c++: Fix up handling of vector CONSTRUCTORs with vectors in it in constexpr.cc [PR104226]

The middle-end uses sometimes VECTOR_TYPE CONSTRUCTORs that contain
some other VECTOR_TYPE elements in it (should be with compatible element
size and smaller number of elements, e.g. a V8SImode vector can be
constructed as { V4SImode_var_1, V4SImode_var_2 }), and expansion of
__builtin_shufflevector emits these early, so constexpr.cc can see those
too.
constexpr.cc already has special cases for NULL index which is typical
for VECTOR_TYPE CONSTRUCTORs, and for VECTOR_TYPE CONSTRUCTORs that
contain just scalar elts that works just fine - init_subob_ctx just
returns on non-aggregate elts and get_or_insert_ctor_field has
  if (TREE_CODE (type) == VECTOR_TYPE && index == NULL_TREE)
    {
      CONSTRUCTOR_APPEND_ELT (CONSTRUCTOR_ELTS (ctor), index, NULL_TREE);
      return &CONSTRUCTOR_ELTS (ctor)->last();
    }
handling for it.  But for the vector in vector case init_subob_ctx would
try to create a sub-CONSTRUCTOR and even didn't handle the NULL index
case well, so instead of creating the sub-CONSTRUCTOR after the elts already
in it overwrote the first one.  So
(V8SImode) { { 0, 0, 0, 0 }, { 0, 0, 0, 0 } }
became
(V8SImode) { 0, 0, 0, 0 }
The following patch fixes it by not forcing a sub-CONSTRUCTOR for this
vector in vector case.

2022-01-26  Jakub Jelinek  <jakub@redhat.com>

	PR c++/104226
	* constexpr.cc (init_subob_ctx): For vector ctors containing
	vector elements, ensure appending to the same ctor instead of
	creating another one.

	* g++.dg/cpp0x/constexpr-104226.C: New test.

2 files changed, 22 insertions, 0 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index dcc1674..7f44253 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -4658,6 +4658,13 @@ init_subob_ctx (const constexpr_ctx *ctx, constexpr_ctx &new_ctx,
   if (!AGGREGATE_TYPE_P (type) && !VECTOR_TYPE_P (type))
     /* A non-aggregate member doesn't get its own CONSTRUCTOR.  */
     return;
+  if (VECTOR_TYPE_P (type)
+      && VECTOR_TYPE_P (TREE_TYPE (ctx->ctor))
+      && index == NULL_TREE)
+    /* A vector inside of a vector CONSTRUCTOR, e.g. when a larger
+       vector is constructed from smaller vectors, doesn't get its own
+       CONSTRUCTOR either.  */
+    return;
 
   /* The sub-aggregate initializer might contain a placeholder;
      update object to refer to the subobject and ctor to refer to
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-104226.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-104226.C
new file mode 100644
index 0000000..a44f417
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-104226.C
@@ -0,0 +1,15 @@
+// PR c++/104226
+// { dg-do compile }
+// { dg-options "-Wno-psabi" }
+
+typedef unsigned short __attribute__((__vector_size__(16))) U;
+typedef unsigned int __attribute__((__vector_size__(16))) V;
+typedef unsigned int __attribute__((__vector_size__(32))) W;
+
+U
+foo (void)
+{
+  return __builtin_convertvector (__builtin_shufflevector ((V){}, (W){},
+							   0, 0, 1, 0,
+							   5, 5, 0, 2), U);
+}