Is the memory alignment by mxCalloc and/or mxMalloc defined

I want to create a MEX function for MATLAB 2014a which uses AVX intrinsics, and the memory used needs to be 32 byte aligned.

Looking at the documentation for the mex memory functions I can't see any specific alignment which is provided by mxCalloc or mxMalloc.

However, running the following example under MATLAB 2014a 64-bit in Linux:

#include <mex.h>
#include <matrix.h>

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
    int len;
    void *data;
    size_t data_addr;
    size_t alignment;

      alignment = 0;
      for (len = 1; len < 0x10001; len++)
      {
          data = mxMalloc (len);
          data_addr = (size_t) data;
          alignment |= data_addr;
      }

      mexPrintf ("mxMalloc alignment=%lx\n", alignment); 

      alignment = 0;
      for (len = 1; len < 0x10001; len++)
      {
          data = mxCalloc (len, 1);
          data_addr = (size_t) data;
          alignment |= data_addr;
      }

      mexPrintf ("mxCalloc alignment=%lx\n", alignment); 
  }

Appears to show that the addresses returned by mxMalloc and mxCalloc have 32 byte alignment (in that the least significant 5 bits of the addresses are always zero):

>> mex c_align_test.c
Building with 'gcc'.
MEX completed successfully.
>> c_align_test
mxMalloc alignment=7f85ffffffe0
mxCalloc alignment=7f85ffffffe0

Is the guaranteed memory alignment provided by mxCalloc and mxAlloc documented anywhere?

Answer 1

[James Tursa]

This topic has come up in the past. I believe the MATLAB API functions use malloc & friends in the background, and those functions are required to return a pointer that is properly aligned for any typical object on the system you are running on. E.g., see a short discussion here:

http://stackoverflow.com/questions/3994035/what-is-aligned-memory-allocation

However, as is pointed out this requirement does not necessarily cover special cases, and your 32-byte alignment would probably fit into that category. I am unaware of any MATLAB documentation that discusses this level of detail to give you the warm fuzzy you want.

[Chester Gillon]

I haven't been able to find anything else which gives the level of detail of the MATLAB memory alignent. However did find that:

1) The MEX functions mxAlloc / mxCalloc end up calling malloc / calloc to perform the allocation.

2) MATAB 2014a under Linux 64-bit uses libstdc++.so.6.0.17 from gcc-4.7.2

3) A C program compiled using the native gcc-4.4.7 gives an alignment of 16 bytes for malloc / calloc

Therefore, I think that later version of glibc may have increased the default alignment to allow for SSE / AVX vector datatypes. e.g. see the discussion on https://gcc.gnu.org/bugzilla/show_bug.cgi?id=15795