Re: Why align all pinned array payloads on 16 bytes?
I don't imagine anyone wants to align to anything that's not a power of 2, or less than a word size. Still, unless the current generality results in extra complication or overheads I wouldn't change it. On Mon, 22 Oct 2018 at 11:44, Ömer Sinan Ağacan wrote: > 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 < > marlo...@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#x15-178.7), > 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. > ___ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
Re: Why align all pinned array payloads on 16 bytes?
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#x37-28400029.1.3), > 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#x15-178.7), > 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. ___ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
Re: Why align all pinned array payloads on 16 bytes?
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#x37-28400029.1.3), 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#x15-178.7), 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. ___ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
Re: Why align all pinned array payloads on 16 bytes?
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 >> >> Ömer >> ___ >> ghc-devs mailing list >> ghc-devs@haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >> > ___ > ghc-devs mailing list > ghc-devs@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > ___ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
Re: Why align all pinned array payloads on 16 bytes?
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 > > Ömer > ___ > ghc-devs mailing list > ghc-devs@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > ___ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
Re: Why align all pinned array payloads on 16 bytes?
while I dont know the original context, some care may be needed ... depending on how this alignment assumption is accidentally used by users... it may result in really gross breakages On Thu, Oct 11, 2018 at 1:44 PM 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 > > Ömer > ___ > ghc-devs mailing list > ghc-devs@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > ___ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
