Re: [RFC] c-family: Add __builtin_noassoc

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

On Fri, Jul 16, 2021 at 2:00 PM Matthias Kretz <m.kretz@gsi.de> wrote:
>
> On Friday, 16 July 2021 11:31:29 CEST Richard Biener wrote:
> > On Fri, Jul 16, 2021 at 10:57 AM Matthias Kretz <m.kretz@gsi.de> wrote:
> > > On Wednesday, 14 July 2021 10:14:55 CEST Richard Biener wrote:
> > > > I think implementing it similar to how we do __builtin_shufflevector
> > > > would
> > > > be easily possible.  PAREN_EXPR is a tree code.
> > >
> > > Like this? If you like it, I'll write the missing documentation and do
> > > real
> > > regression testing.
> >
> > Yes, like this.  Now, __builtin_noassoc (a + b + c) might suggest that
> > it prevents a + b + c from being re-associated - but it does not.
> > PAREN_EXPR is a barrier for association, so for 'a + b + c + PAREN_EXPR <d
> > + e + f>' the a+b+c and d+e+f chains will not mix but they individually can
> > be re-associated.  That said __builtin_noassoc might be a bad name,
> > maybe __builtin_assoc_barrier is better?
>
> Yes, I agree with renaming it. And assoc_barrier sounds intuitive to me.
>
> > To fully prevent association of a a + b + d + e chain you need at least
> > two PAREN_EXPRs, for example (a+b) + (d+e) would do.
> >
> > One could of course provide __builtin_noassoc (a+b+c+d) with the
> > implied semantics and insert PAREN_EXPRs around all operands
> > when lowering it.
>
> I wouldn't want to go there. __builtin_noassoc(f(x, y, z))? We probably both
> agree that it would be a no-op, but it reads like f should be evaluated with -
> fno-associative-math.

Ah, true - yeah, let's not go down this rathole.

> > Not sure what's more useful in practice - directly exposing the middle-end
> > PAREN_EXPR or providing a way to mark a whole expression as to be
> > not re-associated?  Maybe both?
>
> I think this is a tool for specialists. Give them the low-level tool and
> they'll build whatever higher level abstractions they need on top of it. Like

OK, fine.

> float sum_noassoc(RangeOfFloats auto x) {
>   float sum = 0;
>   for (float v : x)
>     sum = __builtin_assoc_barrier(v + x);
>   return sum;
> }
>
> --
> ──────────────────────────────────────────────────────────────────────────
>  Dr. Matthias Kretz                           https://mattkretz.github.io
>  GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
>  std::experimental::simd              https://github.com/VcDevel/std-simd
> ──────────────────────────────────────────────────────────────────────────