Go patch committed: Fix passing zero-sized global variable to function

[Ian Lance Taylor]

The GNU linker doesn't support a zero-sized global variable that is
dynamically exported, so gccgo generates those global variables with a
size of 1 byte.  Unfortunately, that means that passing a global
variable to a function that takes an argument of a zero-sized type
will actually pass 1 byte, taking up an argument slot, rather than a
zero-sized argument that should be skipped.  This patch fixes the
problem in the Go frontend -> GCC interface by adding a conversion to
the real type for any such global variables, and undoing the
conversion where necessary.  Bootstrapped and ran Go testsuite on
x86_64-pc-linux-gnu.  Committed to mainline and gccgo branch.

Ian

2015-12-21  Ian Lance Taylor  <i...@google.com>

* go-gcc.cc (Gcc_backend::global_variable): If type is zero-sized,
add a VIEW_CONVERT_EXPR to the tree.
(Gcc_backend::global_variable_set_init): Remove any
VIEW_CONVERT_EXPR.
(Gcc_backend::write_global_definitions): Likewise.

Index: go-gcc.cc
===================================================================
--- go-gcc.cc   (revision 231887)
+++ go-gcc.cc   (working copy)
@@ -2390,6 +2390,7 @@ Gcc_backend::global_variable(const std::
     return this->error_variable();
 
   // The GNU linker does not like dynamic variables with zero size.
+  tree orig_type_tree = type_tree;
   if ((is_external || !is_hidden) && int_size_in_bytes(type_tree) == 0)
     type_tree = this->non_zero_size_type(type_tree);
 
@@ -2419,6 +2420,10 @@ Gcc_backend::global_variable(const std::
 
   go_preserve_from_gc(decl);
 
+  if (orig_type_tree != type_tree)
+    decl = fold_build1_loc(location.gcc_location(), VIEW_CONVERT_EXPR,
+                          orig_type_tree, decl);
+
   return new Bvariable(decl);
 }
 
@@ -2434,6 +2439,10 @@ Gcc_backend::global_variable_set_init(Bv
   tree var_decl = var->get_tree();
   if (var_decl == error_mark_node)
     return;
+  // Undo the VIEW_CONVERT_EXPR that may have been added by
+  // global_variable.
+  if (TREE_CODE(var_decl) == VIEW_CONVERT_EXPR)
+    var_decl = TREE_OPERAND(var_decl, 0);
   DECL_INITIAL(var_decl) = expr_tree;
 
   // If this variable goes in a unique section, it may need to go into
@@ -3030,7 +3039,12 @@ Gcc_backend::write_global_definitions(
     {
       if ((*p)->get_tree() != error_mark_node)
         {
-          defs[i] = (*p)->get_tree();
+         tree t = (*p)->get_tree();
+         // Undo the VIEW_CONVERT_EXPR that may have been added by
+         // global_variable.
+         if (TREE_CODE(t) == VIEW_CONVERT_EXPR)
+           t = TREE_OPERAND(t, 0);
+          defs[i] = t;
           go_preserve_from_gc(defs[i]);
           ++i;
         }