#4441: NCG miscompiles Double -> Float -> Double
---------------------------------+------------------------------------------
Reporter: dterei | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (NCG)
Version: 7.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: x86 | Failure: Incorrect result at runtime
---------------------------------+------------------------------------------
Description changed by dterei:
Old description:
> I have a need to truncate a double to only float precision but then store
> it back in a double as I'm printing from the double to a hexadecimal
> representation.
>
> To achieve this I basically do two calls to realToFrac. A test case for
> this is below. When I compile it with -fasm -O it appears as if
> '(realToFrac . realToFrac)' is changed to 'id'. (Not what actually
> happens as bug occurs in NCG, after Core stage, but thats how the
> programs behaviour is changed). This bug is specific to -fasm, compiling
> the program below with -fllvm or -fvia-C produces the correct results.
>
> Test Case:
> {{{
> module Main where
>
> import Data.Array.ST
> import Control.Monad.ST
>
> import Data.Word
> import Data.Char
> import Numeric
> import System.IO
>
> -- Test runner
> main = do
> putStr "Enter a double: "
> hFlush stdout
> d <- double
> print $ "Float Version : " ++ (fToStr $ realToFrac d)
> print $ "Double Version: " ++ (dToStr d)
>
> double :: IO Double
> double = do
> x <- getLine
> return $ read x
>
> -----------------------------------------------------------------------------
> -- * Floating point conversion
> --
>
> dToStr :: Double -> String
> dToStr d
> = let bs = doubleToBytes d
> hex d' = case showHex d' "" of
> [] -> error "dToStr: too few hex digits for float"
> [x] -> ['0',x]
> [x,y] -> [x,y]
> _ -> error "dToStr: too many hex digits for
> float"
> str = map toUpper $ concat . reverse . (map hex) $ bs
> in "0x" ++ str
>
> fToStr :: Float -> String
> fToStr = (dToStr . realToFrac)
>
> --
> -----------------------------------------------------------------------------
> -- Converting floating-point literals to integrals for printing
>
> castDoubleToWord8Array :: STUArray s Int Double -> ST s (STUArray s Int
> Word8)
> castDoubleToWord8Array = castSTUArray
>
> doubleToBytes :: Double -> [Int]
> doubleToBytes d
> = runST (do
> arr <- newArray_ ((0::Int),7)
> writeArray arr 0 d
> arr <- castDoubleToWord8Array arr
> i0 <- readArray arr 0
> i1 <- readArray arr 1
> i2 <- readArray arr 2
> i3 <- readArray arr 3
> i4 <- readArray arr 4
> i5 <- readArray arr 5
> i6 <- readArray arr 6
> i7 <- readArray arr 7
> return (map fromIntegral [i0,i1,i2,i3,i4,i5,i6,i7])
> )
> }}}
>
> Output with '-fasm -O':
> {{{
> $ ./TestCase
> Enter a double: 2.0e-2
> Float Version : 0x3F947AE147AE147B
> Double Version: 0x3F947AE147AE147B
> }}}
>
> Output with '-fvia-C -O':
> {{{
> $ ./TestCase
> Enter a double: 2.0e-2
> Float Version : 0x3F947AE140000000
> Double Version: 0x3F947AE147AE147B
> }}}
New description:
I have a need to truncate a double to only float precision but then store
it back in a double as I'm printing from the double to a hexadecimal
representation.
To achieve this I basically do two calls to realToFrac. A test case for
this is below. When I compile it with -fasm -O it appears as if
'(realToFrac . realToFrac)' is changed to 'id'. (Not what actually happens
as bug occurs in NCG, after Core stage, but thats how the programs
behaviour is changed). This bug is specific to -fasm, compiling the
program below with -fllvm or -fvia-C produces the correct results.
Test Case:
{{{
module Main where
import Numeric
import System.IO
-- Test runner
main = do
putStr "Enter a double: "
hFlush stdout
d <- double
print $ "Float Version : " ++ (fToStr $ realToFrac d)
print $ "Double Version: " ++ (dToStr d)
double :: IO Double
double = do
x <- getLine
return $ read x
dToStr :: Double -> String
dToStr d = show d
fToStr :: Float -> String
fToStr = (dToStr . realToFrac)
}}}
Output with '-fasm -O':
{{{
$ ./TestCase
Enter a double: 2.0e-2
Float Version : 2.0e-2
Double Version: 2.0e-2
}}}
Output with '-fvia-C -O':
{{{
$ ./TestCase
Enter a double: 2.0e-2
Float Version : 1.9999999552965164e-2
Double Version: 2.0e-2
}}}
--
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4441#comment:1>
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