NCG miscompiles Double -> Float -> Double
|Reported by:||dterei||Owned by:||simonmar|
|Type of failure:||Incorrect result at runtime||Difficulty:|
|Test Case:||Blocked By:|
Description (last modified by dterei)
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.
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