md.texi: Mention that a target shouldn't implement vec_widen_(s|u)mul_even/odd pair if...

2014-01-29  Bingfeng Mei  <bmei@broadcom.com>

	* doc/md.texi: Mention that a target shouldn't implement 
	vec_widen_(s|u)mul_even/odd pair if it is less efficient
	than hi/lo pair.

From-SVN: r207228

gcc/ChangeLog

+2014-01-29  Bingfeng Mei  <bmei@broadcom.com>
+
+	* doc/md.texi: Mention that a target shouldn't implement 
+	vec_widen_(s|u)mul_even/odd pair if it is less efficient
+	than hi/lo pair.
+
 2014-01-29  Jakub Jelinek  <jakub@redhat.com>
 
 	PR tree-optimization/59594

gcc/doc/md.texi

@@ -4918,7 +4918,8 @@ the output vector (operand 0).
 Signed/Unsigned widening multiplication.  The two inputs (operands 1 and 2)
 are vectors with N signed/unsigned elements of size S@.  Multiply the high/low
 or even/odd elements of the two vectors, and put the N/2 products of size 2*S
-in the output vector (operand 0).
+in the output vector (operand 0). A target shouldn't implement even/odd pattern
+pair if it is less efficient than lo/hi one.
 
 @cindex @code{vec_widen_ushiftl_hi_@var{m}} instruction pattern
 @cindex @code{vec_widen_ushiftl_lo_@var{m}} instruction pattern