[gcc r13-2292] Improve union of ranges containing NAN.

[gcc r13-2292] Improve union of ranges containing NAN.

[Aldy Hernandez]

https://gcc.gnu.org/g:75f59441cd63a1d07e86d70d59c518049f53904f

commit r13-2292-g75f59441cd63a1d07e86d70d59c518049f53904f
Author: Aldy Hernandez <aldyh@redhat.com>
Date:   Tue Aug 30 15:46:43 2022 +0200

    Improve union of ranges containing NAN.
    
    Previously [5,6] U NAN would just drop to VARYING.  With this patch,
    the resulting range becomes [5,6] with the NAN bit set to unknown.
    
    [I still have yet to decide what to do with intersections.  ISTM, the
    intersection of a known NAN with anything else should be a NAN, but it
    could also be undefined (the empty set).  I'll have to run some tests
    and see.  Currently, we drop to VARYING cause well... it's always safe
    to give up;-).]
    
    gcc/ChangeLog:
    
            * value-range.cc (early_nan_resolve): Change comment.
            (frange::union_): Handle union when one side is a NAN.
            (range_tests_nan): Add tests for NAN union.

Diff:
---
 gcc/value-range.cc | 44 ++++++++++++++++++++++++++++++++++----------
 1 file changed, 34 insertions(+), 10 deletions(-)

diff --git a/gcc/value-range.cc b/gcc/value-range.cc
index b6d6c62c06c..473139c6dbd 100644
--- a/gcc/value-range.cc
+++ b/gcc/value-range.cc
@@ -403,7 +403,7 @@ early_nan_resolve (frange &r, const frange &other)
   // There's nothing to do for both NANs.
   if (r.get_nan ().yes_p () == other.get_nan ().yes_p ())
     return false;
-  // But only one NAN complicates things.
+  // ?? Perhaps the intersection of a NAN and anything is a NAN ??.
   r.set_varying (r.type ());
   return true;
 }
@@ -420,10 +420,22 @@ frange::union_ (const vrange &v)
       *this = r;
       return true;
     }
-  // ?? We could do better here.  [5,6] U NAN should be [5,6] with the
-  // NAN maybe bits set.  For now, this is conservatively correct.
-  if (get_nan ().yes_p () || r.get_nan ().yes_p ())
-    return early_nan_resolve (*this, r);
+
+  // If one side has a NAN, the union is just the other side plus the
+  // NAN bit.
+  if (get_nan ().yes_p ())
+    {
+      *this = r;
+      // NOP if NAN already set.
+      set_nan (fp_prop::VARYING);
+      return true;
+    }
+  if (r.get_nan ().yes_p ())
+    {
+      // NOP if NAN already set.
+      set_nan (fp_prop::VARYING);
+      return true;
+    }
 
   bool changed = m_props.union_ (r.m_props);
 
@@ -3520,6 +3532,7 @@ static void
 range_tests_nan ()
 {
   frange r0, r1;
+  REAL_VALUE_TYPE q, r;
 
   // Equal ranges but with differing NAN bits are not equal.
   r1 = frange_float ("10", "12");
@@ -3537,13 +3550,24 @@ range_tests_nan ()
   ASSERT_FALSE (r0 == r0);
   ASSERT_TRUE (r0 != r0);
 
-  // Make sure that combining NAN and INF doesn't give any crazy results.
+  // [5,6] U NAN is [5,6] with an unknown NAN bit.
+  r0 = frange_float ("5", "6");
+  r0.set_nan (fp_prop::NO);
+  r1 = frange_nan (float_type_node);
+  r0.union_ (r1);
+  real_from_string (&q, "5");
+  real_from_string (&r, "6");
+  ASSERT_TRUE (real_identical (&q, &r0.lower_bound ()));
+  ASSERT_TRUE (real_identical (&r, &r0.upper_bound ()));
+  ASSERT_TRUE (r0.get_nan ().varying_p ());
+
+  // NAN U NAN = NAN
   r0 = frange_nan (float_type_node);
-  ASSERT_TRUE (r0.get_nan ().yes_p ());
-  r1 = frange_float ("+Inf", "+Inf");
+  r1 = frange_nan (float_type_node);
   r0.union_ (r1);
-  // [INF, INF] U NAN = VARYING
-  ASSERT_TRUE (r0.varying_p ());
+  ASSERT_TRUE (real_isnan (&r0.lower_bound ()));
+  ASSERT_TRUE (real_isnan (&r1.upper_bound ()));
+  ASSERT_TRUE (r0.get_nan ().yes_p ());
 
   // [INF, INF] ^ NAN = VARYING
   r0 = frange_nan (float_type_node);