Can Haskell optimize function calls the same way Clang / GCC does?

Question

I want to ask you if Haskell and C++ compilers can optimize function calls the same way. Please look at following codes. In the following example Haskell is significantly faster than C++.

I have heard that Haskell can compile to LLVM and can be optimized by the LLVM passes. Additionally I have heard that Haskell has some heavy optimizations under the hood. But the following examples should be able to work with the same performance. I want to ask:

1. Why my sample benchmark in C++ is slower than the on in Haskell?
2. is it possible to further optimize the codes?

(I'm using LLVM-3.2 and GHC-7.6).

C++ code:

#include <cstdio>
#include <cstdlib>

int b(const int x){
    return x+5;
}

int c(const int x){
    return b(x)+1;
}

int d(const int x){
    return b(x)-1;
}

int a(const int x){
    return c(x) + d(x);
}

int main(int argc, char* argv[]){
    printf("Starting...\n");
    long int iternum = atol(argv[1]);
    long long int out = 0;
    for(long int i=1; i<=iternum;i++){
        out += a(iternum-i);
    }
    printf("%lld\n",out);
    printf("Done.\n");
}

compiled with clang++ -O3 main.cpp

haskell code:

module Main where
import qualified Data.Vector as V
import System.Environment
b :: Int -> Int
b x = x + 5
c x = b x + 1
d x = b x - 1
a x = c x + d x
main = do
   putStrLn "Starting..."
   args <- getArgs
   let iternum = read (head args) :: Int in do
      putStrLn $ show $ V.foldl' (+) 0 $ V.map (\i -> a (iternum-i))
         $ V.enumFromTo 1 iternum
      putStrLn "Done."

compiled with ghc -O3 --make -fforce-recomp -fllvm ghc-test.hs

speed results:

Running testcase for program 'cpp/a.out'
-------------------
cpp/a.out 100000000      0.0%   avg time: 105.05 ms 
cpp/a.out 200000000      11.11% avg time: 207.49 ms 
cpp/a.out 300000000      22.22% avg time: 309.22 ms 
cpp/a.out 400000000      33.33% avg time: 411.7 ms 
cpp/a.out 500000000      44.44% avg time: 514.07 ms 
cpp/a.out 600000000      55.56% avg time: 616.7 ms 
cpp/a.out 700000000      66.67% avg time: 718.69 ms
cpp/a.out 800000000      77.78% avg time: 821.32 ms 
cpp/a.out 900000000      88.89% avg time: 923.18 ms 
cpp/a.out 1000000000     100.0% avg time: 1025.43 ms


Running testcase for program 'hs/main'
-------------------
hs/main 100000000    0.0%   avg time: 70.97 ms (diff: 34.08)
hs/main 200000000    11.11% avg time: 138.95 ms (diff: 68.54)
hs/main 300000000    22.22% avg time: 206.3 ms (diff: 102.92)
hs/main 400000000    33.33% avg time: 274.31 ms (diff: 137.39)
hs/main 500000000    44.44% avg time: 342.34 ms (diff: 171.73)
hs/main 600000000    55.56% avg time: 410.65 ms (diff: 206.05)
hs/main 700000000    66.67% avg time: 478.25 ms (diff: 240.44)
hs/main 800000000    77.78% avg time: 546.39 ms (diff: 274.93)
hs/main 900000000    88.89% avg time: 614.12 ms (diff: 309.06)
hs/main 1000000000   100.0% avg time: 682.32 ms (diff: 343.11)

EDIT Of course we cannot compare speed of languages, but the speed of implementiations.

But I'm curious if Ghc and C++ compilers can optimize function calls the same way

I've edited the question with new benchmark and codes based on your help :)

Answer 1

If your goal is to get this running as quickly as your C++ compiler, then you would want to use a data structure that the compiler can have its way with.

module Main where

import qualified Data.Vector as V

b :: Int -> Int
b x = x + 5
c x = b x + 1
d x = b x - 1

a x = c x + d x

main = do
    putStrLn "Starting..."
    putStrLn $ show $ V.foldl' (+) 0 $ V.map a $ V.enumFromTo 1 100000000
    putStrLn "Done."

GHC is able to completely eliminate the loop and just inserts a constant into the resulting assembly. On my computer, this now has a runtime of < 0.002s, when using the same optimization flags as you originally specified.

As a follow up based on the comments by @Yuras, the core produced by the vector based solution and the stream-fusion solution are functionally identical.

Vector

main_$s$wfoldlM'_loop [Occ=LoopBreaker]
  :: Int# -> Int# -> Int#

main_$s$wfoldlM'_loop =
  \ (sc_s2hW :: Int#) (sc1_s2hX :: Int#) ->
    case <=# sc1_s2hX 100000000 of _ {
      False -> sc_s2hW;
      True ->
        main_$s$wfoldlM'_loop
          (+#
             sc_s2hW
             (+#
                (+# (+# sc1_s2hX 5) 1)
                (-# (+# sc1_s2hX 5) 1)))
          (+# sc1_s2hX 1)
    }

stream-fusion

$wloop_foldl [Occ=LoopBreaker]
  :: Int# -> Int# -> Int#

$wloop_foldl =
  \ (ww_s1Rm :: Int#) (ww1_s1Rs :: Int#) ->
    case ># ww1_s1Rs 100000000 of _ {
      False ->
        $wloop_foldl
          (+#
             ww_s1Rm
             (+#
                (+# (+# ww1_s1Rs 5) 1)
                (-# (+# ww1_s1Rs 5) 1)))
          (+# ww1_s1Rs 1);
      True -> ww_s1Rm
    }

The only real difference is the choice of comparison operation for the termination condition. Both versions compile to tight tail recursive loops that can be easily optimized by LLVM.

Answer 2

ghc doesn't fuse lists (avoiding success at all costs?)

Here is version that uses stream-fusion package:

module Main where

import Prelude hiding (map, foldl)
import Data.List.Stream
import Data.Stream (enumFromToInt, unstream)
import Text.Printf
import Control.Exception
import System.CPUTime

b :: Int -> Int
b x = x + 5
c x = b x + 1
d x = b x - 1

a x = c x + d x

main = do
    putStrLn "Starting..."
    putStrLn $ show $ foldl (+) 0 $ map (\z -> a z) $ unstream $ enumFromToInt 1 100000000 
    putStrLn "Done."

I don't have llvm installed to compare with your results, but it is 10x faster then your version (compiled without llvm).

I think vector fusion should perform even faster.