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@@ -2327,6 +2327,12 @@ constraints is through machine-dependent
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