Re: [Haskell-cafe] vector, alignment and SIMD through FFI

On Fri, 2012-07-06 at 13:43 -0700, Thomas DuBuisson wrote:

On Fri, Jul 6, 2012 at 1:06 PM, Nicolas Trangez wrote:

...

-- This fails: -- Ambiguous type variable `a0' in the constraint: -- (Storable a0) arising from a use of `sizeOf'

Here you can either tie a type knot using proxy types or you can use the scoped type variable language extension.

Guess I'll have to do some reading ;-) Thanks.

Perhaps I'm missing something specific to your use, but for the alignment issue you should be OK just calling allocBytes or one of its variants. I made some noise about this a bit ago and it resulted in some extra words in the report under mallocBytes:

""" The block of memory is sufficiently aligned for any of the basic foreign types that fits into a memory block of the allocated size. """

Which I'm pretty sure GHC did, and still does, follow.

Hmh... as far as I could find, mallocBytes basically does what malloc(3) does, which is 8-byte alignment if I'm not mistaken on my x86-64 Linux system. I could use those and the over-allocate-and-offset tricks, but... that's ugly unless strictly necessary ;-) Normally posix_memalign or memalign or valloc or _mm_malloc should provide what I need as-is. Except, when using those and vector's unsafeFromForeignPtr0, all I get is a "Vector a", which no longer has any alignment information in the type, so I can't write a function which only accepts N-aligned vectors. As a result, I'd need to be very careful only to pass aligned vectors to it (checking manually), add code to handle pre/post-alignment bytes in my SIMD functions (slow and stupid), or live with it and let my application crash at random. I found some work by Oleg Kiselyov and Chung-chieh Shan at [1] which might be related, yet as of now I feel like that's too general for my purpose (e.g. I don't see how to integrate it with vector). Thanks, Nicolas [1] http://okmij.org/ftp/Haskell/types.html#ls-resources