forwprop: Fix ICE when building an identity constructor [PR94784]

In the testcase for PR94784, we have two vectors with the same ABI identity
but with different TYPE_MODEs. It would be better to flip the assert around
so that it checks that the two vectors have equal TYPE_VECTOR_SUBPARTS and
that converting the corresponding element types is a useless_type_conversion_p.

2020-04-27  Felix Yang  <felix.yang@huawei.com>

gcc/
	PR tree-optimization/94784
	* tree-ssa-forwprop.c (simplify_vector_constructor): Flip the
	assert around so that it checks that the two vectors have equal
	TYPE_VECTOR_SUBPARTS and that converting the corresponding element
	types is a useless_type_conversion_p.

gcc/testsuite/
	PR tree-optimization/94784
	* gcc.dg/pr94784.c: New test.

gcc/ChangeLog

+2020-04-27  Felix Yang  <felix.yang@huawei.com>
+
+	PR tree-optimization/94784
+	* tree-ssa-forwprop.c (simplify_vector_constructor): Flip the
+	assert around so that it checks that the two vectors have equal
+	TYPE_VECTOR_SUBPARTS and that converting the corresponding element
+	types is a useless_type_conversion_p.
+
 2020-04-27  Szabolcs Nagy  <szabolcs.nagy@arm.com>
 
 	PR target/94515

gcc/testsuite/ChangeLog

+2020-04-27  Felix Yang  <felix.yang@huawei.com>
+
+	PR tree-optimization/94784
+	* gcc.dg/pr94784.c: New test.
+
 2020-04-27  Szabolcs Nagy  <szabolcs.nagy@arm.com>
 
 	PR target/94515

gcc/testsuite/gcc.dg/pr94784.c

+/* { dg-do compile { target aarch64*-*-* } } */
+/* { dg-options "-O2 -ftree-slp-vectorize -march=armv8.2-a+sve -msve-vector-bits=256" } */
+
+typedef short __attribute__((vector_size (8))) v4hi;
+
+typedef union U4HI { v4hi v; short a[4]; } u4hi;
+
+short a[4];
+
+void pass_v4hi (v4hi v) {
+    int j;
+    u4hi u;
+    u.v = v;
+    for (j = 0; j < 4; j++)
+      a[j] = u.a[j];
+};

gcc/tree-ssa-forwprop.c

@@ -2479,7 +2479,10 @@ simplify_vector_constructor (gimple_stmt_iterator *gsi)
 	  tree src_type = TREE_TYPE (orig[0]);
 	  if (!useless_type_conversion_p (type, src_type))
 	    {
-	      gcc_assert (!targetm.compatible_vector_types_p (type, src_type));
+	      gcc_assert (known_eq (TYPE_VECTOR_SUBPARTS (type),
+				    TYPE_VECTOR_SUBPARTS (src_type))
+			  && useless_type_conversion_p (TREE_TYPE (type),
+							TREE_TYPE (src_type)));
 	      tree rhs = build1 (VIEW_CONVERT_EXPR, type, orig[0]);
 	      orig[0] = make_ssa_name (type);
 	      gassign *assign = gimple_build_assign (orig[0], rhs);
@@ -2611,7 +2614,10 @@ simplify_vector_constructor (gimple_stmt_iterator *gsi)
 	res = gimple_build (&stmts, conv_code, type, res);
       else if (!useless_type_conversion_p (type, TREE_TYPE (res)))
 	{
-	  gcc_assert (!targetm.compatible_vector_types_p (type, perm_type));
+	  gcc_assert (known_eq (TYPE_VECTOR_SUBPARTS (type),
+				TYPE_VECTOR_SUBPARTS (perm_type))
+		      && useless_type_conversion_p (TREE_TYPE (type),
+						    TREE_TYPE (perm_type)));
 	  res = gimple_build (&stmts, VIEW_CONVERT_EXPR, type, res);
 	}
       /* Blend in the actual constant.  */