Here's a Holiday present for Fortranners.

2018-12-25  Steven G. Kargl  <ka...@gcc.gnu.org>

        * cpp.c (gfc_cpp_init):  Add pre-defined macros for INTEGER(1)
        INTEGER(2), INTEGER(8) and INTEGER(16) if supported.  Add pre-defined
        macros for REAL(10) and REAL(16) if available.
        * gfortran.texi: Document new macros.

As a Fortran compiler is required to have default integer kind, 
default real kind, and double precision, I did not include macros
for these types (ie., there are no __GFC_INTEGER_4__, __GFC_REAL_4__,
and __GFC_REAL_8__).

This allows code like

% cat a.F90
program foo
#ifdef __GFC_REAL_16__
   real(16) x
   x = 1
   print *, x
#endif
end program foo
%  gfcx -o z a.F90 && ./z
   1.00000000000000000000000000000000000

-- 
Steve
Index: gcc/fortran/cpp.c
===================================================================
--- gcc/fortran/cpp.c	(revision 267418)
+++ gcc/fortran/cpp.c	(working copy)
@@ -570,6 +570,8 @@ void
 gfc_cpp_init (void)
 {
   int i;
+  gfc_integer_info *int_info;
+  gfc_real_info *real_info;
 
   if (gfc_option.flag_preprocessed)
     return;
@@ -607,6 +609,26 @@ gfc_cpp_init (void)
       else if (opt->code == OPT_MT || opt->code == OPT_MQ)
 	deps_add_target (cpp_get_deps (cpp_in),
 			 opt->arg, opt->code == OPT_MQ);
+    }
+
+  for (int_info = gfc_integer_kinds; int_info->kind != 0; int_info++)
+    {
+      if (int_info->kind == 1)
+	cpp_define (cpp_in, "__GFC_INT_1__=1");
+      if (int_info->kind == 2)
+	cpp_define (cpp_in, "__GFC_INT_2__=1");
+      if (int_info->kind == 8)
+	cpp_define (cpp_in, "__GFC_INT_8__=1");
+      if (int_info->kind == 8)
+	cpp_define (cpp_in, "__GFC_INT_16__=1");
+    }
+
+  for (real_info = gfc_real_kinds; real_info->kind != 0; real_info++)
+    {
+      if (real_info->kind == 10)
+	cpp_define (cpp_in, "__GFC_REAL_10__=1");
+      if (real_info->kind == 16)
+	cpp_define (cpp_in, "__GFC_REAL_16__=1");
     }
 
   if (gfc_cpp_option.working_directory
Index: gcc/fortran/gfortran.texi
===================================================================
--- gcc/fortran/gfortran.texi	(revision 267418)
+++ gcc/fortran/gfortran.texi	(working copy)
@@ -418,9 +418,17 @@ statement, the included file is not preprocessed.  To 
 files, use the equivalent preprocessor statement @code{#include}.
 
 If GNU Fortran invokes the preprocessor, @code{__GFORTRAN__}
-is defined and @code{__GNUC__}, @code{__GNUC_MINOR__} and
+is defined.  The macros @code{__GNUC__}, @code{__GNUC_MINOR__} and
 @code{__GNUC_PATCHLEVEL__} can be used to determine the version of the
 compiler.  See @ref{Top,,Overview,cpp,The C Preprocessor} for details.
+
+GNU Fortran supports a number of @code{INTEGER} and @code{REAL} kind types
+in additional to the kind types required by the Fortran standard.
+The availability of any given kind type is architecture dependent.  The
+following pre-defined preprocessor macros can be used to conditional
+include code for these additional kind types: @code{__GFC_INTEGER_1__},
+@code{__GFC_INTEGER_2__}, @code{__GFC_INTEGER_8__}, @code{__GFC_INTEGER_16__},
+@code{__GFC_REAL_10__}, and @code{__GFC_REAL_16__}.
 
 While CPP is the de-facto standard for preprocessing Fortran code,
 Part 3 of the Fortran 95 standard (ISO/IEC 1539-3:1998) defines

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