Sure, but we don't typically do sophisticated machine learning on them. The vast majority of modern CPUs have vector instructions. Even a Raspberry Pi's ARM has NEON.
Not necessarily true. You might want to run ANNs on micro-controllers at the edge for many different reasons.
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.
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.