Unshare DR_STEP before gimplifying it

In this testcase we use an unmasked SVE loop with an Advanced SIMD
epilogue (because we don't yet support fully-masked downward loops).
The main loop uses a gather load for the strided access while the
epilogue loop builds the access from scalars instead.  In both cases
we gimplify expressions based on the DR_STEP and insert them in the
loop preheader.

The problem was that the gather load code didn't copy the DR_STEP before
gimplifying it, meaning that the epilogue loop tried to reuse a result
from the (non-dominating) main loop preheader.

It looks at first glance like there could be other instances of this too,
but this patch just deals with the gather/scatter case.

2019-12-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-stmts.c (vect_get_strided_load_store_ops): Copy
	DR_STEP before gimplifying it.

gcc/testsuite/
	* gcc.dg/vect/vect-strided-epilogue-1.c: New test.

From-SVN: r279753

gcc/ChangeLog

 2019-12-29  Richard Sandiford  <richard.sandiford@arm.com>
 
+	* tree-vect-stmts.c (vect_get_strided_load_store_ops): Copy
+	DR_STEP before gimplifying it.
+
+2019-12-29  Richard Sandiford  <richard.sandiford@arm.com>
+
 	* tree-vect-stmts.c (vectorizable_condition): For extract-last
 	reductions, check that the target supports the required comparison
 	operation.

gcc/testsuite/ChangeLog

 2019-12-29  Richard Sandiford  <richard.sandiford@arm.com>
 
+	* gcc.dg/vect/vect-strided-epilogue-1.c: New test.
+
+2019-12-29  Richard Sandiford  <richard.sandiford@arm.com>
+
 	* gcc.dg/vect/vect-cond-12.c: New test.
 
 2019-12-27  Richard Sandiford  <richard.sandiford@arm.com>

gcc/testsuite/gcc.dg/vect/vect-strided-epilogue-1.c

+/* { dg-do compile } */
+
+void
+f (int *x, short *y, int z)
+{
+  for (int i = 0; i < 0x82; ++i)
+    x[-i] += x[z * i];
+}

gcc/tree-vect-stmts.c

@@ -2993,7 +2993,7 @@ vect_get_strided_load_store_ops (stmt_vec_info stmt_info,
   gimple_seq stmts;
 
   tree bump = size_binop (MULT_EXPR,
-			  fold_convert (sizetype, DR_STEP (dr)),
+			  fold_convert (sizetype, unshare_expr (DR_STEP (dr))),
 			  size_int (TYPE_VECTOR_SUBPARTS (vectype)));
   *dataref_bump = force_gimple_operand (bump, &stmts, true, NULL_TREE);
   if (stmts)
@@ -3005,7 +3005,7 @@ vect_get_strided_load_store_ops (stmt_vec_info stmt_info,
   offset_type = TREE_TYPE (gs_info->offset_vectype);
 
   /* Calculate X = DR_STEP / SCALE and convert it to the appropriate type.  */
-  tree step = size_binop (EXACT_DIV_EXPR, DR_STEP (dr),
+  tree step = size_binop (EXACT_DIV_EXPR, unshare_expr (DR_STEP (dr)),
 			  ssize_int (gs_info->scale));
   step = fold_convert (offset_type, step);
   step = force_gimple_operand (step, &stmts, true, NULL_TREE);