[patch, Fortran] Make REAL(KIND=16) detection more robust

[FX <fxcoudert@gmail.com>]

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Hi,

Right now, the logic in libgfortran for the detection of REAL(KIND=16) is in kinds-override.h:

/* What are the C types corresponding to the real(kind=10) and
   real(kind=16) types? We currently rely on the following assumptions:
     -- if real(kind=10) exists, i.e. if HAVE_GFC_REAL_10 is defined,
        then it is necessarily the "long double" type
     -- if real(kind=16) exists, then:
         * if HAVE_GFC_REAL_10, real(kind=16) is "__float128"
        * otherwise, real(kind=16) is "long double"
   To allow to change this in the future, we create the
   GFC_REAL_16_IS_FLOAT128 macro that is used throughout libgfortran.  */


Well, this may not be true of all platforms, and it’s possible to have other combinations. On the aarch64-apple-darwin port, I’m currently playing with enabling a binary128 floating-point mode, and that target has double == long double… so the assumptions above are not true.

Funnily, we already have more fine-grained logic in the mk-kinds-h.sh script, where we actually check the Fortran kind corresponding to C’s long double. We just have to use it, and emit the GFC_REAL_16_IS_FLOAT128 / GFC_REAL_16_IS_LONG_DOUBLE macros there.


Bootstrapped and regtested on x86_64-linux, checked that no symbols were introduced or removed.
(and tested on a port to aarch64-apple-darwin).

OK to commit?

FX


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commit 6b2a78f168ecf9c6f47e2cbebac121470e81b558
Author: Francois-Xavier Coudert <fxcoudert@gmail.com>
Date:   2021-12-07 12:25:12 +0100

    Allow __float128 on tergets where long double is not REAL(KIND=10)
    
    libgfortran/ChangeLog:
    
            * kinds-override.h: Move GFC_REAL_16_IS_* macros...
            * mk-kinds-h.sh: ... here.

diff --git a/libgfortran/kinds-override.h b/libgfortran/kinds-override.h
index c9e874a3f38..5f7840b0c89 100644
--- a/libgfortran/kinds-override.h
+++ b/libgfortran/kinds-override.h
@@ -23,24 +23,9 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 <http://www.gnu.org/licenses/>.  */
 
 
-/* What are the C types corresponding to the real(kind=10) and
-   real(kind=16) types? We currently rely on the following assumptions:
-     -- if real(kind=10) exists, i.e. if HAVE_GFC_REAL_10 is defined,
-        then it is necessarily the "long double" type
-     -- if real(kind=16) exists, then:
-         * if HAVE_GFC_REAL_10, real(kind=16) is "__float128"
-	 * otherwise, real(kind=16) is "long double"
-   To allow to change this in the future, we create the
-   GFC_REAL_16_IS_FLOAT128 macro that is used throughout libgfortran.  */
-
-#if defined(HAVE_GFC_REAL_16)
-# if defined(HAVE_GFC_REAL_10)
-#  define GFC_REAL_16_IS_FLOAT128
-#  if !defined(HAVE_FLOAT128)
-#   error "Where has __float128 gone?"
-#  endif
-# else
-#  define GFC_REAL_16_IS_LONG_DOUBLE
-# endif
+/* Ensure that TFmode is available under.  */
+
+#if defined(GFC_REAL_16_IS_FLOAT128) && !defined(HAVE_FLOAT128)
+# error "Where has __float128 gone?"
 #endif
 
diff --git a/libgfortran/mk-kinds-h.sh b/libgfortran/mk-kinds-h.sh
index 249619061c6..572878ce891 100755
--- a/libgfortran/mk-kinds-h.sh
+++ b/libgfortran/mk-kinds-h.sh
@@ -64,15 +64,19 @@ for k in $possible_real_kinds; do
     case $k in
       4) ctype="float" ; cplxtype="complex float" ; suffix="f" ;;
       8) ctype="double" ; cplxtype="complex double" ; suffix="" ;;
+      # If we have a REAL(KIND=10), it is always long double
       10) ctype="long double" ; cplxtype="complex long double" ; suffix="l" ;;
-      16) if [ $long_double_kind -eq 10 ]; then
+      # If we have a REAL(KIND=16), it is either long double or __float128
+      16) if [ $long_double_kind -ne 16 ]; then
 	    ctype="__float128"
 	    cplxtype="_Complex float __attribute__((mode(TC)))"
 	    suffix="q"
+	    echo "#define GFC_REAL_16_IS_FLOAT128"
 	  else
 	    ctype="long double"
 	    cplxtype="complex long double"
 	    suffix="l"
+	    echo "#define GFC_REAL_16_IS_LONG_DOUBLE"
 	  fi ;;
       *) echo "$0: Unknown type" >&2 ; exit 1 ;;
     esac