Re: ANNOUNCE: protocol-buffers-0.2.9 for Haskell is ready
I am cross-posting this message to several lists. I had learned the trick before the documentation was updated. It seems I have used a very unreliable trick. And the "use castToSTUArray" suggested alternative is a really poor one since I am not using arrays at all. Who can suggest a way to cast from Float to Word32 and Double to Word64 using ghc? The actual task is that I need to write out the Float as a little endian sequence of four bytes and also be able to read it back in. The writing and reading are done in Put and Get monads to ByteString (from the "binary" package). The alloca/poke/peek work around I have looks like castWord32ToFloat :: Word32 -> Float castWord32ToFloat x = unsafePerformIO $ alloca $ \p -> poke p x >> peek (castPtr p) castFloatToWord32 :: Float -> Word32 castFloatToWord32 x = unsafePerformIO $ alloca $ \p -> poke p x >> peek (castPtr p) The unsafeCoerce trick that is no longer working looks like: castWord64ToDouble :: Word64 -> Double castWord64ToDouble (W64# w) = D# (unsafeCoerce# w) castDoubleToWord64 :: Double -> Word64 castDoubleToWord64 (D# d) = W64# (unsafeCoerce# d) Any ideas? Or is the alloca trick the only way to do this? Chris Ian Lynagh wrote:
Hi Chris,
On Sun, Sep 21, 2008 at 05:37:33PM +0100, Chris Kuklewicz wrote:
Also, I tried two tricks: (D# x) <-> (W64# x) which works fine (F# x) <-> (W32# x) which produced garbage, so I had to replace it with alloca/poke/peek.
This isn't supported, and I suspect is the cause of the -fasm problems.
Please see http://hackage.haskell.org/trac/ghc/ticket/2209 for more details and suggested alternative.
Thanks Ian
Chris Kuklewicz wrote:
Who can suggest a way to cast from Float to Word32 and Double to Word64 using ghc? The actual task is that I need to write out the Float as a little endian sequence of four bytes and also be able to read it back in. The writing and reading are done in Put and Get monads to ByteString (from the "binary" package).
I think alloca-like hacks is really the wrong direction and asking for trouble. You are trying to translate between platform-dependent native floats, and IEEE floats in a specified platform-independent binary format for Google. So use encodeFloat/decodeFloat - fast primitives in GHC - on the native side, and a hand-written Binary instance for the exact format you need on the Google side. My opinion, YMMV. Regards, Yitz
Hello Chris, Monday, September 22, 2008, 2:48:16 PM, you wrote:
used a very unreliable trick. And the "use castToSTUArray" suggested alternative is a really poor one since I am not using arrays at all.
castToSTUArray does the same as your code, only in ST monad so you can skip unsafePerformIO trick if you dn't know, ST is a subset of IO monad with a limited set of operations guaranteed to not have side-effects. so, cvt x = unsafePerformIO $ do alloca $ \place -> do poke place x peek place and cvt x = runST $ do place <- newArray (0,1) writeArray place 0 x readArray place 0 generates almost the same code (the only difference is kind of memory allocated) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com
Chris Kuklewicz wrote:
I am cross-posting this message to several lists.
I had learned the trick before the documentation was updated. It seems I have used a very unreliable trick. And the "use castToSTUArray" suggested alternative is a really poor one since I am not using arrays at all.
castSTUArray is the way GHC does it - the idea is to allocate a small array, store the Float/Double in it, cast the type of the array to Word32 or whatever, and then read out the contents. It's more or less equivalent to the peek/poke solution, except that it doesn't need unsafePerformIO. GHC's code is here: (look for floatToWord): http://darcs.haskell.org/ghc/compiler/cmm/PprC.hs Cheers, Simon
participants (4)
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Bulat Ziganshin -
Chris Kuklewicz -
Simon Marlow -
Yitzchak Gale