diff --git a/gcc/combine.cc b/gcc/combine.cc
index 0106092e456..5aa0ec5c45a 100644
--- a/gcc/combine.cc
+++ b/gcc/combine.cc
@@ -5631,6 +5631,28 @@ maybe_swap_commutative_operands (rtx x)
       SUBST (XEXP (x, 0), XEXP (x, 1));
       SUBST (XEXP (x, 1), temp);
     }
+
+  unsigned n_elts = 0;
+  if (GET_CODE (x) == VEC_MERGE
+      && CONST_INT_P (XEXP (x, 2))
+      && GET_MODE_NUNITS (GET_MODE (x)).is_constant (&n_elts)
+      && (swap_commutative_operands_p (XEXP (x, 0), XEXP (x, 1))
+	  /* Two operands have same precedence, then
+	     first bit of mask select first operand.  */
+	  || (!swap_commutative_operands_p (XEXP (x, 1), XEXP (x, 0))
+	      && !(UINTVAL (XEXP (x, 2)) & 1))))
+    {
+      rtx temp = XEXP (x, 0);
+      unsigned HOST_WIDE_INT sel = UINTVAL (XEXP (x, 2));
+      unsigned HOST_WIDE_INT mask = HOST_WIDE_INT_1U;
+      if (n_elts == HOST_BITS_PER_WIDE_INT)
+	mask = -1;
+      else
+	mask = (HOST_WIDE_INT_1U << n_elts) - 1;
+      SUBST (XEXP (x, 0), XEXP (x, 1));
+      SUBST (XEXP (x, 1), temp);
+      SUBST (XEXP (x, 2), GEN_INT (~sel & mask));
+    }
 }
 
 /* Simplify X, a piece of RTL.  We just operate on the expression at the
diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
index 37e537733c9..8ebce31ba78 100644
--- a/gcc/doc/md.texi
+++ b/gcc/doc/md.texi
@@ -8224,6 +8224,13 @@ second operand.  If a machine only supports a constant as the second
 operand, only patterns that match a constant in the second operand need
 be supplied.
 
+@cindex @code{vec_merge}, canonicalization of
+@item
+For the @code{vec_merge} with constant mask(the third operand), the first
+and the second operand can be exchanged by inverting the mask. In such cases,
+a constant is always made the second operand, otherwise the least significant
+bit of the mask is always set(select the first operand first).
+
 @item
 For associative operators, a sequence of operators will always chain
 to the left; for instance, only the left operand of an integer @code{plus}