Why align all pinned array payloads on 16 bytes?

[Sven Panne]

Am Di., 16. Okt. 2018 um 23:18 Uhr schrieb Simon Marlow <marlowsd at gmail.com
>:

> I vaguely recall that this was because 16 byte alignment is the minimum
> you need for certain foreign types, and it's what malloc() does.  Perhaps
> check the FFI spec and the guarantees that mallocForeignPtrBytes and
> friends provide?
>

mallocForeignPtrBytes is defined in terms of malloc (
https://www.haskell.org/onlinereport/haskell2010/haskellch29.html#x37-28400029.1.3),
which in turn has the following guarantee (
https://www.haskell.org/onlinereport/haskell2010/haskellch31.html#x39-28700031.1
):

   "... All storage allocated by functions that allocate based on a size in
bytes must be sufficiently aligned for any of the basic foreign types that
fits into the newly allocated storage. ..."

The largest basic foreign types are Word64/Double and probably
Ptr/FunPtr/StablePtr (
https://www.haskell.org/onlinereport/haskell2010/haskellch8.html#x15-1700008.7),
so per spec you need at least an 8-byte alignement. But in an SSE-world I
would be *very* reluctant to use an alignment less strict than 16 bytes,
otherwise people will probably hate you... :-]

Cheers,
   S.
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