Re: [PATCH] [range-ops] Add ability to represent open intervals in frange.

[Aldy Hernandez <aldyh@redhat.com>]

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Passes tests for all languages. Passes lapack tests.

So.... ready to be installed unless you have any issues. Oh... I should
write some tests..

Aldy

On Fri, Nov 11, 2022, 19:11 Aldy Hernandez <aldyh@redhat.com> wrote:

> Currently we represent < and > with a closed interval.  So < 3.0 is
> represented as [-INF, +3.0].  This means 3.0 is included in the range,
> and though not ideal, is conservatively correct.  Jakub has found a
> couple cases where properly representing < and > would help
> optimizations and tests, and this patch allows representing open
> intervals with real_nextafter.
>
> There are a few caveats.
>
> First, we leave MODE_COMPOSITE_P types pessimistically as a closed
> interval.
>
> Second, for -ffinite-math-only, real_nextafter will will saturate the
> maximum representable number into +INF.  However, this will still do
> the right thing, as frange::set() will crop things appropriately.
>
> Finally, and most frustratingly, representing < and > -+0.0 is
> problematic because we flush denormals to zero.  Let me explain...
>
> real_nextafter(+0.0, +INF) gives 0x0.8p-148 as expected, but setting a
> range to this value yields [+0.0, 0x0.8p-148] because of the flushing.
>
> On the other hand, real_nextafter(+0.0, -INF) (surprisingly) gives
> -0.0.8p-148, but setting a range to that value yields [-0.0x8p-148,
> -0.0].  I say surprising, because according to cppreference.com,
> std::nextafter(+0.0, -INF) should give -0.0.  But that's neither here
> nor there because our flushing denormals to zero prevents us from even
> representing ranges involving small values around 0.0.  We ultimately
> end up with ranges looking like this:
>
>         > +0.0          => [+0.0, INF]
>         > -0.0          => [+0.0, INF]
>         < +0.0          => [-INF, -0.0]
>         < -0.0          => [-INF, -0.0]
>
> I suppose this is no worse off that what we had before with closed
> intervals.  One could even argue that we're better because we at least
> have the right sign now ;-).
>
> All other (non-zero) values look sane.
>
> Lightly tested.
>
> Thoughts?
>
> gcc/ChangeLog:
>
>         * range-op-float.cc (build_lt): Adjust with frange_nextafter
>         instead of default to a closed range.
>         (build_gt): Same.
> ---
>  gcc/range-op-float.cc | 23 +++++++++++++++++++----
>  1 file changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/gcc/range-op-float.cc b/gcc/range-op-float.cc
> index 380142b4c14..402393097b2 100644
> --- a/gcc/range-op-float.cc
> +++ b/gcc/range-op-float.cc
> @@ -381,9 +381,17 @@ build_lt (frange &r, tree type, const frange &val)
>         r.set_undefined ();
>        return false;
>      }
> -  // We only support closed intervals.
> +
>    REAL_VALUE_TYPE ninf = frange_val_min (type);
> -  r.set (type, ninf, val.upper_bound ());
> +  REAL_VALUE_TYPE prev = val.upper_bound ();
> +  machine_mode mode = TYPE_MODE (type);
> +  // Default to the conservatively correct closed ranges for
> +  // MODE_COMPOSITE_P, otherwise use nextafter.  Note that for
> +  // !HONOR_INFINITIES, nextafter will yield -INF, but frange::set()
> +  // will crop the range appropriately.
> +  if (!MODE_COMPOSITE_P (mode))
> +    frange_nextafter (mode, prev, ninf);
> +  r.set (type, ninf, prev);
>    return true;
>  }
>
> @@ -424,9 +432,16 @@ build_gt (frange &r, tree type, const frange &val)
>        return false;
>      }
>
> -  // We only support closed intervals.
>    REAL_VALUE_TYPE inf = frange_val_max (type);
> -  r.set (type, val.lower_bound (), inf);
> +  REAL_VALUE_TYPE next = val.lower_bound ();
> +  machine_mode mode = TYPE_MODE (type);
> +  // Default to the conservatively correct closed ranges for
> +  // MODE_COMPOSITE_P, otherwise use nextafter.  Note that for
> +  // !HONOR_INFINITIES, nextafter will yield +INF, but frange::set()
> +  // will crop the range appropriately.
> +  if (!MODE_COMPOSITE_P (mode))
> +    frange_nextafter (mode, next, inf);
> +  r.set (type, next, inf);
>    return true;
>  }
>
> --
> 2.38.1
>
>