Re: [eigen] Optimization advice for a specific expression

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 2016-02-05 17:50, Alberto Luaces wrote:
> Thanks a lot, Christoph.  This is very helpful.  Regarding those
> assertions: do you have any rule of thumb to know what is getting
> vectorized and what is not?

For SSE vectorization the matrix must be accessible in blocks of 16 
bytes which are aligned. I.e., the inner dimension must be even for 
doubles and a multiple of 4 for floats. It's a bit complicated for 
..block() and for dynamic-sized objects. Also, in some cases it is 
sufficient that only one side is aligned.

> I get quickly swamped by all of those mul... and add... SSE instructions
> in the assembler output, and cannot clearly see if they are just
> performing scalar or vector operations.  Is it maybe a matter of
> checking that "packet" ops are mostly used ("P" prefix?)

mulPd/addPd are packet (double precision) operations, mulSd/addSd are 
scalar operations.
For float operations the instructions are called mulps/addps and 
mulss/addss. Div and sub are analogue, of course.

Here is a nice reference over all x86 instructions:
  http://www.felixcloutier.com/x86/

If you look at SSE-assembly a lot, you'll soon notice that you only need 
to remember 20ish of these to understand what's going on.


Christoph




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