libstdc++: Fix std::abs(__float128) for -NaN and -0.0 [PR109758]

The current implementation of this non-standard overload of std::abs
incorrectly returns a negative value for negative NaNs and negative
zero, because x < 0 is false in both cases.

Use fabsl(long double) or fabsf128(_Float128) if those do the right
thing.  Otherwise, use __builtin_signbit(x) instead of x < 0 to detect
negative inputs. This assumes that __builtin_signbit handles __float128
correctly, but that seems to be true for all of GCC, clang and icc.

libstdc++-v3/ChangeLog:

	PR libstdc++/109758
	* include/bits/std_abs.h (abs(__float128)): Handle negative NaN
	and negative zero correctly.
	* testsuite/26_numerics/headers/cmath/109758.cc: New test.

2 files changed, 63 insertions, 2 deletions

diff --git a/libstdc++-v3/include/bits/std_abs.h b/libstdc++-v3/include/bits/std_abs.h
index 1bb7ffbc..c70c8e4 100644
--- a/libstdc++-v3/include/bits/std_abs.h
+++ b/libstdc++-v3/include/bits/std_abs.h
@@ -135,11 +135,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   __extension__ inline _GLIBCXX_CONSTEXPR
   __float128
   abs(__float128 __x)
-  { return __x < 0 ? -__x : __x; }
+  {
+#if defined(_GLIBCXX_LDOUBLE_IS_IEEE_BINARY128)
+    return __builtin_fabsl(__x);
+#elif defined(_GLIBCXX_HAVE_FLOAT128_MATH)
+    return __builtin_fabsf128(__x);
+#else
+    // Assume that __builtin_signbit works for __float128.
+    return __builtin_signbit(__x) ? -__x : __x;
+#endif
+  }
 #endif
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
-} // extern "C"++"
+} // extern "C++"
 
 #endif // _GLIBCXX_BITS_STD_ABS_H
diff --git a/libstdc++-v3/testsuite/26_numerics/headers/cmath/109758.cc b/libstdc++-v3/testsuite/26_numerics/headers/cmath/109758.cc
new file mode 100644
index 0000000..c9716d3
--- /dev/null
+++ b/libstdc++-v3/testsuite/26_numerics/headers/cmath/109758.cc
@@ -0,0 +1,52 @@
+// { dg-do run }
+// PR libstdc++/109758 std::abs(__float128) doesn't support NaN
+
+#include <cmath>
+#include <testsuite_hooks.h>
+
+#if !defined(__STRICT_ANSI__) && defined(_GLIBCXX_USE_FLOAT128)
+void
+test_nan()
+{
+  __float128 nan = __builtin_nanl("");
+  VERIFY( !__builtin_signbit(std::abs(nan)) );
+  VERIFY( !__builtin_signbit(std::abs(-nan)) );
+}
+
+void
+test_zero()
+{
+  __float128 zero = 0.0;
+  VERIFY( !__builtin_signbit(std::abs(zero)) );
+  VERIFY( !__builtin_signbit(std::abs(zero * -2.0)) );
+}
+
+void
+test_neg()
+{
+  VERIFY( std::abs((__float128)-1.0) == -1.0 );
+  VERIFY( std::abs((__float128)-2e9) == -2e9 );
+  VERIFY( std::abs((__float128)-3e-4) == 3e-4 );
+}
+
+void
+test_inf()
+{
+  __float128 inf = __builtin_huge_vall();
+  VERIFY( std::abs(inf) == inf );
+  VERIFY( std::abs(-inf) == inf );
+}
+
+#if __cplusplus >= 201103L
+static_assert( std::abs((__float128)-1.0) == (__float128)1.0,
+	       "std::abs(__float128) is usable in constant expressions" );
+#endif
+
+int main()
+{
+  test_nan();
+  test_zero();
+}
+#else
+int main() { }
+#endif