Loop optimisation with identical counters

[Christian Hoener zu Siederdissen]

Is there a ticket for this (didn't find one)? Or should there be?
For some reason, I'd like to see this in ghc ;-)

Gruss,
Christian

On 11/03/2010 11:54 AM, Roman Leshchinskiy wrote:
> LLVM doesn't eliminate the counters. FWIW, fixing this would improve performance of stream fusion code quite a bit. It's very easy to do in Core.
> 
> Roman
> 
> On 3 Nov 2010, at 10:45, Christian Hoener zu Siederdissen <choener at tbi.univie.ac.at> wrote:
> 
>> 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
>>>>
>>>>
>>>>
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>>>
>>
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