Addr# field in ForeignPtr

2011-06-01 Thread Roman Leshchinskiy 
Hi all,

GHC defines ForeignPtr as:

data ForeignPtr a = ForeignPtr Addr# ForeignPtrContents
-- we cache the Addr# in the ForeignPtr object, but attach
-- the finalizer to the IORef (or the MutableByteArray# in
-- the case of a MallocPtr).  The aim of the representation
-- is to make withForeignPtr efficient; in fact, withForeignPtr
-- should be just as efficient as unpacking a Ptr, and multiple
-- withForeignPtrs can share an unpacked ForeignPtr.  Note
-- that touchForeignPtr only has to touch the ForeignPtrContents
-- object, because that ensures that whatever the finalizer is
-- attached to is kept alive.

Is it ok to modify the Addr# field? Or do the libraries assume that it always 
points to the start of the block described by ForeignPtrContents? Changing the 
Addr# field would be quite useful for pointing to the middle of a memory block. 
For instance, Storable vectors are currently defined as:

data Vector a = Vector {-# UNPACK #-} !(Ptr a)
   {-# UNPACK #-} !Int
   {-# UNPACK #-} !(ForeignPtr a)

If I could use the Addr# field of the ForeignPtr, I could get rid of the Ptr.

Alternatively, I could define my own version of ForeignPtr if only 
ForeignPtrContents was exported (abstractedly would be enough).

Roman



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Re: Addr# field in ForeignPtr

2011-06-01 Thread Simon Marlow 

On 01/06/2011 08:29, Roman Leshchinskiy wrote:


GHC defines ForeignPtr as:

data ForeignPtr a = ForeignPtr Addr# ForeignPtrContents
 -- we cache the Addr# in the ForeignPtr object, but attach
 -- the finalizer to the IORef (or the MutableByteArray# in
 -- the case of a MallocPtr).  The aim of the representation
 -- is to make withForeignPtr efficient; in fact, withForeignPtr
 -- should be just as efficient as unpacking a Ptr, and multiple
 -- withForeignPtrs can share an unpacked ForeignPtr.  Note
 -- that touchForeignPtr only has to touch the ForeignPtrContents
 -- object, because that ensures that whatever the finalizer is
 -- attached to is kept alive.

Is it ok to modify the Addr# field? Or do the libraries assume that
it always points to the start of the block described by
ForeignPtrContents? Changing the Addr# field would be quite useful
for pointing to the middle of a memory block. For instance, Storable
vectors are currently defined as:


It should be fine to use whatever Addr# you like.  The only requirement 
is that calling touch# on the ForeignPtrContents is enough to keep the 
memory alive.



data Vector a = Vector {-# UNPACK #-} !(Ptr a)
{-# UNPACK #-} !Int
{-# UNPACK #-} !(ForeignPtr a)

If I could use the Addr# field of the ForeignPtr, I could get rid of the Ptr.


Good idea!

Cheers,
Simon

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