Re: [RFC] c-family: Add __builtin_noassoc

[Matthias Kretz <m.kretz@gsi.de>]

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.

> 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

float sum_noassoc(RangeOfFloats auto x) {
  float sum = 0;
  for (float v : x)
    sum = __builtin_assoc_barrier(v + x);
  return sum;
}

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 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
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