compiler, runtime: support and use single argument go:linkname

    
    The gc compiler has started permitting go:linkname comments with a
    single argument to mean that a function should be externally visible
    outside the package.  Implement this in the Go frontend.
    
    Change the libgo runtime package to use it, rather than repeating the
    name just to export a function.
    
    Remove a couple of unnecessary go:linkname comments on declarations.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/192197

From-SVN: r275239

5 files changed, 38 insertions, 9 deletions

diff --git a/gcc/go/gofrontend/MERGE b/gcc/go/gofrontend/MERGE
index 888e96d..025e66e 100644
--- a/gcc/go/gofrontend/MERGE
+++ b/gcc/go/gofrontend/MERGE
@@ -1,4 +1,4 @@
-80403eb9e95c9642ebabb4d7c43deedaa763211f
+289d94b9e6303ec74649d1f08d418300f2b4d0fd
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
diff --git a/gcc/go/gofrontend/gogo.cc b/gcc/go/gofrontend/gogo.cc
index f8114ece..d39a4fb 100644
--- a/gcc/go/gofrontend/gogo.cc
+++ b/gcc/go/gofrontend/gogo.cc
@@ -2531,9 +2531,22 @@ Gogo::add_linkname(const std::string& go_name, bool is_exported,
   if (no == NULL)
     go_error_at(loc, "%s is not defined", go_name.c_str());
   else if (no->is_function())
-    no->func_value()->set_asm_name(ext_name);
+    {
+      if (ext_name.empty())
+	no->func_value()->set_is_exported_by_linkname();
+      else
+	no->func_value()->set_asm_name(ext_name);
+    }
   else if (no->is_function_declaration())
-    no->func_declaration_value()->set_asm_name(ext_name);
+    {
+      if (ext_name.empty())
+	go_error_at(loc,
+		    ("//%<go:linkname%> missing external name "
+		     "for declaration of %s"),
+		    go_name.c_str());
+      else
+	no->func_declaration_value()->set_asm_name(ext_name);
+    }
   else
     go_error_at(loc,
 		("%s is not a function; "
@@ -5465,7 +5478,8 @@ Function::Function(Function_type* type, Named_object* enclosing, Block* block,
     calls_recover_(false), is_recover_thunk_(false), has_recover_thunk_(false),
     calls_defer_retaddr_(false), is_type_specific_function_(false),
     in_unique_section_(false), export_for_inlining_(false),
-    is_inline_only_(false), is_referenced_by_inline_(false)
+    is_inline_only_(false), is_referenced_by_inline_(false),
+    is_exported_by_linkname_(false)
 {
 }
 
@@ -6220,6 +6234,11 @@ Function::get_or_make_decl(Gogo* gogo, Named_object* no)
       if (this->is_referenced_by_inline_)
 	flags |= Backend::function_is_visible;
 
+      // A go:linkname directive can be used to force a function to be
+      // visible.
+      if (this->is_exported_by_linkname_)
+	flags |= Backend::function_is_visible;
+
       // If a function calls the predeclared recover function, we
       // can't inline it, because recover behaves differently in a
       // function passed directly to defer.  If this is a recover
diff --git a/gcc/go/gofrontend/gogo.h b/gcc/go/gofrontend/gogo.h
index 087e890..e742b6e 100644
--- a/gcc/go/gofrontend/gogo.h
+++ b/gcc/go/gofrontend/gogo.h
@@ -547,7 +547,9 @@ class Gogo
   { this->file_block_names_[name] = location; }
 
   // Add a linkname, from the go:linkname compiler directive.  This
-  // changes the externally visible name of go_name to be ext_name.
+  // changes the externally visible name of GO_NAME to be EXT_NAME.
+  // If EXT_NAME is the empty string, GO_NAME is unchanged, but the
+  // symbol is made publicly visible.
   void
   add_linkname(const std::string& go_name, bool is_exported,
 	       const std::string& ext_name, Location location);
@@ -1359,6 +1361,11 @@ class Function
   set_asm_name(const std::string& asm_name)
   { this->asm_name_ = asm_name; }
 
+  // Mark this symbol as exported by a linkname directive.
+  void
+  set_is_exported_by_linkname()
+  { this->is_exported_by_linkname_ = true; }
+
   // Return the pragmas for this function.
   unsigned int
   pragmas() const
@@ -1706,6 +1713,9 @@ class Function
   // True if this function is referenced from an inlined body that
   // will be put into the export data.
   bool is_referenced_by_inline_ : 1;
+  // True if we should make this function visible to other packages
+  // because of a go:linkname directive.
+  bool is_exported_by_linkname_ : 1;
 };
 
 // A snapshot of the current binding state.
diff --git a/gcc/go/gofrontend/lex.cc b/gcc/go/gofrontend/lex.cc
index 3276de4..f023613 100644
--- a/gcc/go/gofrontend/lex.cc
+++ b/gcc/go/gofrontend/lex.cc
@@ -1996,7 +1996,7 @@ Lex::skip_cpp_comment()
 
 	      while (ps < pend && *ps != ' ' && *ps != '\t')
 		++ps;
-	      if (ps < pend)
+	      if (ps <= pend)
 		go_name = std::string(pg, ps - pg);
 	      while (ps < pend && (*ps == ' ' || *ps == '\t'))
 		++ps;
@@ -2015,8 +2015,8 @@ Lex::skip_cpp_comment()
 	      ext_name.clear();
 	    }
 	}
-      if (go_name.empty() || ext_name.empty())
-	go_error_at(loc, "usage: %<//go:linkname%> localname linkname");
+      if (go_name.empty())
+	go_error_at(loc, "usage: %<//go:linkname%> localname [linkname]");
       else
 	{
 	  if (this->linknames_ == NULL)
diff --git a/gcc/go/gofrontend/lex.h b/gcc/go/gofrontend/lex.h
index 59b770c..3be3806 100644
--- a/gcc/go/gofrontend/lex.h
+++ b/gcc/go/gofrontend/lex.h
@@ -380,7 +380,7 @@ class Lex
 
   struct Linkname
   {
-    std::string ext_name;	// External name.
+    std::string ext_name;	// External name; empty to just export.
     bool is_exported;		// Whether the internal name is exported.
     Location loc;		// Location of go:linkname directive.