Popcount benchmark

This benchmark suite compares the code generation of Clang and GCC, with a native Haskell as a baseline. The point is not to compare FFI & native Haskell, but rather to see how Clang does loop unrolling & AVX2 comapared to GCC.

Please read the following files:

• gcc-popcount.S;
• clang-popcount.S;
• This README.

Implementations

• GHC-defined popcount on Word8 is a combination of popCnt8# and ByteString's foldl'.
• C-defined popcount:

size_t popcount (char const * src, size_t len) {
   size_t result = 0; 
   for (size_t i = 0; i < len; i++) {
       result += __builtin_popcount(src[i]);
   }
   return result;
}

Result

With a clang-enabled GHC 9.6:

1Mo
  ByteString.foldl: OK
    1.32 ms ± 121 μs
  FFI popcount:     OK
    50.0 ns ± 1.8 ns
10Mo
  ByteString.foldl: OK
    13.2 ms ± 723 μs
  FFI popcount:     OK
    61.2 ns ± 5.9 ns
100Mo
  ByteString.foldl: OK
    131  ms ± 3.2 ms
  FFI popcount:     OK
    38.8 ns ± 2.7 ns

With a GCC-enabled GHC 9.4.6:

1Mo
  ByteString.foldl: OK
    1.38 ms ± 106 μs
  FFI popcount:     OK
    63.6 ns ± 1.5 ns
10Mo
  ByteString.foldl: OK
    13.7 ms ± 1.4 ms
  FFI popcount:     OK
    88.5 ns ± 5.3 ns
100Mo
  ByteString.foldl: OK
    136  ms ± 2.8 ms
  FFI popcount:     OK
    39.5 ns ± 3.3 ns

Toolchains

Use ghcup to install multiple toolchains:

Install a gcc-enabled GHC (Find the URL for your system here)

• ghcup install ghc -u 'https://downloads.haskell.org/~ghc/9.6.4/ghc-9.6.4-x86_64-fedora33-linux.tar.xz' 9.6.4-gcc
• CC=clang CXX=clang++ ghcup install ghc --force 9.6.4

Then run:

$ cabal bench -w ghc-9.6.4
$ cabal clean # important!
$ cabal bench -w ghc-9.6.4-gcc