Re: [eigen] Intermediate Packet Storage

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 18/12/2019 12.20, Joel Holdsworth wrote:
> [...]
> const Packet<uint32_t> E = a * b; // Some expensive calculation
> const Packet<uint32_t> x = E + c;
> const Packet<uint32_t> y = E + c;
>
> This results in the value of E being stored onto the stack, and then 
> reloaded twice to calculate x and y.
>
> But of course, I just want E to stay in register.

I'm pretty sure that as long as `E` fits into a (set of) register(s) no 
reasonable compiler will store this on the stack, unless it actually 
runs out of register space, see e.g.:
https://godbolt.org/z/WxEeNF

Even if the small intermediate was stored on the stack, I assume the 
overhead should be negligible.

The problem is different, if you would want to apply your expressions at 
once on a set of large arrays. Something like the following will very 
likely require `E` getting stored or evaluated twice (unless the 
compiler is really smart detecting duplicated code or load after store).

    ArrayXi a,b,c,d; // input from somewhere

    ArrayXi E = a*b; // some expensive operations
    ArrayXi x = E+c, y=E+d;

For solving that problem you may be interested in:
https://gitlab.com/libeigen/eigen/issues/984


> [...]
>
> I was wonder if there needs to be a new intermediate-friendly variant of 
> Eigen::Array where the data is stored in arrays of SIMD types instead of 
> plain-old-C arrays.

A while ago I started thinking about implementing Meta-Packets, but 
mostly to clean-up the current SIMD code base:
  https://gitlab.com/libeigen/eigen/issues/692
This includes a proof-of-concept implementation, but I never actually 
continued working on that.

Cheers,
Christoph



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