# Ticket #1094 (closed feature request: duplicate)

Opened 6 years ago

## loop optimization by removing constants

Reported by: Owned by: Azmo normal 6.8.1 Compiler 6.6 loop constants optimization Unknown/Multiple Unknown/Multiple Unknown

### Description

the current version of Data.List.foldl' is:

```foldl'           :: (a -> b -> a) -> a -> [b] -> a
foldl' f a []     = a
foldl' f a (x:xs) = let a' = f a x in a' `seq` foldl' f a' xs
```

which for some reason is faster if removing the first parameter, e.g:

```foldl2'           :: (a -> b -> a) -> a -> [b] -> a
foldl2' f = loop
where loop a []     = a
loop a (x:xs) = let a' = f a x in a' `seq` loop a' xs

```

Testing speed difference:

```module Main (main) where

import System (getArgs)

sumFoldl' :: Int -> Int
sumFoldl' n = foldl' (+) 0 [1..n]

sumFoldl2' :: Int -> Int
sumFoldl2' n = foldl2' (+) 0 [1..n]

-- copy of Data.List.foldl'
foldl'           :: (a -> b -> a) -> a -> [b] -> a
foldl' f a []     = a
foldl' f a (x:xs) = let a' = f a x in a' `seq` foldl' f a' xs

-- altered version of foldl'
foldl2'           :: (a -> b -> a) -> a -> [b] -> a
foldl2' f = loop
where loop a []     = a
loop a (x:xs) = let a' = f a x in a' `seq` loop a' xs

main = do
[v,n] <- getArgs
case v of
"1" -> print (sumFoldl' (read n))
"2" -> print (sumFoldl2' (read n))

```

I have no idea how the optimization of loops is done, but the request is that this kind of loop rewriting should be done by the compiler, not by the user (to get a faster loop). It seems to just be a matter of finding and removing constants (identifiers just passed along).

## Change History

### Changed 6 years ago by kirsten

The optimization you suggest is called the "static argument transformation". There's already a ticket to implement it at:  http://hackage.haskell.org/trac/ghc/ticket/888

and if you're interested, there's an explanation there of why this isn't easy.

### Changed 6 years ago by simonpj

• status changed from new to closed
• resolution set to duplicate

Indeed. I've copied the example over to #888 (always good to have more examples), and am closing this one.

Simon

### Changed 6 years ago by igloo

• milestone changed from 6.8 branch to 6.8.1

### Changed 5 years ago by simonmar

• architecture changed from Multiple to Unknown/Multiple

### Changed 5 years ago by simonmar

• os changed from Multiple to Unknown/Multiple
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