RFC [MIPS, RS6000] Mangling of IBM long double template literals

[Alan Modra]

As noted in the comment below, IBM long double is really an array of
two doubles.  In little-endian mode this means the words of each
double should be reversed in write_real_cst, but the large magnitude
double remains the first element of the array.

This patch specially treats IBM long double so that mangling for
template literal args of a given long double value is the same for
both little and big endian.  Bootstrapped etc. powerpc64-linux.

This is of course an ABI change for any existing little-endian users
of IBM long double literals in templates.  On powerpc, I think we can
safely say there are no such users.  However it does look like MIPS
also uses a variant of IBM long double, and I'm less certain there.

        * mangle.c (write_real_cst): Specially treat IBM long double.

Index: gcc/cp/mangle.c
===================================================================
--- gcc/cp/mangle.c     (revision 199975)
+++ gcc/cp/mangle.c     (working copy)
@@ -1591,28 +1591,35 @@ write_real_cst (const tree value)
 {
   if (abi_version_at_least (2))
     {
+      const struct real_format *fmt;
       long target_real[4];  /* largest supported float */
       char buffer[9];       /* eight hex digits in a 32-bit number */
-      int i, limit, dir;
+      int i, limit, dir, twid;
 
       tree type = TREE_TYPE (value);
       int words = GET_MODE_BITSIZE (TYPE_MODE (type)) / 32;
 
-      real_to_target (target_real, &TREE_REAL_CST (value),
-                     TYPE_MODE (type));
+      fmt = REAL_MODE_FORMAT (TYPE_MODE (type));
+      real_to_target_fmt (target_real, &TREE_REAL_CST (value), fmt);
 
       /* The value in target_real is in the target word order,
         so we must write it out backward if that happens to be
         little-endian.  write_number cannot be used, it will
         produce uppercase.  */
       if (FLOAT_WORDS_BIG_ENDIAN)
-       i = 0, limit = words, dir = 1;
+       i = 0, limit = words, dir = 1, twid = 0;
+      else if (fmt->pnan < fmt->p)
+       /* This is an IBM extended double format made up of two IEEE
+          doubles.  When little-endian, the doubles are in
+          little-endian word order, but the array order stays the
+          same.  */
+       i = 0, limit = words, dir = 1, twid = 1;
       else
-       i = words - 1, limit = -1, dir = -1;
+       i = words - 1, limit = -1, dir = -1, twid = 0;
 
       for (; i != limit; i += dir)
        {
-         sprintf (buffer, "%08lx", (unsigned long) target_real[i]);
+         sprintf (buffer, "%08lx", (unsigned long) target_real[i ^ twid]);
          write_chars (buffer, 8);
        }
     }

-- 
Alan Modra
Australia Development Lab, IBM