gcc/
	* doc/tm.texi: Documentation on new macro
	PREFERRED_RENAME_CLASS.

diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index e1e0957..40fc204 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -2581,6 +2581,19 @@ only if neither labeling works.
 This macro also has strict and non-strict variants.
 @end defmac
 
+@defmac PREFERRED_RENAME_CLASS (@var{class})
+A C expression that places additional restrictions on the register class
+to use when it is necessary to rename a register in class
+@var{class} to another class, or perhaps still @var{class},
+if you don't define this macro.
+
+Sometimes returning a more restrictive class makes better code.  For
+example, on ARM, thumb-2 instructions using @code{LOW_REGS} may be smaller than
+instructions using @code{GENERIC_REGS}.  By returning @code{LOW_REGS} from
+@code{PREFERRED_RENAME_CLASS}, code size can be reduced.
+
+@end defmac
+
 @defmac PREFERRED_RELOAD_CLASS (@var{x}, @var{class})
 A C expression that places additional restrictions on the register class
 to use when it is necessary to copy value @var{x} into a register in class