C(++) struct force extra padding

Question

I've seen countless questions of the form "I don't like padding how do I turn it off", but have yet to find anything about forcing the compiler to provide extra padding.

The specific case that I have looks like

struct particle{
  vect2 s;
  vect2 v;
  int rX;
  int rY;
  double mass;
  int boxNum;
};

Where vect2 is a simple struct {double x; double y;} vect2. In order to use SSE2, I need to be able to load a pair of doubles, aligned to 16 byte boundaries. This used to work, until I added the extra int, pushing my struct size from 48 bytes to 56 bytes. The result is segfaults.

Is there some kind of compiler directive I can use that either says "pad this struct to make it a multiple of 16 bytes long", or "this struct has an alignment of 16-bytes"? I know I could do it manually (tacking on an extra char[12], for example), but I'd really rather just tell the compiler(GCC, preferably ICC compatible), and not have to do it manually if I change the struct in future.

Answer 1

You can nest two structures to pad it automatically without needing to keep track of the size yourself.

struct particle
{
    // ...
};

{
    particle p;
    char padding[16-(sizeof(particle)%16)];
};

This version unfortunately adds 16 bytes if the structure is already a multiple of 16. It's unavoidable because the standard doesn't allow arrays of zero length.

Some compilers do allow zero length arrays as an extension, and in that case you can do this instead:

struct particle_wrapper
{
    particle p;
    char padding[sizeof(particle)%16 ? 16-(sizeof(particle)%16) : 0];
};

This version does not add any bytes of padding if the structure is already a multiple of 16.