What is the difference between non-packed and packed instruction in the context of SIMD-operations?

问题

What is the difference between non-packed and packed instruction in the context of SIMD-operations?

I was reading an article on optimizing your code for SSE:

http://www.cortstratton.org/articles/OptimizingForSSE.php#batch

and this question arose when I read

"As an added bonus, movss is a non-packed instruction, which allows us to make better use of the parallel instruction decoders.."

So what is the difference?

回答1:

To my understanding, packed means that conceptually more than one value is transferred or used as an operand, whereas non-packed means that only one value is is processed; non-packed means that no parallel processing takes place.

回答2:

SSE supports two modes of operation:

• Packed mode - instructions operate in parallel on all data operands
• Scalar mode - instructions operate on the least significant pairs of packed data operands.

Source

来源：https://stackoverflow.com/questions/30027707/what-is-the-difference-between-non-packed-and-packed-instruction-in-the-context