Index: gfortran.texi
===================================================================
--- gfortran.texi	(revision 172773)
+++ gfortran.texi	(working copy)
@@ -1237,6 +1237,7 @@ without warning.
 * Missing period in FORMAT specifications::
 * I/O item lists::
 * BOZ literal constants::
+* @code{Q} exponent-letter::
 * Real array indices::
 * Unary operators::
 * Implicitly convert LOGICAL and INTEGER values::
@@ -1427,6 +1428,23 @@ To support legacy codes, GNU Fortran all
 of the @code{READ} statement, and the output item lists of the
 @code{WRITE} and @code{PRINT} statements, to start with a comma.
 
+@node @code{Q} exponent-letter
+@subsection @code{Q} exponent-letter
+@cindex @code{Q} exponent-letter
+
+GNU Fortran accepts real literal constants with an exponent-letter
+of @code{Q}, for example, @code{1.23Q45}.  Prior to version 4.6.1,
+GNU Fortran silently accepted @code{Q} as an alias for the single
+precision exponent-letter @code{E}.  With the introduction of software
+support for @code{REAL(16)} (i.e., quadruple precision) on i386 and
+x86_64 targets, the interpretation of @code{Q} has been updated to
+mean a @code{REAL(16)} real-literal-constant.  This aligns GNU Fortran
+with many commercially available compilers.  If the target does not
+support @code{REAL(16)} but has a @code{REAL(10)} type, then the
+real-literal-constant will be interpreted as a @code{REAL(10)} entity.
+In the absence of @code{REAL(16)} and @code{REAL(10)}, an error will
+occur.
+
 @node BOZ literal constants
 @subsection BOZ literal constants
 @cindex BOZ literal constants
Index: primary.c
===================================================================
--- primary.c	(revision 172773)
+++ primary.c	(working copy)
@@ -541,6 +541,17 @@ match_real_constant (gfc_expr **result, 
     goto done;
   exp_char = c;
 
+
+  if (c == 'q')
+    {
+      if (gfc_notify_std (GFC_STD_GNU, "Extension: exponent-letter 'q' in "
+			 "real-literal-constant at %C") == FAILURE)
+	return MATCH_ERROR;
+      else
+	gfc_warning("Extension: exponent-letter 'q' in real-literal-constant "
+		    "at %C");
+    }
+
   /* Scan exponent.  */
   c = gfc_next_ascii_char ();
   count++;
@@ -616,6 +627,29 @@ done:
       kind = gfc_default_double_kind;
       break;
 
+    case 'q':
+      if (kind != -2)
+	{
+	  gfc_error ("Real number at %C has a 'q' exponent and an explicit "
+		     "kind");
+	  goto cleanup;
+	}
+
+      /* The maximum possible real kind type parameter is 16.  First, try
+	 that for the kind, then fallback to trying kind=10 (Intel 80 bit)
+	 extended precision.  If neither value works, just given up.  */
+      kind = 16;
+      if (gfc_validate_kind (BT_REAL, kind, true) < 0)
+	{
+	  kind = 10;
+          if (gfc_validate_kind (BT_REAL, kind, true) < 0)
+	    {
+	      gfc_error ("Invalid real kind %d at %C", kind);
+	      goto cleanup;
+	    }
+	}
+      break;
+
     default:
       if (kind == -2)
 	kind = gfc_default_real_kind;