Performance problem with Euler problem and recursion on Int64 types

Question

I\'m currently learning Haskell using the project Euler problems as my playground. I was astound by how slow my Haskell programs turned out to be compared to similar program

Answer 1

Hm, I installed a fresh Haskell platform with 7.0.3, and get roughly the following core for your program (-ddump-simpl):

Main.$warcLength' =
  \ (ww_s1my :: GHC.Prim.Int64#) (ww1_s1mC :: GHC.Prim.Int64#)
    (ww2_s1mG :: GHC.Prim.Int64#) (ww3_s1mK :: GHC.Prim.Int64#) ->
    case {__pkg_ccall ghc-prim hs_gtInt64 [...]
           ww_s1my ww1_s1mC GHC.Prim.realWorld#
[...]

So GHC has realized that it can unpack your integers, which is good. But this hs_getInt64 call looks suspiciously like a C call. Looking at the assembler output (-ddump-asm), we see stuff like:

pushl %eax
movl 76(%esp),%eax
pushl %eax
call _hs_gtInt64
addl $16,%esp

So this looks very much like every operation on the Int64 get turned into a full-blown C call in the backend. Which is slow, obviously.

The source code of GHC.IntWord64 seems to verify that: In a 32-bit build (like the one currently shipped with the platform), you will have only emulation via the FFI interface.