Nevertheless, I think even without the tricks I'm using in GHC, the case where a ForeignPtr is used in conjunction with malloc()/free() is one which is likely to be optimisable in any system with its own memory management.
I wasn't meaning so much that only GHC could take advantage of it (though I think that is true at present) but that someone might come along next week with a technique which avoids the problem altogether.
[...] using a ForeignPtr here, with free() as its finalizer, adds so much overhead that [...]
Where is the overhead coming from? Is it the cost of a C call or the cost of the standard malloc library? If the latter, I imagine that a custom allocator would have similar performance to using pinned objects. (I'm sort of assuming that pinned objects are more expensive than normal objects.) btw I don't know if it's relevant but there's an important semantic difference between allocating on the GHC heap and allocating on the C heap. The C version can be manipulated by the malloc.h functions. In particular, you can call realloc on it. Do that on the GHC heap version and... well, it won't be pretty. I don't know if this is relevant because using realloc with ForeignPtr'd memory is likely to be a delicate procedure no matter how it got allocated. -- Alastair