Strongly Specify Alignment for FFI Allocation

[Duncan Coutts]

On Thu, 2009-09-24 at 23:13 +0100, Don Stewart wrote:

> > It would be beneficial if this wording was applied to all allocation
> > routines - such as mallocForeignPtrBytes, mallocForeignPtrArray, etc.
> > For the curious, this proposal was born from the real-world issue of
> > pulling Word32's from a ByteString in an efficient but portable manner
> > (binary is portable but inefficient, a straight forward
> > unsafePerformIO/peek is efficient but need alignment).
> 
> As a side issue, the get/put primitives on Data.Binary should be
> efficient (though they're about twice as fast when specialized to a
> strict bytestring... stay tuned for a package in this area).

They are efficient within the constraint of doing byte reads and
reconstructing a multi-byte word using bit twiddling.

eg:

getWord16be :: Get Word16
getWord16be = do
    s <- readN 2 id
    return $! (fromIntegral (s `B.index` 0) `shiftl_w16` 8) .|.
              (fromIntegral (s `B.index` 1))

Where as reading an aligned word directly is rather faster. The problem
is that the binary API cannot guarantee alignment so we have to be
pessimistic. We could do better on machines that are tolerant of
misaligned memory accesses such as x86. We'd need to use cpp to switch
between two implementations depending on if the arch supports misaligned
memory access and if it's big or little endian.

#ifdef ARCH_ALLOWS_MISALIGNED_MEMORY_ACCESS

#ifdef ARCH_LITTLE_ENDIAN
getWord32le = getWord32host
#else
getWord32le = ...
#endif

etc

Note also that currently the host order binary ops are not documented as
requiring alignment, but they do. They will fail eg on sparc or ppc for
misaligned access.

Duncan