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

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

[Ian Lance Taylor]

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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  <iant@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.

[-- Attachment #2: patch.txt --]
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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;
         }

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

[Andrew Pinski]

On Mon, Dec 21, 2015 at 4:10 PM, Ian Lance Taylor <iant@golang.org> wrote:
> 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.

VIEW_CONVERT_EXPR on different size types is invalid for GCC's IR.
So this patch cause other issues.

Thanks,
Andrew Pinski

>
> Ian
>
> 2015-12-21  Ian Lance Taylor  <iant@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.

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

[Ian Lance Taylor]

On Mon, Dec 21, 2015 at 4:19 PM, Andrew Pinski <pinskia@gmail.com> wrote:
> On Mon, Dec 21, 2015 at 4:10 PM, Ian Lance Taylor <iant@golang.org> wrote:
>> 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.
>
> VIEW_CONVERT_EXPR on different size types is invalid for GCC's IR.
> So this patch cause other issues.

Hmmm, thanks for the pointer.  Any suggestions?  The constraints are
that the variable has to be non-zero sized (or the GNU linker
complains) and I can't entirely drop the reference to the variable (or
the linker might fail to pull in an object from an archive) and the
resulting expression has to be zero-sized (or the function call is
miscompiled).

ian

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

[Ian Lance Taylor]

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On Mon, Dec 21, 2015 at 4:19 PM, Andrew Pinski <pinskia@gmail.com> wrote:
> On Mon, Dec 21, 2015 at 4:10 PM, Ian Lance Taylor <iant@golang.org> wrote:
>> 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.
>
> VIEW_CONVERT_EXPR on different size types is invalid for GCC's IR.
> So this patch cause other issues.

Thanks again.  This patch should fix that by taking a somewhat
different approach.  Instead of a VIEW_CONVERT_EXPR it generates
*(orig_type*)&decl.  Bootstrapped and ran Go testsuite on
x86_64-pc-linux-gnu.  Committed to mainline and gccgo branch.

Ian

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

* go-gcc.cc (class Bvariable): Remove Gcc_tree parent class.  Add
t_ and orig_type_ fields.  Add new two parameter constructor.  Add
get_tree and get_decl methods.
(Gcc_backend::var_expression): Pass location to var get_tree.
(Gcc_backend::global_variable): Don't add VIEW_CONVERT_EXPR.  Use
two parameter constructor for Bvariable.
(Gcc_backend::global_variable_set_init): Don't remove
VIEW_CONVERT_EXPR.  Use var get_decl, not get_tree.
(Gcc_backend::write_global_definitions): Likewise.
(Gcc_backend::init_statement): Call var get_decl, not get_tree.
(Gcc_backend::block): Likewise.
(Gcc_backend::implicit_variable_set_init): Likewise.
(Gcc_backend::immutable_struct_set_init): Likewise.
(Gcc_backend::function_set_parameters): Likewise.

[-- Attachment #2: patch.txt --]
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Index: go-gcc.cc
===================================================================
--- go-gcc.cc	(revision 231888)
+++ go-gcc.cc	(working copy)
@@ -109,22 +109,65 @@ class Bblock : public Gcc_tree
   { }
 };
 
-class Bvariable : public Gcc_tree
+class Blabel : public Gcc_tree
 {
  public:
-  Bvariable(tree t)
+  Blabel(tree t)
     : Gcc_tree(t)
   { }
 };
 
-class Blabel : public Gcc_tree
+// Bvariable is a bit more complicated, because of zero-sized types.
+// The GNU linker does not permit dynamic variables with zero size.
+// When we see such a variable, we generate a version of the type with
+// non-zero size.  However, when referring to the global variable, we
+// want an expression of zero size; otherwise, if, say, the global
+// variable is passed to a function, we will be passing a
+// non-zero-sized value to a zero-sized value, which can lead to a
+// miscompilation.
+
+class Bvariable
 {
  public:
-  Blabel(tree t)
-    : Gcc_tree(t)
+  Bvariable(tree t)
+    : t_(t), orig_type_(NULL)
   { }
+
+  Bvariable(tree t, tree orig_type)
+    : t_(t), orig_type_(orig_type)
+  { }
+
+  // Get the tree for use as an expression.
+  tree
+  get_tree(Location) const;
+
+  // Get the actual decl;
+  tree
+  get_decl() const
+  { return this->t_; }
+
+ private:
+  tree t_;
+  tree orig_type_;
 };
 
+// Get the tree of a variable for use as an expression.  If this is a
+// zero-sized global, create an expression that refers to the decl but
+// has zero size.
+tree
+Bvariable::get_tree(Location location) const
+{
+  if (this->orig_type_ == NULL
+      || this->t_ == error_mark_node
+      || TREE_TYPE(this->t_) == this->orig_type_)
+    return this->t_;
+  // Return *(orig_type*)&decl.  */
+  tree t = build_fold_addr_expr_loc(location.gcc_location(), this->t_);
+  t = fold_build1_loc(location.gcc_location(), NOP_EXPR,
+		      build_pointer_type(this->orig_type_), t);
+  return build_fold_indirect_ref_loc(location.gcc_location(), t);
+}
+
 // This file implements the interface between the Go frontend proper
 // and the gcc IR.  This implements specific instantiations of
 // abstract classes defined by the Go frontend proper.  The Go
@@ -1158,9 +1201,9 @@ Gcc_backend::zero_expression(Btype* btyp
 // An expression that references a variable.
 
 Bexpression*
-Gcc_backend::var_expression(Bvariable* var, Location)
+Gcc_backend::var_expression(Bvariable* var, Location location)
 {
-  tree ret = var->get_tree();
+  tree ret = var->get_tree(location);
   if (ret == error_mark_node)
     return this->error_expression();
   return this->make_expression(ret);
@@ -1894,7 +1937,7 @@ Gcc_backend::expression_statement(Bexpre
 Bstatement*
 Gcc_backend::init_statement(Bvariable* var, Bexpression* init)
 {
-  tree var_tree = var->get_tree();
+  tree var_tree = var->get_decl();
   tree init_tree = init->get_tree();
   if (var_tree == error_mark_node || init_tree == error_mark_node)
     return this->error_statement();
@@ -2264,7 +2307,7 @@ Gcc_backend::block(Bfunction* function,
        pv != vars.end();
        ++pv)
     {
-      *pp = (*pv)->get_tree();
+      *pp = (*pv)->get_decl();
       if (*pp != error_mark_node)
 	pp = &DECL_CHAIN(*pp);
     }
@@ -2420,11 +2463,7 @@ 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);
+  return new Bvariable(decl, orig_type_tree);
 }
 
 // Set the initial value of a global variable.
@@ -2436,13 +2475,9 @@ Gcc_backend::global_variable_set_init(Bv
   if (expr_tree == error_mark_node)
     return;
   gcc_assert(TREE_CONSTANT(expr_tree));
-  tree var_decl = var->get_tree();
+  tree var_decl = var->get_decl();
   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
@@ -2668,7 +2703,7 @@ Gcc_backend::implicit_variable_set_init(
 					Btype*, bool, bool, bool is_common,
 					Bexpression* init)
 {
-  tree decl = var->get_tree();
+  tree decl = var->get_decl();
   tree init_tree;
   if (init == NULL)
     init_tree = NULL_TREE;
@@ -2762,7 +2797,7 @@ Gcc_backend::immutable_struct_set_init(B
 				       bool, bool is_common, Btype*, Location,
 				       Bexpression* initializer)
 {
-  tree decl = var->get_tree();
+  tree decl = var->get_decl();
   tree init_tree = initializer->get_tree();
   if (decl == error_mark_node || init_tree == error_mark_node)
     return;
@@ -2981,7 +3016,7 @@ Gcc_backend::function_set_parameters(Bfu
        pv != param_vars.end();
        ++pv)
     {
-      *pp = (*pv)->get_tree();
+      *pp = (*pv)->get_decl();
       gcc_assert(*pp != error_mark_node);
       pp = &DECL_CHAIN(*pp);
     }
@@ -3037,14 +3072,10 @@ Gcc_backend::write_global_definitions(
        p != variable_decls.end();
        ++p)
     {
-      if ((*p)->get_tree() != error_mark_node)
+      tree v = (*p)->get_decl();
+      if (v != error_mark_node)
         {
-	  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;
+          defs[i] = v;
           go_preserve_from_gc(defs[i]);
           ++i;
         }