Just a quick note that may be useful to someone else. As you may know SSE2 introduced a new instruction: MOVDQA (MOVe Double Quadword Aligned). This is used to move 128 bit (16 bytes) of data from/to memory/xmm registers. This instruction only works if the data is aligned the the 16 byte boundary. There is also another instruction for the unaligned case, but the aligned version is way faster. So let’s summarize some techniques to get an aligned memory area
- For local, static and member variables you can append __attribute__ (( aligned (16 ) ) to the type definition. Example:
- For dynamically allocated memory the usual malloc is not enough, but there is a posix_memalign which has the semantics that we need. It is defined as:
struct A { int val; } __attribute__ ((aligned ( 16 ) );
int posix_memalign(void **memptr, size_t alignment, size_t size);
So we have to pass a pointer to the pointer that will receive the newly allocated memory, the required alignment (which has to be a power of two) and the allocation size. Memory allocated this way can (at least on the glibc implementation) be freed using the usual free function.
