compiler: pad structs ending with zero-sized field

For a struct with zero-sized last field, the address of the
    field falls out of the object boundary, which confuses the
    garbage collector. Pad an extra byte in this case.
    
    Reviewed-on: https://go-review.googlesource.com/c/157557

From-SVN: r267861

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-960637781ca9546ea2db913e48afd7eccbdadfa9
+0d64279c01a37b2579c0c62ca4f2c3e3f81de07c
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -13082,6 +13082,12 @@ Struct_construction_expression::do_get_backend(Translate_context* context)
 	  ++pv;
 	}
     }
+  if (this->type_->struct_type()->has_padding())
+    {
+      // Feed an extra value if there is a padding field.
+      Btype *fbtype = Type::lookup_integer_type("uint8")->get_backend(gogo);
+      init.push_back(gogo->backend()->zero_expression(fbtype));
+    }
   return gogo->backend()->constructor_expression(btype, init, this->location());
 }
 

gcc/go/gofrontend/types.cc

@@ -24,8 +24,7 @@
 // backend.h.
 
 static void
-get_backend_struct_fields(Gogo* gogo, const Struct_field_list* fields,
-			  bool use_placeholder,
+get_backend_struct_fields(Gogo* gogo, Struct_type* type, bool use_placeholder,
 			  std::vector<Backend::Btyped_identifier>* bfields);
 
 static void
@@ -1162,8 +1161,7 @@ Type::get_backend_placeholder(Gogo* gogo)
       // struct field.
       {
 	std::vector<Backend::Btyped_identifier> bfields;
-	get_backend_struct_fields(gogo, this->struct_type()->fields(),
-				  true, &bfields);
+	get_backend_struct_fields(gogo, this->struct_type(), true, &bfields);
 	bt = gogo->backend()->struct_type(bfields);
       }
       break;
@@ -6140,12 +6138,14 @@ Struct_type::interface_method_table(Interface_type* interface,
 // backend.h.
 
 static void
-get_backend_struct_fields(Gogo* gogo, const Struct_field_list* fields,
-			  bool use_placeholder,
+get_backend_struct_fields(Gogo* gogo, Struct_type* type, bool use_placeholder,
 			  std::vector<Backend::Btyped_identifier>* bfields)
 {
+  const Struct_field_list* fields = type->fields();
   bfields->resize(fields->size());
   size_t i = 0;
+  int64_t lastsize = 0;
+  bool saw_nonzero = false;
   for (Struct_field_list::const_iterator p = fields->begin();
        p != fields->end();
        ++p, ++i)
@@ -6155,8 +6155,24 @@ get_backend_struct_fields(Gogo* gogo, const Struct_field_list* fields,
 			     ? p->type()->get_backend_placeholder(gogo)
 			     : p->type()->get_backend(gogo));
       (*bfields)[i].location = p->location();
+      lastsize = gogo->backend()->type_size((*bfields)[i].btype);
+      if (lastsize != 0)
+        saw_nonzero = true;
     }
   go_assert(i == fields->size());
+  if (saw_nonzero && lastsize == 0)
+    {
+      // For nonzero-sized structs which end in a zero-sized thing, we add
+      // an extra byte of padding to the type. This padding ensures that
+      // taking the address of the zero-sized thing can't manufacture a
+      // pointer to the next object in the heap. See issue 9401.
+      size_t n = fields->size();
+      bfields->resize(n + 1);
+      (*bfields)[n].name = "_";
+      (*bfields)[n].btype = Type::lookup_integer_type("uint8")->get_backend(gogo);
+      (*bfields)[n].location = (*bfields)[n-1].location;
+      type->set_has_padding();
+    }
 }
 
 // Get the backend representation for a struct type.
@@ -6165,7 +6181,7 @@ Btype*
 Struct_type::do_get_backend(Gogo* gogo)
 {
   std::vector<Backend::Btyped_identifier> bfields;
-  get_backend_struct_fields(gogo, this->fields_, false, &bfields);
+  get_backend_struct_fields(gogo, this, false, &bfields);
   return gogo->backend()->struct_type(bfields);
 }
 
@@ -10504,8 +10520,7 @@ Named_type::convert(Gogo* gogo)
     case TYPE_STRUCT:
       {
 	std::vector<Backend::Btyped_identifier> bfields;
-	get_backend_struct_fields(gogo, base->struct_type()->fields(),
-				  true, &bfields);
+	get_backend_struct_fields(gogo, base->struct_type(), true, &bfields);
 	if (!gogo->backend()->set_placeholder_struct_type(bt, bfields))
 	  bt = gogo->backend()->error_type();
       }

gcc/go/gofrontend/types.h

@@ -2432,7 +2432,7 @@ class Struct_type : public Type
   Struct_type(Struct_field_list* fields, Location location)
     : Type(TYPE_STRUCT),
       fields_(fields), location_(location), all_methods_(NULL),
-      is_struct_incomparable_(false)
+      is_struct_incomparable_(false), has_padding_(false)
   { }
 
   // Return the field NAME.  This only looks at local fields, not at
@@ -2552,6 +2552,17 @@ class Struct_type : public Type
   set_is_struct_incomparable()
   { this->is_struct_incomparable_ = true; }
 
+  // Return whether this struct's backend type has padding, due to
+  // trailing zero-sized field.
+  bool
+  has_padding() const
+  { return this->has_padding_; }
+
+  // Record that this struct's backend type has padding.
+  void
+  set_has_padding()
+  { this->has_padding_ = true; }
+
   // Write the hash function for this type.
   void
   write_hash_function(Gogo*, Named_type*, Function_type*, Function_type*);
@@ -2656,6 +2667,9 @@ class Struct_type : public Type
   // True if this is a generated struct that is not considered to be
   // comparable.
   bool is_struct_incomparable_;
+  // True if this struct's backend type has padding, due to trailing
+  // zero-sized field.
+  bool has_padding_;
 };
 
 // The type of an array.