Remove constant vec_select restriction.

The vec_select operator is documented to require a const_int for the lane
selector operand, but GCN has an instruction that can select the lane at
runtime, so it seems reasonable to remove this restriction.

This patch simply replaces assertions that the operand is constant with early
exits from the optimizers.  I think it's reasonable that vec_select with a
non-constant operand cannot be optimized, yet.

Also included is the necessary documentation tweak.

2018-09-19  Andrew Stubbs  <ams@codesourcery.com>

	gcc/
	* doc/rtl.texi: Adjust vec_select description.
	* simplify-rtx.c (simplify_binary_operation_1): Allow VEC_SELECT to use
	non-constant selectors.

From-SVN: r264423

gcc/ChangeLog

+2018-09-19  Andrew Stubbs  <ams@codesourcery.com>
+
+	* doc/rtl.texi: Adjust vec_select description.
+	* simplify-rtx.c (simplify_binary_operation_1): Allow VEC_SELECT to use
+	non-constant selectors.
+
 2018-09-19  Matthew Malcomson  <matthew.malcomson@arm.com>
 
 	* config/aarch64/aarch64-protos.h

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})

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));
 		}