#1138: The -fexcess-precision flag is ignored if supplied on the command line.
----------------------------+-----------------------------------------------
Reporter: dons | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Driver | Version: 6.6
Severity: normal | Keywords: numerics, excess-precision
Difficulty: Easy (1 hr) | Testcase:
Architecture: x86 | Os: Unknown
----------------------------+-----------------------------------------------
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.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1138>
GHC <http://www.haskell.org/ghc/>
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