Re: [PATCH] vect: while_ult for integer mask

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Sep 29, 2022 at 12:17 PM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Andrew Stubbs <ams@codesourcery.com> writes:
> > On 29/09/2022 10:24, Richard Sandiford wrote:
> >> Otherwise:
> >>
> >>    operand0[0] = operand1 < operand2;
> >>    for (i = 1; i < operand3; i++)
> >>      operand0[i] = operand0[i - 1] && (operand1 + i < operand2);
> >>
> >> looks like a "length and mask" operation, which IIUC is also what
> >> RVV wanted?  (Wasn't at the Cauldron, so not entirely sure.)
> >>
> >> Perhaps the difference is that in this case the length must be constant.
> >> (Or is that true for RVV as well?)
> >
> > I too saw that presentation and I have compared notes with Juzhe before
> > posting this.
> >
> > As he has posted, what they want is different because their config
> > register has an explicit length field whereas GCN just uses a mask to
> > limit the length (more like AArch64, I think).
> >
> > The RVV solution uses different logic in the gimple IR; this proposal is
> > indistinguishable from the status quo at that point.
>
> Hmm, OK.  (And thanks to Juzhe for the summary.)
>
> I can't think of any existing examples of optabs that have a variable
> number of operands.  But maybe this is a good reason to change that.
>
> Having to add what amounts to a vector type descriptor to make up for
> the lack of mode information seems like a bit of a hack.  But it's
> possibly a hack that we'll need to do again (for other patterns),
> if we keep representing multiple distinct vector/predicate types
> using the same integer mode.  I guess this establishes a way of
> coping with the situation in general.
>
> So personally I'm OK with the patch, if Richi agrees.

It's not the nicest way of carrying the information but short of inventing
new modes I can't see something better (well, another optab).  I see
the GCN backend expects a constant in operand 3 but the docs don't
specify the operand has to be a CONST_INT, can you adjust them
accordingly?

Otherwise I'm fine with it.  It might even prove useful for x86.

Richard.

> Richard