middle-end/99281 - avoid bitfield stores into addressable types

This avoids doing bitfield stores into the return object of calls
when using return-slot optimization and the type is addressable.
Instead we have to pass down the original target RTX to the call
expansion which otherwise tries to create a new temporary.

2021-02-26  Richard Biener  <rguenther@suse.de>

	PR middle-end/99281
	* expr.c (store_field): For calls with return-slot optimization
	and addressable return type expand the store directly.

	* g++.dg/pr99218.C: New testcase.

2 files changed, 38 insertions, 1 deletions

diff --git a/gcc/expr.c b/gcc/expr.c
index 86dc1b6..92035c7 100644
--- a/gcc/expr.c
+++ b/gcc/expr.c
@@ -7214,7 +7214,13 @@ store_field (rtx target, poly_int64 bitsize, poly_int64 bitpos,
 	      || !multiple_p (bitpos, BITS_PER_UNIT)
 	      || !poly_int_tree_p (DECL_SIZE (TREE_OPERAND (exp, 1)),
 				   &decl_bitsize)
-	      || maybe_ne (decl_bitsize, bitsize)))
+	      || maybe_ne (decl_bitsize, bitsize))
+	  /* A call with an addressable return type and return-slot
+	     optimization must not need bitfield operations but we must
+	     pass down the original target.  */
+	  && (TREE_CODE (exp) != CALL_EXPR
+	      || !TREE_ADDRESSABLE (TREE_TYPE (exp))
+	      || !CALL_EXPR_RETURN_SLOT_OPT (exp)))
       /* If we are expanding a MEM_REF of a non-BLKmode non-addressable
          decl we must use bitfield operations.  */
       || (known_size_p (bitsize)
diff --git a/gcc/testsuite/g++.dg/pr99218.C b/gcc/testsuite/g++.dg/pr99218.C
new file mode 100644
index 0000000..477a6a2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/pr99218.C
@@ -0,0 +1,31 @@
+// { dg-do compile }
+// { dg-require-effective-target c++17 }
+
+struct Data
+{
+  Data() {}
+  ~Data() {}
+
+  long long i;
+};
+
+struct X
+{
+  Data a;
+  int b;
+};
+
+template<class T>
+X get(T const&)
+{
+  return X{};
+}
+
+template<class... Ts>
+struct pack_type : Ts...
+{};
+
+int main()
+{
+  pack_type<X>{get(1)};
+}