diff --git a/gcc/doc/rtl.texi b/gcc/doc/rtl.texi
index 5b1e695..0695ad2 100644
--- a/gcc/doc/rtl.texi
+++ b/gcc/doc/rtl.texi
@@ -2939,11 +2939,12 @@ a set bit indicates it is taken from @var{vec1}.
 @item (vec_select:@var{m} @var{vec1} @var{selection})
 This describes an operation that selects parts of a vector.  @var{vec1} is
 the source vector, and @var{selection} is a @code{parallel} that contains a
-@code{const_int} for each of the subparts of the result vector, giving the
-number of the source subpart that should be stored into it.
-The result mode @var{m} is either the submode for a single element of
-@var{vec1} (if only one subpart is selected), or another vector mode
-with that element submode (if multiple subparts are selected).
+@code{const_int} (or another expression, if the selection can be made at
+runtime) for each of the subparts of the result vector, giving the number of
+the source subpart that should be stored into it.  The result mode @var{m} is
+either the submode for a single element of @var{vec1} (if only one subpart is
+selected), or another vector mode with that element submode (if multiple
+subparts are selected).
 
 @findex vec_concat
 @item (vec_concat:@var{m} @var{x1} @var{x2})
diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index a9f2586..b4c6883 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -3604,7 +3604,10 @@ simplify_binary_operation_1 (enum rtx_code code, machine_mode mode,
 	  gcc_assert (mode == GET_MODE_INNER (GET_MODE (trueop0)));
 	  gcc_assert (GET_CODE (trueop1) == PARALLEL);
 	  gcc_assert (XVECLEN (trueop1, 0) == 1);
-	  gcc_assert (CONST_INT_P (XVECEXP (trueop1, 0, 0)));
+
+	  /* We can't reason about selections made at runtime.  */
+	  if (!CONST_INT_P (XVECEXP (trueop1, 0, 0)))
+	    return 0;
 
 	  if (vec_duplicate_p (trueop0, &elt0))
 	    return elt0;
@@ -3703,7 +3706,9 @@ simplify_binary_operation_1 (enum rtx_code code, machine_mode mode,
 		{
 		  rtx x = XVECEXP (trueop1, 0, i);
 
-		  gcc_assert (CONST_INT_P (x));
+		  if (!CONST_INT_P (x))
+		    return 0;
+
 		  RTVEC_ELT (v, i) = CONST_VECTOR_ELT (trueop0,
 						       INTVAL (x));
 		}