Re: A problem with one instruction multiple latencies and pipelines

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi!

On Mon, Sep 07, 2020 at 09:20:59PM +0100, Richard Sandiford wrote:
> This is just personal opinion, but in general (from the point of view
> of a new port, or a new subport like SVE), I think the best approach
> to handling the "type" attribute is to start with the coarsest
> classification that makes sense, then split these relatively coarse
> types up whenever there's a specific need.

Agreed.

> When taking that approach, it's OK (and perhaps even a good sign)
> for an existing type to sometimes be too coarse for a new CPU.
> 
> So thanks for asking about this, and please don't feel constrained
> by the existing "type" classification.  IMO we should split existing
> types wherever that makes sense for new CPUs.

You can also use some other attributes to classify instructions, you
don't have to put it all in one "type" attribute.  This can of course be
done later, at a time when it is clearer what a good design will be.
Sometimes it is obvious from the start though :-)

(This primarily makes the pipeline descriptions much simpler, but also
custom scheduling code and the like.  If one core has two types of "A"
insn, say "Aa" and "Ab", it isn't nice if all other cores now have to
handle both "Aa" and "Ab" instead of just "A").


Segher