Fix PR ipa/60965 (placement new wrt ipa-devirt)

[Jan Hubicka]

Hi,
this patch fixes unfortunate thinko in get_class_context that invalidates
transitions from non-POD type to a polymorphic type that may happen by virtue
of placement new and apparently breaks openJDK and Qt. I really tried to keep
placement new in mind when implementing ipa-devirt, so hope there are no
similar negaitve surprises.

The patch disables about 200 out of 30000 devirtualizations happening in
Firefox.

I have commited it to 4.9 and will commit to mainline once testing finishes.

Honza

        PR ipa/60965
        * g++.dg/ipa/devirt-31.C: New testcase.
        * g++.dg/ipa/devirt-11.C: Adjust testcase.
        * ipa-devirt.c (get_class_context): Allow POD to change to non-POD.


Index: testsuite/g++.dg/ipa/devirt-31.C
===================================================================
--- testsuite/g++.dg/ipa/devirt-31.C    (revision 0)
+++ testsuite/g++.dg/ipa/devirt-31.C    (revision 0)
@@ -0,0 +1,23 @@
+/* { dg-options "-O2 -std=c++11 -fdump-ipa-inline"  } */
+#include <new>
+
+class EmbeddedObject {
+public:
+  virtual int val() { return 2; }
+};
+
+class Container {
+  alignas(EmbeddedObject) char buffer[sizeof(EmbeddedObject)];
+public:
+  EmbeddedObject *obj() { return (EmbeddedObject*)buffer; }
+  Container() { new (buffer) EmbeddedObject(); }
+};
+
+Container o;
+
+int main()
+{
+  __builtin_printf("%d\n", o.obj()->val());
+}
+/* { dg-final { scan-ipa-dump-not "__builtin_unreachable"  "inline"  } } */
+/* { dg-final { cleanup-ipa-dump "inline" } } */
Index: testsuite/g++.dg/ipa/devirt-11.C
===================================================================
--- testsuite/g++.dg/ipa/devirt-11.C    (revision 210049)
+++ testsuite/g++.dg/ipa/devirt-11.C    (working copy)
@@ -45,5 +45,5 @@ bar ()
 /* While inlining function called once we should devirtualize a new call to fn2
    and two to fn3. While doing so the new symbol for fn2 needs to be
    introduced.  */
-/* { dg-final { scan-ipa-dump-times "Discovered a virtual call to a known 
target" 3 "inline"  } } */
+/* { dg-final { scan-ipa-dump-times "Discovered a virtual call to a known 
target" 1 "inline"  } } */
 /* { dg-final { cleanup-ipa-dump "inline" } } */
Index: ipa-devirt.c
===================================================================
--- ipa-devirt.c        (revision 210049)
+++ ipa-devirt.c        (working copy)
@@ -987,6 +987,17 @@ give_up:
   context->outer_type = expected_type;
   context->offset = 0;
   context->maybe_derived_type = true;
+  context->maybe_in_construction = true;
+  /* POD can be changed to an instance of a polymorphic type by
+     placement new.  Here we play safe and assume that any
+     non-polymorphic type is POD.  */
+  if ((TREE_CODE (type) != RECORD_TYPE
+       || !TYPE_BINFO (type)
+       || !polymorphic_type_binfo_p (TYPE_BINFO (type)))
+      && (TREE_CODE (TYPE_SIZE (type)) != INTEGER_CST
+         || (offset + tree_to_uhwi (TYPE_SIZE (expected_type)) <=
+             tree_to_uhwi (TYPE_SIZE (type)))))
+    return true;
   return false;
 }