Interaction between GMP and the Storage Manager

The Glasgow Haskell Compiler uses GMP to implement Integer. GMP expects a storage model where pointers (newtyped as mpz_t) are explicitly allocated and freed. However, Haskell requires Integers to be garbage collected. This could be done with ForeignPtrs, however (presumably for performance reasons?) GHC uses ByteArray#s from the heap instead. rts/sm/Storage.c initStorage sets three override functions to do this: stgAllocForGMP, stgReallocForGMP, and stgDeallocForGMP.

This requires a fairly subtle interaction in order to work safely, because heap ByteArray#s are normal objects and can be moved by garbage collection. The important thing to note is that the GHC garbage collector is of the stop-the-world variety. This means that all threads must reach a synchronization point before garbage collection can actually begin. However, there are no such synchronization points inside the GMP-based primops. Therefore, garbage collection, and consequent moving of Integers, can only occur when no GMP operations are executing.