[gcc r13-2714] [PR106831] Avoid propagating long doubles that may have multiple representations.

[Aldy Hernandez <aldyh@gcc.gnu.org>]

https://gcc.gnu.org/g:5dba8b2a91376d0518eb21c69a0700f099aa9cc4

commit r13-2714-g5dba8b2a91376d0518eb21c69a0700f099aa9cc4
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Sat Sep 17 08:50:22 2022 +0200

    [PR106831] Avoid propagating long doubles that may have multiple representations.
    
    Long doubles are tricky when it comes to considering singletons
    because small numbers and +-INF can have multiple representations for
    the same number.  So we need to be very careful not to treat those as
    singletons, lest they be incorrectly propagated by VRP.  This is
    similar to the -0.0 and +0.0 duality.
    
    In long doubles +INF can be represented with +INF in the MSB and
    either -0.0 or +0.0 in the LSB.  Similarly for numbers that are exactly
    representable in DF.  For example, 1.0 can be represented as either
    (1.0, +0.0) or (1.0, -0.0).
    
    This patch avoids treating these numbers as singletons.
    
    Note that NANs in long double format have a LSB of don't care, but
    this is irrelevant for singleton_p, because NANs are never considered
    singletons.  Also, internally in the frange we store NANs as a pair of
    boolean flags indicating whether they are +NAN or -NAN, so we don't need
    any special treatment here for comparing range equality etc.  We never
    see anything but the boolean flags.
    
            PR middle-end/106831
    
    gcc/ChangeLog:
    
            * value-range.cc (frange::singleton_p): Avoid propagating long
            doubles that may have multiple representations.

Diff:
---
 gcc/value-range.cc | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/gcc/value-range.cc b/gcc/value-range.cc
index 55a216efd8b..67d5d7fa90f 100644
--- a/gcc/value-range.cc
+++ b/gcc/value-range.cc
@@ -639,6 +639,21 @@ frange::singleton_p (tree *result) const
       if (HONOR_NANS (m_type) && maybe_isnan ())
 	return false;
 
+      if (MODE_COMPOSITE_P (TYPE_MODE (m_type)))
+	{
+	  // For IBM long doubles, if the value is +-Inf or is exactly
+	  // representable in double, the other double could be +0.0
+	  // or -0.0.  Since this means there is more than one way to
+	  // represent a value, return false to avoid propagating it.
+	  // See libgcc/config/rs6000/ibm-ldouble-format for details.
+	  if (real_isinf (&m_min))
+	    return false;
+	  REAL_VALUE_TYPE r;
+	  real_convert (&r, DFmode, &m_min);
+	  if (real_identical (&r, &m_min))
+	    return false;
+	}
+
       if (result)
 	*result = build_real (m_type, m_min);
       return true;