Re: GHC -O2 and unsafePerformIO

2007-05-03 Thread Simon Marlow 

Neil Mitchell wrote:

Hi

Thanks to dcoutts, I have now come up with an answer. I don't
understand why it works now, but not before. I do remember than
browsing either Core or STG is not a fun thing to do...

p_System_IO_hGetChar h   = trace i am here $
   unsafePerformIO  $ getCharIO h


{-# NOINLINE getCharIO #-}
getCharIO h = do
   c - getchar
   print c
   return $ if c == (-1) then 0 else chr_ c


This is clearly a misuse of unsafePerformIO (as I'm sure you're aware).  Just 
out of interest - what's the context?


Cheers,
Simon
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Re: GHC -O2 and unsafePerformIO

2007-05-03 Thread Neil Mitchell 

Hi Simon,


This is clearly a misuse of unsafePerformIO (as I'm sure you're aware).  Just
out of interest - what's the context?


I am writing an optimiser for Yhc, doing whole-program optimisation,
with the intention of keeping it fast and high performance. Since
writing out Yhc bytecode would kill any performance benefits, I am
writing back to Haskell to be compiled with GHC. So the basic sequence
of steps is:

compile Haskell to Yhc Core
transform Yhc Core
convert Yhc Core to Haskell
compile Haskell to GHC

The problem I'm currently overcoming is that Yhc inserts its own IO
monad, which isn't the same as the GHC one. By the time I get to GHC
Haskell, all of the Yhc monad is invisible to GHC, so I have to
unsafePerformIO the getchar/putchar functions. With this as the
primitive getchar, it seems to work for my particular example, but is
clearly a bit fragile.

All the examples I'm doing for now are wc -c, wc -l, so shouldn't
stress the IO much more than getchar/putchar.

Thanks

Neil
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Re: GHC -O2 and unsafePerformIO

2007-05-02 Thread Bulat Ziganshin 
Hello Neil,

Wednesday, May 2, 2007, 7:00:05 PM, you wrote:
 {-# NOINLINE wrapIO #-}
 wrapIO x = unsafePerformIO (x = return)

-fno-cse ? it's usual company for unsafePerformIO+NOINLINE :)


-- 
Best regards,
 Bulatmailto:[EMAIL PROTECTED]

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Re: GHC -O2 and unsafePerformIO

2007-05-02 Thread Neil Mitchell 

Hi Bulat,


Wednesday, May 2, 2007, 7:00:05 PM, you wrote:
 {-# NOINLINE wrapIO #-}
 wrapIO x = unsafePerformIO (x = return)

-fno-cse ? it's usual company for unsafePerformIO+NOINLINE :)


No luck, alas. A slightly tweaked version, which is slightly simpler
and still gives the same behaviour is below.

Thanks

Neil


--



main = p_System_IO_hGetChar undefined `seq` p_System_IO_hGetChar 12
`seq` putStrLn done

foreign import ccall stdio.h getchar getchar :: IO Word8

{-# NOINLINE p_System_IO_hGetChar #-}
p_System_IO_hGetChar h   = trace i am here $
   unsafePerformIO  (getchar = \c - print c  return (if c ==
(-1) then 0 else chr_ c))

chr_ = fromEnum
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Re: GHC -O2 and unsafePerformIO

2007-05-02 Thread Neil Mitchell 

Hi

Thanks to dcoutts, I have now come up with an answer. I don't
understand why it works now, but not before. I do remember than
browsing either Core or STG is not a fun thing to do...

p_System_IO_hGetChar h   = trace i am here $
   unsafePerformIO  $ getCharIO h


{-# NOINLINE getCharIO #-}
getCharIO h = do
   c - getchar
   print c
   return $ if c == (-1) then 0 else chr_ c


Thanks

Neil

On 5/2/07, Neil Mitchell [EMAIL PROTECTED] wrote:

Hi Bulat,

 Wednesday, May 2, 2007, 7:00:05 PM, you wrote:
  {-# NOINLINE wrapIO #-}
  wrapIO x = unsafePerformIO (x = return)

 -fno-cse ? it's usual company for unsafePerformIO+NOINLINE :)

No luck, alas. A slightly tweaked version, which is slightly simpler
and still gives the same behaviour is below.

Thanks

Neil


--



main = p_System_IO_hGetChar undefined `seq` p_System_IO_hGetChar 12
`seq` putStrLn done

foreign import ccall stdio.h getchar getchar :: IO Word8

{-# NOINLINE p_System_IO_hGetChar #-}
p_System_IO_hGetChar h   = trace i am here $
unsafePerformIO  (getchar = \c - print c  return (if c ==
(-1) then 0 else chr_ c))

chr_ = fromEnum


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Re: GHC -O2 and unsafePerformIO

2007-05-02 Thread Duncan Coutts 
On Wed, 2007-05-02 at 16:33 +0100, Neil Mitchell wrote:
 Hi
 
 Thanks to dcoutts, I have now come up with an answer. I don't
 understand why it works now, but not before.

main = p_System_IO_hGetChar 1
 `seq` p_System_IO_hGetChar 2
 `seq` putStrLn done

This is fine (though note that p_System_IO_hGetChar 1 is a constant) the
real problem is here:

{-# NOINLINE p_System_IO_hGetChar #-}
p_System_IO_hGetChar h   = trace i am here $
unsafePerformIO $ 
  getchar = \c - 
  print c 
  return (if c == (-1) then 0 else chr_ c)

You've tried to trick ghc into always calling this by passing a dummy
'h' parameter. Then 'h' is never used in the body. Note however that the
whole body of this function is a constant and so ghc can (and at -O2
does) float it out as a CAF. This means you get the side effects of
p_System_IO_hGetChar at most once.

The solution of course is to use a full data dependency like IO or ST
uses.

 
 I do remember than browsing either Core or STG is not a fun thing to
 do...

So yeah, we see the above CAFs and let floating by looking at the core.
We could do with a prettier pretty printer for core, I agree.

Duncan

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