Some GHC.* module should export word size and heap object header size

Some code, like bytestring, uses magic constants to help it decide how much memory to allocate. This is not a correctness issue, just an optimisation one. It allows the bytestring package to allocate buffers that make optimal use of ghc's pinned allocator strategy which has to use a whole number of 4k pages. The magic is in subtracting the heap object header size:

-- | Currently set to 32k, less the memory management overhead
defaultChunkSize :: Int
defaultChunkSize = 32 * k - chunkOverhead
   where k = 1024

-- | Currently set to 4k, less the memory management overhead
smallChunkSize :: Int
smallChunkSize = 4 * k - chunkOverhead
   where k = 1024

-- | The memory management overhead. Currently this is tuned for GHC only.
chunkOverhead :: Int
chunkOverhead = 2 * sizeOf (undefined :: Int)

This works for normal builds but for profiling builds it is a couple words off which wastes memory. It would also be be nice if it did not have to use magic constants but if it could import them from GHC.Exts or something.

We would want:

• Heap object header size, in words (currently 2 normally or 4 for profiling)
• Word size (4 or 8 bytes)
• Heap block size (currently 4k)

These should all be actual Haskell constants so that they inline perfectly into calculations with no runtime overhead. The heap object header size is obviously different between profiling and normal, however this works out ok because we have to recompile for profiling anyway.

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries (other)
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