[PATCH] Don't optimize away lhs from calls with addressable zero sized return type (PR c++/82159)

[Jakub Jelinek]

Hi!

The expansion relies on lhs being kept for calls that return addressable
types.  On the following testcase (which is a GNU extension, pedantically
we error out on zero sized arrays) we return TREE_ADDRESSABLE
zero_sized_type and optimize away the lhs which we need later on.

Fixed thusly, bootstrapped/regtested on x86_64-linux and i686-linux,
during the bootstrap+regtests it only made a difference on the newly added
testcase.  Ok for trunk?

2017-09-27  Jakub Jelinek  <ja...@redhat.com>

        PR c++/82159
        * gimplify.c (gimplify_modify_expr): Don't optimize away zero sized
        lhs from calls if the lhs has addressable type.

        * g++.dg/opt/pr82159.C: New test.

--- gcc/gimplify.c.jj   2017-09-01 09:25:34.000000000 +0200
+++ gcc/gimplify.c      2017-09-26 13:03:11.614726601 +0200
@@ -5479,7 +5479,12 @@ gimplify_modify_expr (tree *expr_p, gimp
      side as statements and throw away the assignment.  Do this after
      gimplify_modify_expr_rhs so we handle TARGET_EXPRs of addressable
      types properly.  */
-  if (zero_sized_type (TREE_TYPE (*from_p)) && !want_value)
+  if (zero_sized_type (TREE_TYPE (*from_p))
+      && !want_value
+      /* Don't do this for calls that return addressable types, expand_call
+        relies on those having a lhs.  */
+      && !(TREE_ADDRESSABLE (TREE_TYPE (*from_p))
+          && TREE_CODE (*from_p) == CALL_EXPR))
     {
       gimplify_stmt (from_p, pre_p);
       gimplify_stmt (to_p, pre_p);
--- gcc/testsuite/g++.dg/opt/pr82159.C.jj       2017-09-26 13:04:08.711027279 
+0200
+++ gcc/testsuite/g++.dg/opt/pr82159.C  2017-09-26 14:20:01.519361945 +0200
@@ -0,0 +1,18 @@
+// PR c++/82159
+// { dg-do compile }
+// { dg-options "" }
+
+template<int N>
+struct S
+{
+  ~S () {}
+  template<int M> S<M> foo () { return S<M> (); }
+  unsigned char data[N];
+};
+
+int
+main ()
+{
+  S<16> d;
+  S<0> t = d.foo<0> ();
+}

        Jakub