Re: Use of H98 FFI
Peter Thiemann [EMAIL PROTECTED] wrote, I recently had my first exposure to Haskell's FFI when I was trying to compute MD5 and SHA1 hashes using the existing C implementations. In each case, the idea is to make the hash function available as function md5 :: String - String However, the naive implementation md5_init md5_state n - newCString str md5_append md5_state n (fromIntegral (length str)) md5_finish md5_state md5_digest does not scale to computing hashes of really long strings (50 MB, say, as arising from reading a moderately big file), since it tries to create a CString of that size, first! Trying to avoid the allocation of this giant CString requires to split up the original string into smaller parts and convert each part to a CString separately. Clearly, this task involves a lot of allocation, essentially the input string needs to be copied part by part. Hence, I was wondering why the FFI only provides functionality to convert an *entire* list of Char into a CString. For applications like this hash computation, it would be advantageous to be able to specify *how much* of the input string to marshall to the CString and have the conversion function return the rest of the input string and the CString. That is, in addition to newCString :: String - IO CString there should be newCStringPart :: String - Int - IO (CStringLen, String) or even toCStringPart :: String - CStringLen - IO (Int, String) where CStringLen describes a target buffer into which the String argument is to be marshalled. (and similarly for other list types) The idea of the FFI Addendum is to provide the *basic* functionality needed for using foreign code from Haskell. However, it was explicitly never the aim to cover all possibly needed idioms. Instead, we expect that FFI tools (such as GreenCard or C-Haskell) and additional support libraries provide this coverage by mapping more complex marshaling requirements down to the basics described in the FFI Addendum. Clearly, I can program this functionality by hand. But I have to revert to byte-wise processing using pokeByteOff, castCharToCChar, and so on. In addition, the optimizer does not seem to be very effective on such code, so it seems advantageous to provide it in the library already. As Sven already pointed out, it seems like you should use Word8 instead of Char (which is unicode-based). The FFI libraries don't do anything but use pokeElemOff and friends themselves. So, performance-wise there will be no difference. For `pokeArray', the libraries use pokeArray :: Storable a = Ptr a - [a] - IO () #ifndef __GLASGOW_HASKELL__ pokeArray ptr vals = zipWithM_ (pokeElemOff ptr) [0..] vals #else pokeArray ptr vals = go vals 0# where go [] n# = return () go (val:vals) n# = do pokeElemOff ptr (I# n#) val; go vals (n# +# 1#) #endif ie the GHC code makes use of unboxed values, though. Cheers, Manuel PS: It might be useful to provide the functions you describe as part of the hierachical libraries, though. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: Use of H98 FFI
Peter Thiemann wrote: md5 :: String - String Hmmm, this should probably be: md5 :: [Word8] - [Word8] unless you really want the MD5 of the Unicode characters... Cheers, S. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: Use of H98 FFI
On 01 Aug 2003 09:44:14 +0200 Peter Thiemann [EMAIL PROTECTED] wrote: I recently had my first exposure to Haskell's FFI when I was trying to compute MD5 and SHA1 hashes using the existing C implementations. In each case, the idea is to make the hash function available as function md5 :: String - String However, the naive implementation md5_init md5_state n - newCString str md5_append md5_state n (fromIntegral (length str)) md5_finish md5_state md5_digest does not scale to computing hashes of really long strings (50 MB, say, as arising from reading a moderately big file), since it tries to create a CString of that size, first! Trying to avoid the allocation of this giant CString requires to split up the original string into smaller parts and convert each part to a CString separately. Clearly, this task involves a lot of allocation, essentially the input string needs to be copied part by part. Hence, I was wondering why the FFI only provides functionality to convert an *entire* list of Char into a CString. For applications like this hash computation, it would be advantageous to be able to specify *how much* of the input string to marshall to the CString and have the conversion function return the rest of the input string and the CString. That is, in addition to newCString :: String - IO CString there should be newCStringPart :: String - Int - IO (CStringLen, String) or even toCStringPart :: String - CStringLen - IO (Int, String) where CStringLen describes a target buffer into which the String argument is to be marshalled. (and similarly for other list types) Clearly, I can program this functionality by hand. But I have to revert to byte-wise processing using pokeByteOff, castCharToCChar, and so on. In addition, the optimizer does not seem to be very effective on such code, so it seems advantageous to provide it in the library already. But perhaps I'm overlooking something, so I'm appending the code I was using below. -Peter Except that I would probably mapM_ over a list of chunks, I don't see what the problem is with your second version of the code is. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: Use of H98 FFI
Derek == Derek Elkins [EMAIL PROTECTED] writes: Derek Derek Except that I would probably mapM_ over a list of chunks, I don't Derek see what the problem is with your second version of the code is. The second version allocates memory like crazy, so much that the pokeByteOff and castCharToCChar version becomes twice roughly twice as fast. Here are some figures [all compiled with -O2, otherwise version c looses badly]: s - hash in one go a - chunks of 512 characters c - byte-wise [on 51MB String] Main a : 58.730u 1.440s 1:00.13 100.0% 0+0k 0+0io 182pf+0w Main c : 29.370u 0.740s 0:30.07 100.1% 0+0k 0+0io 182pf+0w [on 32MB String] Main a : 37.500u 1.100s 0:38.58 100.0% 0+0k 0+0io 182pf+0w Main c : 18.790u 0.510s 0:19.43 99.3%0+0k 0+0io 182pf+0w [on 500k String] Main s : 0.560u 0.120s 0:00.66 103.0%0+0k 0+0io 182pf+0w Main a : 0.610u 0.010s 0:00.59 105.0%0+0k 0+0io 182pf+0w Main c : 0.290u 0.000s 0:00.27 107.4%0+0k 0+0io 182pf+0w So the generally most efficient seems to be to use c, but I suspect that further gains would be possible if this variant were implemented at a lower level. Hence the proposal for newCStringPart. -Peter ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
