tm.texi.in (Register Classes): Document rule for the narrowest register classes.

	* doc/tm.texi.in (Register Classes): Document rule for the narrowest
	register classes.
	* doc/tm.texi: Regenerate.

From-SVN: r174866

gcc/ChangeLog

+2011-06-10  Hans-Peter Nilsson  <hp@axis.com>
+
+	* doc/tm.texi.in (Register Classes): Document rule for the narrowest
+	register classes.
+	* doc/tm.texi: Regenerate.
+
 2011-06-09  Kaz Kojima  <kkojima@gcc.gnu.org>
 
 	PR target/49307

gcc/doc/tm.texi

@@ -2337,6 +2337,12 @@ constraints is through machine-dependent operand constraint letters.
 You can define such letters to correspond to various classes, then use
 them in operand constraints.
 
+You must define the narrowest register classes for allocatable
+registers, so that each class either has no subclasses, or that for
+some mode, the move cost between registers within the class is
+cheaper than moving a register in the class to or from memory
+(@pxref{Costs}).
+
 You should define a class for the union of two classes whenever some
 instruction allows both classes.  For example, if an instruction allows
 either a floating point (coprocessor) register or a general register for a

gcc/doc/tm.texi.in

@@ -2327,6 +2327,12 @@ constraints is through machine-dependent operand constraint letters.
 You can define such letters to correspond to various classes, then use
 them in operand constraints.
 
+You must define the narrowest register classes for allocatable
+registers, so that each class either has no subclasses, or that for
+some mode, the move cost between registers within the class is
+cheaper than moving a register in the class to or from memory
+(@pxref{Costs}).
+
 You should define a class for the union of two classes whenever some
 instruction allows both classes.  For example, if an instruction allows
 either a floating point (coprocessor) register or a general register for a