The SSE types require 16-byte alignment. Most of the original SSE instructions have versions that accept non-aligned data though. Alexander On Tue, Oct 16, 2018 at 11:18 PM Simon Marlow wrote:

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?

Cheers Simon

On Thu, 11 Oct 2018 at 18:44, Ömer Sinan Ağacan wrote:

...

Hi,

I just found out we currently align all pinned array payloads to 16 bytes and I'm wondering why. I don't see any comments/notes on this, and it's also not part of the primop documentation. We also have another primop for aligned allocation: newAlignedPinnedByteArray#. Given that alignment behavior of newPinnedByteArray# is not documented and we have another one for aligned allocation, perhaps we can remove alignment in newPinnedByteArray#.

Does anyone remember what was the motivation for always aligning pinned arrays?

Thanks

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Thanks for all the answers. Another surprising thing about the pinned object allocation primops is that the aligned allocator allows alignment to bytes, rather than to words (the documentation doesn't say whether it's words or bytes, but it can be seen from the code that it's actually aligning to the given byte). Is there a use case for this or people mostly use alignment on word boundaries? Ömer Sven Panne , 17 Eki 2018 Çar, 10:29 tarihinde şunu yazdı:

Am Di., 16. Okt. 2018 um 23:18 Uhr schrieb Simon Marlow :

...

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#x37-284000...), which in turn has the following guarantee (https://www.haskell.org/onlinereport/haskell2010/haskellch31.html#x39-287000...):

"... 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...), 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.