[Haskell-cafe] Re: Array copying

[ChrisK]

Andrew Coppin wrote:
> ChrisK wrote:
>> For GHC 6.6 I created
>>
>>  
>>> foreign import ccall unsafe "memcpy"
>>>     memcpy :: MutableByteArray# RealWorld -> MutableByteArray#
>>> RealWorld -> Int# -> IO ()
>>>     
>>
>>  
>>> {-# INLINE copySTU #-}
>>> copySTU :: (Show i,Ix i,MArray (STUArray s) e (ST s)) => STUArray s i
>>> e -> STUArray s i e -> ST s ()
>>> copySTU (STUArray _ _ msource) (STUArray _ _ mdest) =
>>> -- do b1 <- getBounds s1
>>> --  b2 <- getBounds s2
>>> --  when (b1/=b2) (error ("\n\nWTF copySTU: "++show (b1,b2)))
>>>   ST $ \s1# ->
>>>     case sizeofMutableByteArray# msource        of { n# ->
>>>     case unsafeCoerce# memcpy mdest msource n# s1# of { (# s2#, () #) ->
>>>     (# s2#, () #) }}
>>>     
>>
>> To allow efficient copying of STUArrays.
>>   
> 
> So... that copies the entire array into another array of the same size?
> (I'm having a lot of trouble understanding the code...)

Yes, that is what it does.  The "STUArray" data type has the STUArray
constructor which I import and pattern match against.  The imports are:

> import Data.Array.Base(unsafeRead,unsafeWrite,STUArray(..))
> import GHC.Prim(MutableByteArray#,RealWorld,Int#,sizeofMutableByteArray#,unsafeCoerce#)

in 6.6.1 this is defined as
> data STUArray s i a = STUArray !i !i (MutableByteArray# s)
in 6.8.1 this is defined as
> data STUArray s i a = STUArray !i !i !Int (MutableByteArray# s)

I use sizeofMutableByteArray# to get the source size, n#.

I have lost track of how unsafeCoerce# and s1# are being used...oops.
It is similar to data-dependency tricks used inside Data.Array.Base, though.

-- 
Chris