Go patch committed: allow multiple blank labels

[Ian Lance Taylor]

This patch by Chris Manghane fixes the Go frontend to permit multiple
blank label definitions.  This fixes https://golang.org/issue/12316 .
Bootstrapped and ran Go testsuite on x86_64-unknown-linux-gnu.
Committed to mainline.

Ian

Index: gcc/go/gofrontend/MERGE
===================================================================
--- gcc/go/gofrontend/MERGE     (revision 227276)
+++ gcc/go/gofrontend/MERGE     (working copy)
@@ -1,4 +1,4 @@
-a1d2cac484f46068b5a6ddf3e041d425a3d25e0c
+9ae5835a010a55fba875103be5f4e61485a97099
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
Index: gcc/go/gofrontend/gogo.h
===================================================================
--- gcc/go/gofrontend/gogo.h    (revision 227276)
+++ gcc/go/gofrontend/gogo.h    (working copy)
@@ -2689,6 +2689,8 @@ class Label
   void
   define(Location location, Bindings_snapshot* snapshot)
   {
+    if (this->is_dummy_label())
+      return;
     go_assert(Linemap::is_unknown_location(this->location_)
               && this->snapshot_ == NULL);
     this->location_ = location;
@@ -2709,6 +2711,11 @@ class Label
   static Label*
   create_dummy_label();
 
+  // Return TRUE if this is a dummy label.
+  bool
+  is_dummy_label() const
+  { return this->name_ == "_"; }
+
  private:
   // The name of the label.
   std::string name_;
Index: gcc/go/gofrontend/statements.cc
===================================================================
--- gcc/go/gofrontend/statements.cc     (revision 226846)
+++ gcc/go/gofrontend/statements.cc     (working copy)
@@ -3080,6 +3080,11 @@ Label_statement::do_traverse(Traverse*)
 Bstatement*
 Label_statement::do_get_backend(Translate_context* context)
 {
+  if (this->label_->is_dummy_label())
+    {
+      Bexpression* bce = 
context->backend()->boolean_constant_expression(false);
+      return context->backend()->expression_statement(bce);
+    }
   Blabel* blabel = this->label_->get_backend_label(context);
   return context->backend()->label_definition_statement(blabel);
 }