Here is the code I normally use to get aligned memory with Visual Studio and GCC
inline void* aligned_malloc(size_t size, size_t align) {
void *result;
#ifdef _MSC_VER
result = _aligned_malloc(size, align);
#else
if(posix_memalign(&result, align, size)) result = 0;
#endif
return result;
}
inline void aligned_free(void *ptr) {
#ifdef _MSC_VER
_aligned_free(ptr);
#else
free(ptr);
#endif
}
Is this code fine in general? I have also seen people use _mm_malloc
, _mm_free
. In most cases that I want aligned memory it's to use SSE/AVX. Can I use those functions in general? It would make my code a lot simpler.
Lastly, it's easy to create my own function to align memory (see below). Why then are there so many different common functions to get aligned memory (many of which only work on one platform)?
This code does 16 byte alignment.
float* array = (float*)malloc(SIZE*sizeof(float)+15);
// find the aligned position
// and use this pointer to read or write data into array
float* alignedArray = (float*)(((unsigned long)array + 15) & (~0x0F));
// dellocate memory original "array", NOT alignedArray
free(array);
array = alignedArray = 0;
See: http://www.songho.ca/misc/alignment/dataalign.html and How to allocate aligned memory only using the standard library?
Edit: In case anyone cares, I got the idea for my aligned_malloc() function from Eigen (Eigen/src/Core/util/Memory.h)
Edit:
I just discovered that posix_memalign
is undefined for MinGW. However, _mm_malloc
works for Visual Studio 2012, GCC, MinGW, and the Intel C++ compiler so it seems to be the most convenient solution in general. It also requires using its own _mm_free
function, although on some implementations you can pass pointers from _mm_malloc
to the standard free
/ delete
.
As long as you're ok with having to call a special function to do the freeing, your approach is okay. I would do your #ifdef
s the other way around though: start with the standards-specified options and fall back to platform-specific ones. For example
__STDC_VERSION__ >= 201112L
use aligned_alloc
._POSIX_VERSION >= 200112L
use posix_memalign
._MSC_VER
is defined, use the Windows stuff.malloc
/free
and disable SSE/AVX code.The problem is harder if you want to be able to pass the allocated pointer to free
; that's valid on all the standard interfaces, but not on Windows and not necessarily with the legacy memalign
function some unix-like systems have.