libstdc++: Disable floating-point std::to_chars on unsupported targets

This patch conditionally disables the floating-point std::to_chars
implementation on targets whose float and double aren't IEEE binary32
and binary64, until a proper fallback can be added for such targets.
This fixes a bootstrap failure on non-IEEE-754 FP targets such as
vax-netbsdelf.

The new preprocessor tests in c++config that detect the binary32 and
binary64 formats were copied from gcc/testsuite/gcc.dg/float-exact-1.c.

libstdc++-v3/ChangeLog:

	* include/bits/c++config (_GLIBCXX_FLOAT_IS_IEEE_BINARY_32):
	Define this macro.
	(_GLIBCXX_DOUBLE_IS_IEEE_BINARY_64): Likewise.
	* include/std/charconv (to_chars): Use these macros to
	conditionally hide the overloads for floating-point types.
	* src/c++17/floating_to_chars.cc: Use the macros to
	conditionally disable this file.
	(floating_type_traits<float>): Remove redundant static assert.
	(floating_type_traits<double>): Likewise.
	* testsuite/20_util/to_chars/double.cc: Run this test only on
	ieee-floats effective targets.
	* testsuite/20_util/to_chars/float.cc: Likewise.
	* testsuite/20_util/to_chars/long_double.cc: Likewise.
	* testsuite/lib/libstdc++.exp
	(check_effective_target_ieee-floats): Define new proc for
	detecting whether float and double have the IEEE binary32 and
	binary64 formats.

libstdc++-v3/include/bits/c++config

@@ -688,6 +688,20 @@ namespace std
 # endif
 #endif
 
+// Define if float has the IEEE binary32 format.
+#if __FLT_MANT_DIG__ == 24 \
+  && __FLT_MIN_EXP__ == -125 \
+  && __FLT_MAX_EXP__ == 128
+# define _GLIBCXX_FLOAT_IS_IEEE_BINARY32 1
+#endif
+
+// Define if double has the IEEE binary64 format.
+#if __DBL_MANT_DIG__ == 53 \
+  && __DBL_MIN_EXP__ == -1021 \
+  && __DBL_MAX_EXP__ == 1024
+# define _GLIBCXX_DOUBLE_IS_IEEE_BINARY64 1
+#endif
+
 #ifdef __has_builtin
 # ifdef __is_identifier
 // Intel and older Clang require !__is_identifier for some built-ins:

libstdc++-v3/include/std/charconv

@@ -702,6 +702,7 @@ namespace __detail
 	     chars_format __fmt = chars_format::general) noexcept;
 #endif
 
+#if _GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
   // Floating-point std::to_chars
 
   // Overloads for float.
@@ -725,6 +726,7 @@ namespace __detail
 			   chars_format __fmt) noexcept;
   to_chars_result to_chars(char* __first, char* __last, long double __value,
 			   chars_format __fmt, int __precision) noexcept;
+#endif
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std

libstdc++-v3/src/c++17/floating_to_chars.cc

@@ -40,6 +40,10 @@
 #include <string_view>
 #include <type_traits>
 
+// This implementation crucially assumes float/double have the
+// IEEE binary32/binary64 formats.
+#if _GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64
+
 // Determine the binary format of 'long double'.
 
 // We support the binary64, float80 (i.e. x86 80-bit extended precision),
@@ -109,8 +113,6 @@ namespace
   template<>
     struct floating_type_traits<float>
     {
-      // We (and Ryu) assume float has the IEEE binary32 format.
-      static_assert(__FLT_MANT_DIG__ == 24);
       static constexpr int mantissa_bits = 23;
       static constexpr int exponent_bits = 8;
       static constexpr bool has_implicit_leading_bit = true;
@@ -124,8 +126,6 @@ namespace
   template<>
     struct floating_type_traits<double>
     {
-      // We (and Ryu) assume double has the IEEE binary64 format.
-      static_assert(__DBL_MANT_DIG__ == 53);
       static constexpr int mantissa_bits = 52;
       static constexpr int exponent_bits = 11;
       static constexpr bool has_implicit_leading_bit = true;
@@ -1565,3 +1565,5 @@ _ZSt8to_charsPcS_eSt12chars_formati(char* first, char* last, double value,
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std
+
+#endif // _GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64

libstdc++-v3/testsuite/20_util/to_chars/double.cc

@@ -32,6 +32,7 @@
 // DEALINGS IN THE SOFTWARE.
 
 // { dg-do run { target c++17 } }
+// { dg-require-effective-target ieee-floats }
 
 #include <charconv>
 

libstdc++-v3/testsuite/20_util/to_chars/float.cc

@@ -32,6 +32,7 @@
 // DEALINGS IN THE SOFTWARE.
 
 // { dg-do run { target c++17 } }
+// { dg-require-effective-target ieee-floats }
 
 #include <charconv>
 

libstdc++-v3/testsuite/20_util/to_chars/long_double.cc

@@ -19,6 +19,7 @@
 // hexadecimal floating-point literals.
 // { dg-do run { target c++17 } }
 // { dg-xfail-run-if "Ryu needs __int128" { large_long_double && { ! int128 } } }
+// { dg-require-effective-target ieee-floats }
 
 #include <charconv>
 

libstdc++-v3/testsuite/lib/libstdc++.exp

@@ -1323,6 +1323,14 @@ proc check_effective_target_futex { } {
     }]
 }
 
+# Return 1 if float and double have the IEEE binary32 and binary64 formats.
+proc check_effective_target_ieee-floats { } {
+    return [check_v3_target_prop_cached et_ieee_floats {
+	set cond "_GLIBCXX_FLOAT_IS_IEEE_BINARY32 && _GLIBCXX_DOUBLE_IS_IEEE_BINARY64"
+	return [v3_check_preprocessor_condition ieee_floats $cond]
+    }]
+}
+
 set additional_prunes ""
 
 if { [info exists env(GCC_RUNTEST_PARALLELIZE_DIR)] \