Cache-aligned stack variables

Question

Using new C++11 alignment tools I wanted to make sure that a set of temporary (stack) variables will lie in one cache line. My first naive attempt was as follows:

int main() {
    alignas(64) int a; // 0x7fffc58aac80, properly aligned at 64
    int b; // 0x7fffc58aac7c
    int c; // 0x7fffc58aac78
    return 0;
}

Stupid me! Stack doesn't allocate the variables this way, thus a will be on different cache line than b and c.

Does this mean that the only way to properly align several variables is through an aggregate?

struct alignas(64) Abc {
   int x;
   int y;
   int z;
};

int main() {
   Abc foo;
   // x 0x7fff40c2d3c0 (aligned at 64)
   // y 0x7fff40c2d3c4
   // z 0x7fff40c2d3c8   
   return 0;
}

Compiler: Clang 3.2

Answer 1

To properly align several variables you must use an aggregate, because the layout for automatic variables is not defined. I can't find anything in the C++11 standard that says variables with automatic storage have to be allocated on the stack in the same order they are defined. Section 5.9 of the standard insists that only a few kinds of pointer comparisons are defined, and comparisons between variables with automatic storage is not among those listed as defined.