Re: [PATCH 1/3] Disable generating store vector pair.

[Michael Meissner <meissner@linux.ibm.com>]

On Tue, Jun 07, 2022 at 07:59:34PM -0500, Peter Bergner wrote:
> On 6/7/22 4:24 PM, Segher Boessenkool wrote:
> > On Tue, Jun 07, 2022 at 04:17:04PM -0500, Peter Bergner wrote:
> >> I think I mentioned this offline, but I'd prefer a negative target flag,
> >> something like TARGET_NO_STORE_VECTOR_PAIR that defaults to off, meaning we'd
> >> generate stxvp by default.
> > 
> > NAK.  All negatives should be -mno-xxx with -mxxx the corresponding
> > positive.  All of them.
> 
> That's not what I was asking for.  I totally agree that -mno-store-vector-pair
> should disable generating stxvp and that -mstore-vector-pair should enable
> generating it.  What I asked for was that the internal flag we use to enable
> and disable it should be a negative flag, where TARGET_NO_STORE_VECTOR_PAIR is
> true when we use -mno-store-vector-pair and false when using -mstore-vector-pair.
> That way we can add that flag to power10's rs6000-cpu.def entry and then we're
> done.  What I don't want to have to do is that if/when power87 is released, we
> still have to add TARGET_STORE_VECTOR_PAIR its rs6000-cpu.def entry just to
> get stxvp insns generated.  That adds a cost to every cpu after power10 since
> we'd have to remember to add that flag to every follow-on cpu.

FWIW, I really dislike having negative flags like that (just talking about the
option mask internals, not the user option).

I don't view the cost to add one postive flag to the next CPU as bad, as it
will be a one time cost.  Presumably it would be set also next++ CPU.  This is
like power8 is all of the power7 flags + new flags.  Power9 is all of the
power8 flags + new flags.  I.e. in general it is cumulative.  Yes, I'm aware
there are times when there are breaks, but hopefully those are rare.

Otherwise it is like the mess with -mpower8-fusion, where going from power8 to
power9 we have to clear the fusion flag.  If store vector pair is a postive
flag, then it isn't set in power10 flags, but it might be set in next cpu
flags.  But if it is a negative flag, we have to explicitly clear it.

We can do it, but I just prefer to go with the positive flag approach.

-- 
Michael Meissner, IBM
PO Box 98, Ayer, Massachusetts, USA, 01432
email: meissner@linux.ibm.com