[gcc r13-2450] Do not fold __builtin_signbit if NAN is a possibility.

[Aldy Hernandez]

https://gcc.gnu.org/g:5f895f22173d78c1995a001d721d2ae1d075797b

commit r13-2450-g5f895f22173d78c1995a001d721d2ae1d075797b
Author: Aldy Hernandez <aldyh@redhat.com>
Date:   Mon Sep 5 20:06:30 2022 +0200

    Do not fold __builtin_signbit if NAN is a possibility.
    
    I had some queued up work to try harder to keep the sign bit up to
    date in the presence of NANs, but after the discussion with Richi
    regarding the representation of NAN and signs in the frange, I've
    decided to put it aside until after Cauldron, since it'll probably get
    rewritten anyhow.  So for now, the sign property in the frange is not
    applicable to NANs.
    
    Right now the only user of the sign bit that affects generated code
    (apart from signed zeros) is __builtin_sign, so I'm just checking for
    NAN there.  Signed zeros continue working as before.
    
    Regstrapped and mpfr checked on x86-64 Linux.
    
    gcc/ChangeLog:
    
            * gimple-range-fold.cc
            (fold_using_range::range_of_builtin_int_call): Ignore sign bit
            when there's the possibility of a NAN.

Diff:
---
 gcc/gimple-range-fold.cc | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/gcc/gimple-range-fold.cc b/gcc/gimple-range-fold.cc
index 3543f0980b8..c9c7a2ccc70 100644
--- a/gcc/gimple-range-fold.cc
+++ b/gcc/gimple-range-fold.cc
@@ -1029,7 +1029,9 @@ fold_using_range::range_of_builtin_int_call (irange &r, gcall *call,
 	frange tmp;
 	if (src.get_operand (tmp, arg))
 	  {
-	    if (tmp.get_signbit ().varying_p ())
+	    if (tmp.get_signbit ().varying_p ()
+		// FIXME: We don't support signed NANs yet.
+		|| !tmp.get_nan ().no_p ())
 	      return false;
 	    if (tmp.get_signbit ().yes_p ())
 	      r.set_nonzero (type);