Re: [PATCH v2 3/3] Consider doloop cmp use in ivopts

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

On Wed, May 15, 2019 at 10:47:31AM +0200, Richard Biener wrote:
> Ah, so the key issue is that the doloop IV is "free"?  That
> is, it doesn't consume a general register and whatnot?  That
> is allocating this IV doesn't really interfere with other IVs?

That is one half of it, yes.

> But can other uses be based on the doloop IV easily (if the
> IV doesn't reside in a general reg?)?

Getting the value of the count reg can be expensive, that is the
other half of it.

> Otherwise I understand that IVOPTs doesn't properly cost
> the doloop IV update and conditional branch.

Currently it doesn't even *know* something is or isn't a doloop.
And yeah that matters a lot for proper costing, on all targets that
have a doloop.

> That's clearly
> something we should fix (maybe even indepenently on other
> changes).  One important thing is that we need to base costs
> on a common base to not compare apples and oranges, didn't
> dig into your patch in detail enough to see whether it
> fits into the general cost model or whether it is a hack
> ontop of everything.

The different cost for a doloop is pretty easy...  Might have to
be a target hook though; on Power the decrement + compare-to-zero
are "free", while on some other targets only the "compare" is.
The cost for using the IV...  For us we could just disallow it
being used at all (except for the looping itself of course), but
not sure what is optimal in general.  Another hook?


Segher