GCC - How to realign stack?

Question

I try to build an application which uses pthreads and __m128 SSE type. According to GCC manual, default stack alignment is 16 bytes. In order to use __m128, the requirement is t

Answer 1

Allocate on the stack an array that is 15-bytes larger than sizeof(__m128), and use the first aligned address in that array. If you need several, allocate them in an array with a single 15-byte margin for alignment.

I do not remember if allocating an unsigned char array makes you safe from strict aliasing optimizations by the compiler or if it only works only the other way round.

#include 

void *f(void *x)
{
   unsigned char y[sizeof(__m128)+15];
   __m128 *py = (__m128*) (((uintptr_t)&y) + 15) & ~(uintptr_t)15);
   ...
}