#4470: Loop optimization: identical counters
---------------------------------+------------------------------------------
Reporter: choenerzs | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: | Keywords: loop optimization
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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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.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4470>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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