Create Memory Aligned Array in C#-CodePudding

I need to create a array that is aligned to a 64 byte boundary. I need to do this as I'm calling a DLL which uses AVX, which requires the data to be aligned. Essentially I need to do this in C#:

void* ptr = _aligned_malloc(64 * 1024, 64);
int8_t* memory_ptr = (int8_t*)ptr;

I'm pretty sure I can't create an array to such a boundary naturally in C#. So one option is to create an byte array that is x 64 long, and then 'create' an array that overlays it, but with an offset at the required boundary.

The problem is how do I accomplish this, and not have a memory leak? (Memory leaking is the reason I'd rather not use the DLL to create a reference to the array and pass it to C#. Unless there is a good way to do so?)

CodePudding user response：

As mentioned, .NET 6 has NativeMemory.AlignedAlloc. You need to make sure to call AlignedFree otherwise you could get a leak.

void* a = default;
try
{
    a = NativeMemory.AlignedAlloc(size * sizeof(long), 64);
    var span = new Span<long>(a, size);
    // fill span
    // call DLL with span
}
finally
{
    NativeMemory.AlignedFree(a);
}

A pinned GCHandle is another option for older versions of .NET. You then need to calculate the starting aligned offset with the following code, where alignment would be 64 in your case.

var ptr = (long)handle.AddrOfPinnedObject();
var offset = (int) ((ptr   alignment - 1) / alignment * alignment - ptr) / sizeof(long);

Again you need to make sure to call handle.Free in a finally.

CodePudding user response：

To avoid the memory leak, first you need to pin the array. Pinning prevents the object pointed to from moving on the garbage-collected heap.

There's an example of something similar to what you're doing here.

However, that example doesn't go far enough as it only pins without controlling the initial memory allocation. To also prevent the memory leak, instead use GCHandle.Alloc with GCHandleType.Pinned. Like this.