[Haskell-cafe] Memory corruption issues when using newAlignedPinnedByteArray, GC kicking in?

[Nicolas Trangez]

All,

While working on my vector-simd library, I noticed somehow memory I'm
using gets corrupted/overwritten. I reworked this into a test case, and
would love to get some help on how to fix this.

Previously I used some custom FFI calls to C to allocate aligned memory,
which yields correct results, but this has a significant (+- 10x)
performance impact on my benchmarks. Later on I discovered the
newAlignedPinnedByteArray# function, and wrote some code using this.

Here's what I did in the test case: I created an MVector instance, with
the exact same implementation as vector's
Data.Vector.Storable.Mutable.MVector instance, except for basicUnsafeNew
where I pass one more argument to mallocVector [1].

I also use 3 different versions of mallocVector (depending on
compile-time flags):

mallocVectorOrig [2]: This is the upstream version, discarding the
integer argument I added.

Then here's my first attempt, very similar to the implementation of
mallocPlainForeignPtrBytes [3] at [4] using GHC.* libraries.

Finally there's something similar at [5] which uses the 'primitive'
library.

The test case creates vectors of increasing size, then checks whether
they contain the expected values. For the default implementation this
works correctly. For both others it fails at some random size, and the
values stored in the vector are not exactly what they should be.

I don't understand what's going on here. I suspect I lack a reference
(or something along those lines) so GC kicks in, or maybe the buffer
gets relocated, whilst it shouldn't.

Basically I'd need something like

GHC.ForeignPtr.mallocPlainAlignedForeignPtrBytes :: Int -> Int -> IO
(ForeignPtr a)

Thanks,

Nicolas

[1] https://gist.github.com/3084806#LC37
[2] https://gist.github.com/3084806#LC119
[3]
http://hackage.haskell.org/packages/archive/base/latest/doc/html/src/GHC-ForeignPtr.html
[4] https://gist.github.com/3084806#LC100
[5] https://gist.github.com/3084806#LC81