[gcc r12-7597] libstdc++: Fix std::strong_order to handle NaN on VAX

[gcc r12-7597] libstdc++: Fix std::strong_order to handle NaN on VAX

[Jonathan Wakely]

https://gcc.gnu.org/g:73f3b8a53e6664c079731c2a183c16621481d039

commit r12-7597-g73f3b8a53e6664c079731c2a183c16621481d039
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Mar 10 14:17:03 2022 +0000

    libstdc++: Fix std::strong_order to handle NaN on VAX
    
    I mistakenly believed that VAX floats do not support NaN, but with GCC
    __builtin_isnan(__builtin_nan("")) is true. That means my previous
    change to <compare> is wrong, because it fails to handle NaN.
    
    When std::numeric_limits<floating-point-type>::is_iec559 is false, as on
    VAX, the standard only requires an ordering that is consistent with the
    ordering observed by comparison operators. With this change the ordering
    is -NaN < numbers < +NaN, and there is no support for different NaN bit
    patterns (as I'm not even sure if GCC supports any for VAX).
    
    libstdc++-v3/ChangeLog:
    
            * libsupc++/compare (_Strong_order::_S_fp_cmp) [__vax__]:
            Handle NaN.

Diff:
---
 libstdc++-v3/libsupc++/compare | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/libsupc++/compare b/libstdc++-v3/libsupc++/compare
index 3c22d9addf1..6e1ed53eeed 100644
--- a/libstdc++-v3/libsupc++/compare
+++ b/libstdc++-v3/libsupc++/compare
@@ -844,8 +844,16 @@ namespace std
 	_S_fp_cmp(_Tp __x, _Tp __y) noexcept
 	{
 #ifdef __vax__
-	  // VAX format has no NaN, only "excess" for Inf, so totally ordered.
-	  return __builtin_bit_cast(strong_ordering, __x <=> __y);
+	  if (__builtin_isnan(__x) || __builtin_isnan(__y))
+	    {
+	      int __ix = (bool) __builtin_isnan(__x);
+	      int __iy = (bool) __builtin_isnan(__y);
+	      __ix *= __builtin_signbit(__x) ? -1 : 1;
+	      __iy *= __builtin_signbit(__y) ? -1 : 1;
+	      return __ix <=> __iy;
+	    }
+	  else
+	    return __builtin_bit_cast(strong_ordering, __x <=> __y);
 #endif
 
 	  auto __ix = _S_fp_bits(__x);