gg !k = ff k 0 0
>>
>> main = do
>>    print \$ g 20
>>    print \$ gg 20
>>
>>
>> Here, 'f' and 'g' are a representation of the code I have. Both counters
>> 'i' and 'j' in 'f' count from the same value with the same step size and
>> terminate at the same time but are not reduced to just one counter. Can
>> I reasonably expect this to be done by the code generator?
>> 'ff' represents what I would like to see.
>
> GHC doesn't have any optimisations that would do this currently,
> although it's possible that LLVM's loop optimisations might do this on
> the generated code for f.
>
> Cheers,
>     Simon
>
>
>
>> Btw. look at the core, to see that indeed 'f' keep four arguments.
>> Functions like 'f' are a result of vector-fusion at work but can be
>> written by oneself as well. The point is that if 'f' gets reduced to
>> 'ff' then I can have this:
>>
>> fun k = zipWith (+) (map f1 \$ mkIdxs k) (map f2 \$ mkIdxs k)
>>
>> which makes for nicer code sometimes; but before rewriting I wanted to
>> ask if that kills performance.
>>
>>
>> Thanks,
>> Christian
>>
>>
>>
>> _______________________________________________