Fortran: provide a fallback implementation of issignaling

For targets with IEEE support but without the issignaling macro in libc
(currently, everywhere except glibc), this allows us to provide a fallback
implementation. In order to keep the code in ieee_helper.c relatively
readable, I've put that new implementation in a separate file,
issignaling_fallback.h.

libgfortran/ChangeLog:

	* ieee/issignaling_fallback.h: New file.
	* ieee/ieee_helper.c: Include issignaling_fallback.h when target
	does not define issignaling macro.

gcc/testsuite/ChangeLog:

	* gfortran.dg/ieee/signaling_1.f90: Do not require issignaling.
	* gfortran.dg/ieee/signaling_2.f90: Add comment.
	* gfortran.dg/ieee/signaling_3.f90: New test.

2 files changed, 241 insertions, 4 deletions

diff --git a/libgfortran/ieee/ieee_helper.c b/libgfortran/ieee/ieee_helper.c
index 794ccec40ee..7e310f2c5b0 100644
--- a/libgfortran/ieee/ieee_helper.c
+++ b/libgfortran/ieee/ieee_helper.c
@@ -26,11 +26,10 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 #include "libgfortran.h"
 
 
-/* Check support for issignaling macro.
-   TODO: In the future, provide fallback implementations for IEEE types,
-   because many libc's do not have issignaling yet.  */
+/* Check support for issignaling macro.  If not, we include our own
+   fallback implementation.  */
 #ifndef issignaling
-# define issignaling(X) 0
+# include "issignaling_fallback.h"
 #endif
 
 
diff --git a/libgfortran/ieee/issignaling_fallback.h b/libgfortran/ieee/issignaling_fallback.h
new file mode 100644
index 00000000000..e824cf8c59b
--- /dev/null
+++ b/libgfortran/ieee/issignaling_fallback.h
@@ -0,0 +1,238 @@
+/* Fallback implementation of issignaling macro.
+   Copyright (C) 2022 Free Software Foundation, Inc.
+   Contributed by Francois-Xavier Coudert <fxcoudert@gcc.gnu.org>
+
+This file is part of the GNU Fortran runtime library (libgfortran).
+
+Libgfortran is free software; you can redistribute it and/or
+modify it under the terms of the GNU General Public
+License as published by the Free Software Foundation; either
+version 3 of the License, or (at your option) any later version.
+
+Libgfortran is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+Under Section 7 of GPL version 3, you are granted additional
+permissions described in the GCC Runtime Library Exception, version
+3.1, as published by the Free Software Foundation.
+
+You should have received a copy of the GNU General Public License and
+a copy of the GCC Runtime Library Exception along with this program;
+see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+<http://www.gnu.org/licenses/>.  */
+
+#include "libgfortran.h"
+
+/* This header provides an implementation of the type-generic issignaling macro.
+   Some points of note:
+
+     - This header is only included if the issignaling macro is not defined.
+     - All targets for which Fortran IEEE modules are supported currently have
+       the high-order bit of the NaN mantissa clear for signaling (and set
+       for quiet), as recommended by IEEE.
+     - We use the __*_IS_IEC_60559__ macros to make sure we only deal with formats
+       we know. For other floating-point formats, we consider all NaNs as quiet.
+
+ */
+
+typedef union
+{
+  float value;
+  uint32_t word;
+} ieee_float_shape_type;
+
+static inline int
+__issignalingf (float x)
+{
+#if __FLT_IS_IEC_60559__
+  uint32_t xi;
+  ieee_float_shape_type u;
+
+  u.value = x;
+  xi = u.word;
+
+  xi ^= 0x00400000;
+  return (xi & 0x7fffffff) > 0x7fc00000;
+#else
+  return 0;
+#endif
+}
+
+
+typedef union
+{
+  double value;
+  uint64_t word;
+} ieee_double_shape_type;
+
+static inline int
+__issignaling (double x)
+{
+#if __DBL_IS_IEC_60559__
+  ieee_double_shape_type u;
+  uint64_t xi;
+
+  u.value = x;
+  xi = u.word;
+
+  xi ^= UINT64_C (0x0008000000000000);
+  return (xi & UINT64_C (0x7fffffffffffffff)) > UINT64_C (0x7ff8000000000000);
+#else
+  return 0;
+#endif
+}
+
+
+#if __LDBL_DIG__ == __DBL_DIG__
+
+/* Long double is the same as double.  */
+static inline int
+__issignalingl (long double x)
+{
+  return __issignaling (x);
+}
+
+#elif (__LDBL_DIG__ == 18) && __LDBL_IS_IEC_60559__
+
+/* Long double is x86 extended type.  */
+
+typedef union
+{
+  long double value;
+  struct
+  {
+#if __FLOAT_WORD_ORDER == __BIG_ENDIAN
+    int sign_exponent:16;
+    unsigned int empty:16;
+    uint32_t msw;
+    uint32_t lsw;
+#elif __FLOAT_WORD_ORDER == __LITTLE_ENDIAN
+    uint32_t lsw;
+    uint32_t msw;
+    int sign_exponent:16;
+    unsigned int empty:16;
+#endif
+  } parts;
+} ieee_long_double_shape_type;
+
+static inline int
+__issignalingl (long double x)
+{
+  int ret;
+  uint32_t exi, hxi, lxi;
+  ieee_long_double_shape_type u;
+
+  u.value = x;
+  exi = u.parts.sign_exponent;
+  hxi = u.parts.msw;
+  lxi = u.parts.lsw;
+
+  /* Pseudo numbers on x86 are always signaling.  */
+  ret = (exi & 0x7fff) && ((hxi & 0x80000000) == 0);
+
+  hxi ^= 0x40000000;
+  hxi |= (lxi | -lxi) >> 31;
+  return ret || (((exi & 0x7fff) == 0x7fff) && (hxi > 0xc0000000));
+}
+
+#elif (__LDBL_DIG__ = 33) && __LDBL_IS_IEC_60559__
+
+/* Long double is 128-bit type.  */
+
+typedef union
+{
+  long double value;
+  struct
+  {
+#if __FLOAT_WORD_ORDER == __BIG_ENDIAN
+    uint64_t msw;
+    uint64_t lsw;
+#elif __FLOAT_WORD_ORDER == __LITTLE_ENDIAN
+    uint64_t lsw;
+    uint64_t msw;
+#endif
+  } parts64;
+} ieee854_long_double_shape_type;
+
+static inline int
+__issignalingl (long double x)
+{
+  uint64_t hxi, lxi;
+  ieee854_long_double_shape_type u;
+
+  u.value = x;
+  hxi = u.parts64.msw;
+  lxi = u.parts64.lsw;
+
+  hxi ^= UINT64_C (0x0000800000000000);
+  hxi |= (lxi | -lxi) >> 63;
+  return (hxi & UINT64_C (0x7fffffffffffffff)) > UINT64_C (0x7fff800000000000);
+}
+
+#else
+
+static inline int
+__issignalingl (long double x)
+{
+  return 0;
+}
+
+#endif
+
+
+#if __FLT128_IS_IEC_60559__
+
+/* We have a _Float128 type.  */
+
+typedef union
+{
+  __float128 value;
+  struct
+  {
+#if __FLOAT_WORD_ORDER == __BIG_ENDIAN
+    uint64_t msw;
+    uint64_t lsw;
+#elif __FLOAT_WORD_ORDER == __LITTLE_ENDIAN
+    uint64_t lsw;
+    uint64_t msw;
+#endif
+  } parts64;
+} ieee854_float128_shape_type;
+
+static inline int
+__issignalingf128 (__float128 x)
+{
+  uint64_t hxi, lxi;
+  ieee854_float128_shape_type u;
+
+  u.value = x;
+  hxi = u.parts64.msw;
+  lxi = u.parts64.lsw;
+
+  hxi ^= UINT64_C (0x0000800000000000);
+  hxi |= (lxi | -lxi) >> 63;
+  return (hxi & UINT64_C (0x7fffffffffffffff)) > UINT64_C (0x7fff800000000000);
+}
+
+#endif
+
+
+/* Define the type-generic macro based on the functions above.  */
+
+#if __FLT128_IS_IEC_60559__
+# define issignaling(X) \
+  _Generic ((X), \
+	    __float128: __issignalingf128, \
+	    float: __issignalingf, \
+	    double: __issignaling, \
+	    long double: __issignalingl)(X)
+#else
+# define issignaling(X) \
+  _Generic ((X), \
+	    float: __issignalingf, \
+	    double: __issignaling, \
+	    long double: __issignalingl)(X)
+#endif
+