[PATCH] Prepare for prefixed instructions on PowerPC

[Segher Boessenkool]

On Wed, Jul 03, 2019 at 12:50:37PM -0400, Michael Meissner wrote:
> On Tue, Jul 02, 2019 at 07:09:20PM -0500, Segher Boessenkool wrote:
> > We'll need to update our insn_cost for prefixed, sure, it currently does
> >   int n = get_attr_length (insn) / 4;
> > to figure out how many machine instructions a pattern is, and then uses
> > "n" differently for the different types of insn.  We'll need to refine
> > this a bit for prefixed instructions.
> 
> Yes, I have some plans with this regard.  In particular, I will be introducing
> a "num_insns" RTL attribute, that if set is the number of instructions that
> will be emitted.

I don't think this is a good idea.  You can set "cost" directly, if that
is the only thing you need this for?

> What I was talking about is I've found some insns that don't set the length,
> and are split.  Using the insn cost mechanism will mean that these instructions
> will be thought of being cheaper than they actually are.

Yes.  Please split RTL insns as early as possible.  It also matters for
scheduling, and it prevents exponential explosion of the number of
patterns you need.  Only sometimes do you need to split late, usually
because RA can put some registers in memory and you want to handle that
optimally, or things depend on what exact register you were allocated
(cr0 vs. crN for example, but could be GPR vs. VSR).

And again, you can set cost directly; length alone is not usually enough
for determining the cost of split patterns.  But you do need length for
accurate costs with -Os, hrm.


Segher