md.texi (define_subst): Document how multiple occurrences of the same argument…

md.texi (define_subst): Document how multiple occurrences of the same argument in the replacement...

	* doc/md.texi (define_subst): Document how multiple occurrences of
	the same argument in the replacement pattern are handled.

From-SVN: r262562

gcc/ChangeLog

+2018-07-08  Paul Koning  <ni1d@arrl.net>
+
+	* doc/md.texi (define_subst): Document how multiple occurrences of
+	the same argument in the replacement pattern are handled.
+
 2018-07-11  Paul Koning  <ni1d@arrl.net>
 
 	* doc/extend.texi (Common Variable Attributes): Move "mode" into

gcc/doc/md.texi

@@ -10263,7 +10263,11 @@ the expression from the original pattern, which matched
 @code{match_operand N} from the input pattern.  As a consequence,
 @code{match_dup} cannot be used to point to @code{match_operand}s from
 the output pattern, it should always refer to a @code{match_operand}
-from the input pattern.
+from the input pattern.  If a @code{match_dup N} occurs more than once
+in the output template, its first occurrence is replaced with the
+expression from the original pattern, and the subsequent expressions
+are replaced with @code{match_dup N}, i.e., a reference to the first
+expression.
 
 In the output template one can refer to the expressions from the
 original pattern and create new ones.  For instance, some operands could