gimplify: prevent some C++ temporary elision

In this testcase, we were optimizing away the temporary for f(), but
C++17 and above are clear that there is a temporary, and because its
destructor has visible side-effects we can't optimize it away under the
as-if rule.  So disable this optimization for TREE_ADDRESSABLE type.

I moved the declaration of volatile_p after the call to
gimple_fold_indirect_ref_rhs to minimize indentation changes; I don't see
any way the value of that flag could be affected by the call.

gcc/ChangeLog:

	* gimplify.cc (gimplify_modify_expr_rhs): Don't optimize
	x = *(A*)&<expr> to x = <expr> for a TREE_ADDRESSABLE type.

gcc/testsuite/ChangeLog:

	* g++.dg/init/elide9.C: New test.

1 files changed, 25 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/init/elide9.C b/gcc/testsuite/g++.dg/init/elide9.C
new file mode 100644
index 0000000..810d60a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/init/elide9.C
@@ -0,0 +1,25 @@
+// The static_cast should prevent temporary elision.
+// { dg-do run { target c++11 } }
+
+int d;
+struct A
+{
+  int i;
+  A() { }
+  ~A() { ++d; }
+};
+
+A f() { return A(); }
+
+struct B
+{
+  A a;
+  B(): a(static_cast<A&&>(f())) {}
+};
+
+int main()
+{
+  { B b; }
+  if (d != 2)
+    return -1;
+}