[PATCH] gfortran -- Map REAL128 to REAL kind type with widest precision

[Steve Kargl]

All,

The attach patch maps REAL128 from iso_fortran_env to 
the REAL kind type with widest precision.  Prior to
this patch, REAL128 is mapped to the first REAL kind
type with a matching storage size of 128 bits.  On
x86_64-*-freebsd both REAL(10) and REAL(16) occupy 
16 bytes, but REAL(10) is mapped to REAL128.  This 
may surprise users that assume REAL128 is mapped to
an IEEE754-2008 128-bit floating point entity with
113 bits of precision.  OK to commit?

2017-02-07  Steven G. Kargl  <ka...@gcc.gnu.org>

        * trans-types.c (gfc_get_int_kind_from_width_isofortranen):  Choose
        REAL type with the widest precision if two (or more) have the same
        storage size.

-- 
Steve
20161221 https://www.youtube.com/watch?v=IbCHE-hONow

Index: gcc/fortran/trans-types.c
===================================================================
--- gcc/fortran/trans-types.c	(revision 245068)
+++ gcc/fortran/trans-types.c	(working copy)
@@ -234,27 +234,42 @@ gfc_get_int_kind_from_width_isofortranen
   return -1;
 }
 
-/* Get the kind number corresponding to a real of given storage size,
-   following the required return values for ISO_FORTRAN_ENV REAL* constants:
-   -2 is returned if we support a kind of larger size, -1 otherwise.  */
+
+/* Get the kind number corresponding to a real of a given storage size.
+   If two real's have the same storage size, then choose the real with
+   the largest precision.  If a kind type is unavailable and a real
+   exists with wider storage, then return -2; otherwise, return -1.  */
+
 int
 gfc_get_real_kind_from_width_isofortranenv (int size)
 {
-  int i;
+  int digits, i, kind;
 
   size /= 8;
 
+  kind = -1;
+  digits = 0;
+
   /* Look for a kind with matching storage size.  */
   for (i = 0; gfc_real_kinds[i].kind != 0; i++)
     if (int_size_in_bytes (gfc_get_real_type (gfc_real_kinds[i].kind)) == size)
-      return gfc_real_kinds[i].kind;
+      {
+	if (gfc_real_kinds[i].digits > digits)
+	  {
+	    digits = gfc_real_kinds[i].digits;
+	    kind = gfc_real_kinds[i].kind;
+	  }
+      }
+
+  if (kind != -1)
+    return kind;
 
   /* Look for a kind with larger storage size.  */
   for (i = 0; gfc_real_kinds[i].kind != 0; i++)
     if (int_size_in_bytes (gfc_get_real_type (gfc_real_kinds[i].kind)) > size)
-      return -2;
+      kind = -2;
 
-  return -1;
+  return kind;
 }