index variable for array traversal worker function is not unboxed

See also http://hackage.haskell.org/trac/ghc/ticket/3606.

In an attempt to write a better comparison function for unboxed arrays, I wrote the following code:

{-# LANGUAGE BangPatterns #-}
{-# OPTIONS -Wall #-}

module BadArrayCmp where

import Data.Array.Base
import Data.Array.Unboxed


cmpArrays :: (IArray a1 e, IArray a2 e, Ix i, Ord e)
          => a1 i e
          -> a2 i e
          -> Ordering
cmpArrays a1 a2 = case compare (bounds a1) (bounds a2) of
                    LT -> LT
                    EQ -> go 0
                    GT -> GT
  where
    iMaxPlusOne :: Int
    iMaxPlusOne = numElements a1
    
    go :: Int -> Ordering
    go !i = if i == iMaxPlusOne
            then EQ
            else case compare (unsafeAt a1 i) (unsafeAt a2 i) of
                   LT -> LT
                   EQ -> go (i+1)
                   GT -> GT
{-# INLINE cmpArrays #-}

I have attached this code as a test case.

Examining the core generated by ghc 6.10.4 using -O2 on Ubuntu 9.04 x86_64, the code generated for 'go' uses a boxed Int for the index variable.

Because of this boxed int (and perhaps other problems in the generated code, I'm not sure), in a particular application of mine, ~60% of the total time is used and %80% of the total allocation is done in 'cmpArrays'. (These percentages obtained via profiling.)

I imagine that 'go' could be compiled so that the only potential allocation done is for the Ordering value at the end. Using a boxed index variable instead of something kept in a register really kills performance!

Trac metadata

Trac field	Value
Version	6.10.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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