compiler: statically allocate constant interface data

    
    When converting a constant to interface, such as interface{}(42)
    or interface{}("hello"), if the interface escapes, we currently
    generate a heap allocation to hold the constant value.
    
    This CL changes it to generate a static allocation instead, as
    the gc compiler does. This reduces allocations in such cases.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/180277

From-SVN: r271945

gcc/go/gofrontend/MERGE

-e4d8ccaed06f81683e79774ede6c61949f6df8b8
+949c3b7aa603bc09e650d62e82c600b3463802f0
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -323,10 +323,15 @@ Expression::convert_type_to_interface(Type* lhs_type, Expression* rhs,
     {
       // We are assigning a non-pointer value to the interface; the
       // interface gets a copy of the value in the heap if it escapes.
+      if (rhs->is_constant())
+        obj = Expression::make_unary(OPERATOR_AND, rhs, location);
+      else
+        {
           obj = Expression::make_heap_expression(rhs, location);
           if (on_stack)
             obj->heap_expression()->set_allocate_on_stack();
         }
+    }
 
   return Expression::make_interface_value(lhs_type, first_field, obj, location);
 }
@@ -4896,6 +4901,18 @@ Unary_expression::do_get_backend(Translate_context* context)
 						     false, btype, loc, bexpr);
           bexpr = gogo->backend()->var_expression(decl, loc);
         }
+      else if (this->expr_->is_constant())
+        {
+          std::string var_name(gogo->initializer_name());
+          std::string asm_name(go_selectively_encode_id(var_name));
+          Bvariable* decl =
+              gogo->backend()->implicit_variable(var_name, asm_name, btype,
+                                                 true, true, false, 0);
+          gogo->backend()->implicit_variable_set_init(decl, var_name, btype,
+                                                      true, true, false,
+                                                      bexpr);
+          bexpr = gogo->backend()->var_expression(decl, loc);
+        }
 
       go_assert(!this->create_temp_ || this->expr_->is_variable());
       ret = gogo->backend()->address_expression(bexpr, loc);