Loop optimisation with identical counters

[Roman Leshchinskiy]

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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>>> Glasgow-haskell-users at haskell.org
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>> 
> 
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