The -fexcess-precision flag is ignored if supplied on the command line.

The numerics/Double-based programs on the great language shootout were performing poorly. Investigations revealed that the -fexcess-precision flag was being silently ignored by GHC when supplied as a command line flag. If it is supplied as a {-# OPTIONS -fexcess-precision #-} pragma, it is respected.

Consider the following shootout entry for the 'mandelbrot' benchmark. It writes the mandelbrot set as bmp format to stdout.

import System
import System.IO
import Foreign
import Foreign.Marshal.Array

main = do
    w <- getArgs >>= readIO . head
    let n      = w `div` 8
        m  = 2 / fromIntegral w
    putStrLn ("P4\n"++show w++" "++show w)
    p <- mallocArray0 n

    unfold n (next_x w m n) p (T 1 0 0 (-1))

unfold :: Int -> (T -> Maybe (Word8,T)) -> Ptr Word8 -> T -> IO ()
unfold !i !f !ptr !x0 = loop x0
  where
    loop !x = go ptr 0 x

    go !p !n !x = case f x of
        Just (w,y) | n /= i -> poke p w >> go (p `plusPtr` 1) (n+1) y
        Nothing             -> hPutBuf stdout ptr i
        _                   -> hPutBuf stdout ptr i >> loop x
{-# NOINLINE unfold #-}

data T = T !Int !Int !Int !Double

next_x !w !iw !bw (T bx x y ci)
    | y  == w   = Nothing
    | bx == bw  = Just (loop_x w x 8 iw ci 0, T 1 0    (y+1)   (iw+ci))
    | otherwise = Just (loop_x w x 8 iw ci 0, T (bx+1) (x+8) y ci)

loop_x !w !x !n !iw !ci !b
    | x < w = if n == 0
                    then b
                    else loop_x w (x+1) (n-1) iw ci (b+b+v)
    | otherwise = b `shiftL` n
  where
    v = fractal 0 0 (fromIntegral x * iw - 1.5) ci 50

fractal :: Double -> Double -> Double -> Double -> Int -> Word8
fractal !r !i !cr !ci !k
    | r2 + i2 > 4 = 0
    | k == 0      = 1
    | otherwise   = fractal (r2-i2+cr) ((r+r)*i+ci) cr ci (k-1)
  where
    (!r2,!i2) = (r*r,i*i)

We can compile and run this as follows:

$ ghc -O -fglasgow-exts -optc-march=pentium4 -fbang-patterns -funbox-strict-fields -optc-O2 -optc-mfpmath=sse -optc-msse2 -fexcess-precision -o m1 mandel3.hs -no-recomp

$ time ./m1 3000 > /dev/null
./m1 3000 > /dev/null  8.12s user 0.00s system 99% cpu 8.143 total

8s is around 3x the speed of C (or worse).

now, if we add the following pragma to the top of the file:

{-# OPTIONS -fexcess-precision #-}

and recompile and rerun:

$ ghc -O -fglasgow-exts -optc-march=pentium4 -fbang-patterns -funbox-strict-fields -optc-O2 -optc-mfpmath=sse -optc-msse2 -fexcess-precision -o m1 mandel3.hs -no-recomp

$ time ./m1 3000 > /dev/null
./m1 3000 > /dev/null  2.94s user 0.00s system 99% cpu 2.945 total

Nearly 3x faster, and competitive with C.

Across the board the -fexcess-precision flag seems to be ignored by GHC, affecting all Double-based entries on the shootout.

A diff on the ghc -v3 output shows that -ffloat-store is not being passed to GCC when -fexcess-precision is supplied on the command line.