Re: Loop optimisation with identical counters

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 wrote:

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

Gruss, Christian

On 11/02/2010 01:23 PM, Simon Marlow wrote:

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On 02/11/2010 08:17, Christian Höner zu Siederdissen wrote:

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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

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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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