Loop optimization: identical counters

Consider the small program below, where 'f' has to counters 'i' and 'j'. Both are completely identical; the only difference is that 'i' is used to change 's', while 'j' changes 'm'. It would be beneficial to have GHC transform 'f' into something like 'ff' so that one register less is required.

Neither GHC nor LLVM perform this optimization.

Code of this kind occurs when one uses the "vector library". See this discussion: http://www.haskell.org/pipermail/glasgow-haskell-users/2010-November/019446.html

{-# LANGUAGE BangPatterns #-}

module Main where

import Criterion.Main

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
-}

main = defaultMain
  [ bench " g" $ whnf g  20 -- 67.9ns
  , bench "gg" $ whnf gg 20 -- 64.5ns
  ]

Function 'f' produces this core:

$wf =
  \ (ww_s1uU :: Int#)
    (ww1_s1uY :: Int#)
    (ww2_s1v2 :: Int#)
    (ww3_s1v6 :: Int#) ->
    case ww_s1uU of wild_B1 {
      __DEFAULT ->
        $wf
          (-# wild_B1 1)
          (-# ww1_s1uY 1)
          (+# (+# ww2_s1v2 wild_B1) 1)
          (+# ww3_s1v6 (*# ww1_s1uY 5));
      0 -> +# ww2_s1v2 ww3_s1v6
    }

'wild_B1' and 'ww1_s1uY' should be merged in this case.

The attached source is above program.

Trac metadata

Trac field	Value
Version
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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CC	choener@tbi.univie.ac.at
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Architecture