expr: Unbreak build of mesa [PR96194]

> > The store to the whole of each volatile object was picked apart
> > like there had been an individual assignment to each of the
> > fields.  Reads were added as part of that; see PR for details.
> > The reads from volatile memory were a clear bug; individual
> > stores questionable.  A separate patch clarifies the docs.

This breaks building of mesa on both the trunk and 10 branch.

The problem is that the middle-end may never create temporaries of non-POD
(TREE_ADDRESSABLE) types, those can be only created when the language says
so and thus only the FE is allowed to create those.

This patch just reverts the behavior to what we used to do before for the
stores to volatile non-PODs.  Perhaps we want to do something else, but
definitely we can't create temporaries of the non-POD type.  It is up to
discussions on what should happen in those cases.

2020-07-14  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/96194
	* expr.c (expand_constructor): Don't create temporary for store to
	volatile MEM if exp has an addressable type.

	* g++.dg/opt/pr96194.C: New test.

(cherry picked from commit b1d389d60d1929c7528ef984925ea010e3bf2c1a)

gcc/expr.c

@@ -8271,7 +8271,9 @@ expand_constructor (tree exp, rtx target, enum expand_modifier modifier,
       || GET_CODE (target) == PARALLEL || modifier == EXPAND_STACK_PARM
       /* Also make a temporary if the store is to volatile memory, to
 	 avoid individual accesses to aggregate members.  */
-      || (GET_CODE (target) == MEM && MEM_VOLATILE_P (target)))
+      || (GET_CODE (target) == MEM
+	  && MEM_VOLATILE_P (target)
+	  && !TREE_ADDRESSABLE (TREE_TYPE (exp))))
     {
       if (avoid_temp_mem)
 	return NULL_RTX;

gcc/testsuite/g++.dg/opt/pr96194.C

+// PR middle-end/96194
+// { dg-do compile }
+// { dg-options "-O2" }
+
+#include <new>
+
+struct A { ~A (); };
+struct B : A { float e[64]; };
+
+B *
+foo ()
+{
+  return new ((void *) 0) B ();
+}
+
+B *
+bar (void *x, bool y)
+{
+  void *p = y ? x : (void *) 0;
+  return new (p) B ();
+}