Partial fix for #2917
- add newAlignedPinnedByteArray# for allocating pinned BAs with
arbitrary alignment
- the old newPinnedByteArray# now aligns to 16 bytes
Foreign.alloca will use newAlignedPinnedByteArray#, and so might end
up wasting less space than before (we used to align to 8 by default).
Foreign.allocaBytes and Foreign.mallocForeignPtrBytes will get 16-byte
aligned memory, which is enough to avoid problems with SSE
instructions on x86, for example.
There was a bug in the old newPinnedByteArray#: it aligned to 8 bytes,
but would have failed if the header was not a multiple of 8
(fortunately it always was, even with profiling). Also we
occasionally wasted some space unnecessarily due to alignment in
allocatePinned().
I haven't done anything about Foreign.malloc/mallocBytes, which will
give you the same alignment guarantees as malloc() (8 bytes on
Linux/x86 here).