[Qu3tzal]

Excuse me if this sounds stupid but, vector instructions are assembly. I know we can use inline assembly or compile and link assembly alongside C but isn't it the compiler that is in charge of using vector instructions ?

IIRC GCC has -mmx, -sse(2|3|4) options to enable this kind of instructions.

[WoodenChair]

Sure, if the compiler can find optimizations by inserting vector instructions, it will. But, typically you'll want to specifically format your code using matrices/a library like BLAS to maximize performance and use as many vector instructions as possible.