Re: [PATCH] frange: flush denormals to zero for -funsafe-math-optimizations.

[Aldy Hernandez <aldyh@redhat.com>]

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On Mon, Sep 19, 2022 at 3:45 PM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Mon, Sep 19, 2022 at 3:04 PM Aldy Hernandez <aldyh@redhat.com> wrote:
> >
> > On Mon, Sep 19, 2022 at 9:37 AM Richard Biener
> > <richard.guenther@gmail.com> wrote:
> > >
> > > On Sat, Sep 17, 2022 at 10:25 AM Aldy Hernandez via Gcc-patches
> > > <gcc-patches@gcc.gnu.org> wrote:
> > > >
> > > > Jakub has mentioned that for -funsafe-math-optimizations we may flush
> > > > denormals to zero, in which case we need to be careful to extend the
> > > > ranges to the appropriate zero.  This patch does exactly that.  For a
> > > > range of [x, -DENORMAL] we flush to [x, -0.0] and for [+DENORMAL, x]
> > > > we flush to [+0.0, x].
> > > >
> > > > It is unclear whether we should do this for Alpha, since I believe
> > > > flushing to zero is the default, and the port requires -mieee for IEEE
> > > > sanity.  If so, perhaps we should add a target hook so backends are
> > > > free to request flushing to zero.
> > > >
> > > > Thoughts?
> > >
> > > I'm not sure what the intention of this is - it effectively results in
> > > more conservative ranges for -funsafe-math-optimizations.  That is,
> > > if -funsafe-math-optimizations says that denormals don't exist and
> > > are all zero then doesn't that mean we can instead use the smalles
> > > non-denormal number less than (or greater than) zero here?
> > >
> > > That said, the flushing you do is also "valid" for
> > > -fno-unsafe-math-optimizations
> > > in case we don't want to deal with denormals in range boundaries.
> > >
> > > It might also be a correctness requirement in case we don't know how
> > > targets handle denormals (IIRC even OS defaults might matter here) so
> > > we conservatively have to treat them as being flushed to zero.
> >
> > Actually, rth suggested we always flush to zero because we don't know
> > what the target would do.  Again, I'm happy to do whatever you agree
> > on.  I have no opinion.  My main goal here is correctness.
>
> Yes, I think we should do this.

Ok, I'm pushing the attached patch because it's conservatively correct
everywhere.  We can tweak it when y'all reach consensus.

>
> > >
> > > So ... if we want to be on the "safe" side then please always do this.
> > >
> > > At the same point you could do
> > >
> > >  if (!HONOR_SIGNED_ZEROS ())
> > >    if (real_iszero (&m_max))
> > >      {
> > >         if (real_iszero (&m_min))
> > >            m.min.sign = 1;
> > >         m_max.sign = 1;
> > >      }
> >
> > But wouldn't that set [-0.0, -0.0] when encountering [+0, +0] ??
>
> Yeah, that's my laziness not adding a special case for m_min == m_max.
>
> >
> > >    else if (real_iszero (&m_min))
> > >      m_min.sign = 0;
> >
> > Jakub actually suggested something completely different...just set +0
> > always for !HONOR_SIGNED_ZEROS.
>
> Hmm, but the [-1, -0.] with known sign becomes [-1, +0.] with unknown sign?

Huh.  I guess that's true.  Does that happen often enough in practice
to matter?  I suppose we're going into uncharted territory with folks
asking for no signs on zeros, but them appearing in the source code?
My gut feeling is that we should take whatever signs are in the source
code (similar to what we're doing for NANs), but don't introduce any
additional ones.

Again, no strong opinions.  Feel free to add whatever adjustment you
think is necessary.

Aldy

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From 2b61ed838c7f3f4bf54d4530c0f053b420623beb Mon Sep 17 00:00:00 2001
From: Aldy Hernandez <aldyh@redhat.com>
Date: Sat, 17 Sep 2022 10:17:13 +0200
Subject: [PATCH] frange: flush denormals to zero

For some architectures (or for -funsafe-math-optimizations) we may
flush denormals to zero, in which case we need to be careful to
extend the ranges to the appropriate zero.  This patch does exactly that.
For a range of [x, -DENORMAL] we flush to [x, -0.0] and for [+DENORMAL, x]
we flush to [+0.0, x].

gcc/ChangeLog:

	* value-range.cc (frange::flush_denormals_to_zero): New.
	(frange::set): Call flush_denormals_to_zero.
	* value-range.h (class frange): Add flush_denormals_to_zero.
---
 gcc/value-range.cc | 23 +++++++++++++++++++++++
 gcc/value-range.h  |  1 +
 2 files changed, 24 insertions(+)

diff --git a/gcc/value-range.cc b/gcc/value-range.cc
index 67d5d7fa90f..a8e3bb39bab 100644
--- a/gcc/value-range.cc
+++ b/gcc/value-range.cc
@@ -267,6 +267,26 @@ tree_compare (tree_code code, tree op1, tree op2)
   return !integer_zerop (fold_build2 (code, integer_type_node, op1, op2));
 }
 
+// Flush denormal endpoints to the appropriate 0.0.
+
+void
+frange::flush_denormals_to_zero ()
+{
+  if (undefined_p () || known_isnan ())
+    return;
+
+  // Flush [x, -DENORMAL] to [x, -0.0].
+  if (real_isdenormal (&m_max) && real_isneg (&m_max))
+    {
+      m_max = dconst0;
+      if (HONOR_SIGNED_ZEROS (m_type))
+	m_max.sign = 1;
+    }
+  // Flush [+DENORMAL, x] to [+0.0, x].
+  if (real_isdenormal (&m_min) && !real_isneg (&m_min))
+    m_min = dconst0;
+}
+
 // Setter for franges.
 
 void
@@ -317,6 +337,9 @@ frange::set (tree min, tree max, value_range_kind kind)
   gcc_checking_assert (tree_compare (LE_EXPR, min, max));
 
   normalize_kind ();
+
+  flush_denormals_to_zero ();
+
   if (flag_checking)
     verify_range ();
 }
diff --git a/gcc/value-range.h b/gcc/value-range.h
index 3a401f3e4e2..795b1f00fdc 100644
--- a/gcc/value-range.h
+++ b/gcc/value-range.h
@@ -327,6 +327,7 @@ private:
   bool union_nans (const frange &);
   bool intersect_nans (const frange &);
   bool combine_zeros (const frange &, bool union_p);
+  void flush_denormals_to_zero ();
 
   tree m_type;
   REAL_VALUE_TYPE m_min;
-- 
2.37.1