On Dec 21, 2007 2:55 PM, Bertram Felgenhauer
If you look at the generated machine code, you'll find that f and g are identical functions. The sole purpose of the int2Word# and word2Int# operations is to satisfy the type checker. (This is even true at the core level. The core language is typed, so you still need to do the conversions there. No code is generated for them though.)
Good to know. They are scary!
The I# deconstruction has a cost, but with proper strictness annotations ghc should optimize those away - check the intermediate Core to see whether it actually does.
If I see things like GHC.Prim.Intzh, is that a clue its the "unlifted" type?
In fact most of the speedup you got seems to come from the use of uncheckedShiftL# and uncheckedShiftRL# - just using
shiftL, shiftR :: Int -> Int -> Int I# a `shiftR` I# b = I# (word2Int# (int2Word# a `uncheckedShiftRL#` b)) I# a `shiftL` I# b = I# (word2Int# (int2Word# a `uncheckedShiftL#` b))
speeds up the stepWithUArray code by a factor of 2 here, starting with the first program at http://hpaste.org/4151.
That is great. I tried your speedup and you are right - just redefining those makes the "lifted" version faster than the unlifted. Too bad there isn't an "unsafe" version of the shifts available. Justin