#9125: int-to-float conversion broken on ARM - 7.8.1-rc2 ------------------------------------------------+-------------------------- Reporter: Ansible | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.1 Resolution: | Keywords: Operating System: Linux | Architecture: arm Type of failure: Incorrect result at runtime | Difficulty: Test Case: | Unknown Blocking: | Blocked By: | Related Tickets: ------------------------------------------------+-------------------------- Comment (by amurrayc): I have the same problem on iOS (simulator or device) with 7.8.2 or 7.8.3 compiled from source tarballs. I'm not sure if it should be a separate ticket. I don't think this is an `Integer` to `Float` problem per se. If I run {{{ print (F# 29.0#) -- to pick a value at random }}} I get {{{ 2109.0 }}} as in the original ticket. This should be using a direct `Float` literal, no? A little digging showed that {{{ print (floor (29.0 :: Float) :: Int) }}} shows {{{ 2109 }}} with no optimisation, but {{{ 29 }}} with `-O` Another clue is that `floor :: Float -> Integer` doesn't give the correct result, even with `-O`. Optimised `floor :: Float -> Int` uses the primop `float2Int#` while its unoptimised version and both versions of `floor :: Float -> Integer` use `decodeFloat_Int#` as does `show :: Float -> String` which last explains the original ticket. running {{{ let (m,e) = decodeFloat (29.0 :: Float) mstr = printf "%#x" m putStrLn $ "(" ++ mstr ++ ", " ++ show e ++ ")" }}} gives {{{ (0x41e80000, -19) }}} instead of the correct {{{ (0xe80000, -19) }}} Notably the erroneous `0x41e80000` is the correct value for the entire bitfield of `29.0 :: Float#` rather than just the mantissa. This all suggests that the problem lies within `decodeFloat_Int#`. I looked at `__decodeFloat_Int` in `rts/StgPrimFloat.c`, even inserting a couple of `assert`s at the end to check the values for 29.0 which passed. That led me to look at `stg_decodeFloatzuIntzh` in `rts/PrimOps.cmm` but at that point I was getting a bit lost. Any ideas anyone? For the record I'm running the above snippets in a simple Haskell `Main.hs` like: {{{ {-# LANGUAGE ForeignFunctionInterface #-} module Main where import Foreign import Text.Printf ( printf ) foreign import ccall safe "c_main" c_main :: IO () main = do let (m,e) = decodeFloat (29.0 :: Float) mstr = printf "%#x" m putStrLn $ "(" ++ mstr ++ ", " ++ show e ++ ")" c_main }}} with a skeleton Xcode 5.1.1 project and observing the results in the debug window. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9125#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler