Re: [COMMITTED] Drop -0.0 in frange::set() for !HONOR_SIGNED_ZEROS.

[Aldy Hernandez <aldyh@redhat.com>]

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On Fri, Oct 14, 2022 at 4:33 PM Jakub Jelinek <jakub@redhat.com> wrote:
>
> On Fri, Oct 14, 2022 at 04:26:50PM +0200, Aldy Hernandez via Gcc-patches wrote:
> > Similar to what we do for NANs when !HONOR_NANS and Inf when
> > flag_finite_math_only, we can remove -0.0 from the range at creation
> > time.
> >
> > We were kinda sorta doing this because there is a bug in
> > real_isdenormal that is causing flush_denormals_to_zero to saturate
> > [x, -0.0] to [x, +0.0] when !HONOR_SIGNED_ZEROS.  Fixing this bug
> > (upcoming), causes us to leave -0.0 in places where we aren't
> > expecting it (the intersection code).
> >
> > gcc/ChangeLog:
> >
> >       * value-range.cc (frange::set): Drop -0.0 for !HONOR_SIGNED_ZEROS.
>
> This looks wrong to me.
> !HONOR_NANS is different from !HONOR_SIGNED_ZEROS.
> The former says that either NaNs aren't supported or if they appear,
> it will be UB.
> The latter says that either -0.0 doesn't exist, or user doesn't care
> if -0.0 or 0.0 is used.
>
> So, what you do is ok for !MODE_HAS_SIGNED_ZEROS (TYPE_MODE (m_type)),
> but otherwise we want to canonicalize [x, -0.0] to [x, 0.0] and
> [0.0, y] to [-0.0, y].

If the user doesn't care, I would expect  they'd be ok with it being
+0.0, but I must say, this is way beyond my paygrade.

How does this patch in testing look?

Thanks.
Aldy

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From 045d57b979722d15ce7fce733616bbf4ab0e0459 Mon Sep 17 00:00:00 2001
From: Aldy Hernandez <aldyh@redhat.com>
Date: Fri, 14 Oct 2022 16:49:33 +0200
Subject: [PATCH] Implement distinction between HONOR_SIGNED_ZEROS and
 MODE_HAS_SIGNED_ZEROS.

gcc/ChangeLog:

	* value-range.cc (frange::set): Implement distinction between
	HONOR_SIGNED_ZEROS and MODE_HAS_SIGNED_ZEROS.
---
 gcc/value-range.cc | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/gcc/value-range.cc b/gcc/value-range.cc
index 6b4f3dddcd5..e5b4c1565d4 100644
--- a/gcc/value-range.cc
+++ b/gcc/value-range.cc
@@ -324,13 +324,20 @@ frange::set (tree type,
       m_neg_nan = false;
     }
 
-  if (!HONOR_SIGNED_ZEROS (m_type))
+  if (!MODE_HAS_SIGNED_ZEROS (TYPE_MODE (m_type)))
     {
       if (real_iszero (&m_min, 1))
 	m_min.sign = 0;
       if (real_iszero (&m_max, 1))
 	m_max.sign = 0;
     }
+  else if (!HONOR_SIGNED_ZEROS (m_type))
+    {
+      if (real_iszero (&m_max, 1))
+	m_max.sign = 0;
+      if (real_iszero (&m_min, 0))
+	m_min.sign = 1;
+    }
 
   // For -ffinite-math-only we can drop ranges outside the
   // representable numbers to min/max for the type.
-- 
2.37.3