compiler: avoid introducing redundant write barriers

    
    The traversal used by the write barrier insertion phase can sometimes
    wind up visiting new statements inserted during the traversal, which
    then results in duplicate / redundant write barrier guards. Example
    program to reproduce:
    
      package small
      type S struct {
            N *S
            K int
      }
      var G *S = &S{N: nil, K: 101}
    
    This patch changes the traversal code to keep track of statements
    already added and avoid processing them again later in the traversal.
    
    Fixes golang/go#25867
    
    Reviewed-on: https://go-review.googlesource.com/118637

From-SVN: r261568

gcc/go/gofrontend/MERGE

-1f07926263b6d14edb6abd6a00e6385190d30d0e
+c3ef5bbf4e4271216b6f22621269d458599e8087
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/gogo.cc

@@ -8444,6 +8444,9 @@ Traverse::function_declaration(Named_object*)
 void
 Statement_inserter::insert(Statement* s)
 {
+  if (this->statements_added_ != NULL)
+    this->statements_added_->insert(s);
+
   if (this->block_ != NULL)
     {
       go_assert(this->pindex_ != NULL);

gcc/go/gofrontend/gogo.h

@@ -3419,19 +3419,24 @@ class Traverse
 class Statement_inserter
 {
  public:
+  typedef Unordered_set(Statement*) Statements;
+
   // Empty constructor.
   Statement_inserter()
-    : block_(NULL), pindex_(NULL), gogo_(NULL), var_(NULL)
+      : block_(NULL), pindex_(NULL), gogo_(NULL), var_(NULL),
+        statements_added_(NULL)
   { }
 
   // Constructor for a statement in a block.
-  Statement_inserter(Block* block, size_t *pindex)
-    : block_(block), pindex_(pindex), gogo_(NULL), var_(NULL)
+  Statement_inserter(Block* block, size_t *pindex, Statements *added = NULL)
+      : block_(block), pindex_(pindex), gogo_(NULL), var_(NULL),
+        statements_added_(added)
   { }
 
   // Constructor for a global variable.
-  Statement_inserter(Gogo* gogo, Variable* var)
-    : block_(NULL), pindex_(NULL), gogo_(gogo), var_(var)
+  Statement_inserter(Gogo* gogo, Variable* var, Statements *added = NULL)
+      : block_(NULL), pindex_(NULL), gogo_(gogo), var_(var),
+        statements_added_(added)
   { go_assert(var->is_global()); }
 
   // We use the default copy constructor and assignment operator.
@@ -3451,6 +3456,8 @@ class Statement_inserter
   Gogo* gogo_;
   // The global variable, when looking at an initializer expression.
   Variable* var_;
+  // If non-null, a set to record new statements inserted (non-owned).
+  Statements* statements_added_;
 };
 
 // When translating the gogo IR into the backend data structure, this

gcc/go/gofrontend/wb.cc

@@ -213,7 +213,7 @@ class Write_barriers : public Traverse
  public:
   Write_barriers(Gogo* gogo)
     : Traverse(traverse_functions | traverse_variables | traverse_statements),
-      gogo_(gogo), function_(NULL)
+      gogo_(gogo), function_(NULL), statements_added_()
   { }
 
   int
@@ -230,6 +230,8 @@ class Write_barriers : public Traverse
   Gogo* gogo_;
   // Current function.
   Function* function_;
+  // Statements introduced.
+  Statement_inserter::Statements statements_added_;
 };
 
 // Traverse a function.  Just record it for later.
@@ -298,9 +300,10 @@ Write_barriers::variable(Named_object* no)
   Location loc = init->location();
   Expression* ref = Expression::make_var_reference(no, loc);
 
-  Statement_inserter inserter(this->gogo_, var);
+  Statement_inserter inserter(this->gogo_, var, &this->statements_added_);
   Statement* s = this->gogo_->assign_with_write_barrier(NULL, NULL, &inserter,
 							ref, init, loc);
+  this->statements_added_.insert(s);
 
   var->add_preinit_statement(this->gogo_, s);
   var->clear_init();
@@ -313,6 +316,9 @@ Write_barriers::variable(Named_object* no)
 int
 Write_barriers::statement(Block* block, size_t* pindex, Statement* s)
 {
+  if (this->statements_added_.find(s) != this->statements_added_.end())
+    return TRAVERSE_SKIP_COMPONENTS;
+
   switch (s->classification())
     {
     default:
@@ -355,7 +361,7 @@ Write_barriers::statement(Block* block, size_t* pindex, Statement* s)
 
 	Function* function = this->function_;
 	Location loc = init->location();
-	Statement_inserter inserter(block, pindex);
+	Statement_inserter inserter(block, pindex, &this->statements_added_);
 
 	// Insert the variable declaration statement with no
 	// initializer, so that the variable exists.
@@ -370,6 +376,7 @@ Write_barriers::statement(Block* block, size_t* pindex, Statement* s)
 								   &inserter,
 								   ref, init,
 								   loc);
+        this->statements_added_.insert(assign);
 
 	// Replace the old variable declaration statement with the new
 	// initialization.
@@ -391,12 +398,14 @@ Write_barriers::statement(Block* block, size_t* pindex, Statement* s)
 	// Change the assignment to use a write barrier.
 	Function* function = this->function_;
 	Location loc = as->location();
-	Statement_inserter inserter = Statement_inserter(block, pindex);
+	Statement_inserter inserter =
+            Statement_inserter(block, pindex, &this->statements_added_);
 	Statement* assign = this->gogo_->assign_with_write_barrier(function,
 								   block,
 								   &inserter,
 								   lhs, rhs,
 								   loc);
+        this->statements_added_.insert(assign);
 	block->replace_statement(*pindex, assign);
       }
       break;