[Haskell-cafe] looking for optimization advice

David Roundy droundy at jdj5.mit.edu
Thu Mar 25 13:09:07 EST 2004

Hello everyone,

I've got a function that I can't think how it can be optimized any more,
and perhaps it can't, but just in case, I figured I'd run it by you all,
since it's currently taking 70% of my CPU time.  The main problem is an
unrelated scaling issue (i.e. it's getting called too many times... this is
a separate issue I'm working on), but even in "reasonable" cases, it takes
20% of the processor time.

The function is a simple packed string compare, and I basically just call
the C standard library function memcmp for this.  Without further ado:

data PackedString = PS !(ForeignPtr Word8) !Int !Int

instance Eq PackedString where
   (==) = psniceq

foreign import ccall unsafe "static string.h memcmp" c_memcmp
    :: Ptr Word8 -> Ptr Word8 -> Int -> IO Int

{-# INLINE psniceq #-}
psniceq :: PackedString -> PackedString -> Bool
psniceq a b | nullPS a && nullPS b = True
psniceq (PS x1 s1 l1) (PS x2 s2 l2) =
    (l1 == l2 &&) $ unsafePerformIO $ withForeignPtr x1 $ \p1->
    withForeignPtr x2 $ \p2 ->
        liftM (==0) $ c_memcmp (p1 `plusPtr` s1) (p2 `plusPtr` s2) l1

Is there anything obvious that I might have missed? Or is there anything
incredibly subtle that I missed? :)

BTW, I compile this with -O -funbox-strict-fields, so the packed string
contents are unboxed, if that makes any difference.
David Roundy

More information about the Haskell-Cafe mailing list