glsl: Do not allow reads from write-only buffer variables

The error location won't be right, but fixing that would require to check
for this as we process each type of AST node that can involve a variable
read.

v2:
  - Limit the check to buffer variables, image variables have different
    semantics involved.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>

src/glsl/ast_to_hir.cpp

@@ -67,6 +67,48 @@ static void
 remove_per_vertex_blocks(exec_list *instructions,
                          _mesa_glsl_parse_state *state, ir_variable_mode mode);
 
+/**
+ * Visitor class that finds the first instance of any write-only variable that
+ * is ever read, if any
+ */
+class read_from_write_only_variable_visitor : public ir_hierarchical_visitor
+{
+public:
+   read_from_write_only_variable_visitor() : found(NULL)
+   {
+   }
+
+   virtual ir_visitor_status visit(ir_dereference_variable *ir)
+   {
+      if (this->in_assignee)
+         return visit_continue;
+
+      ir_variable *var = ir->variable_referenced();
+      /* We can have image_write_only set on both images and buffer variables,
+       * but in the former there is a distinction between reads from
+       * the variable itself (write_only) and from the memory they point to
+       * (image_write_only), while in the case of buffer variables there is
+       * no such distinction, that is why this check here is limited to
+       * buffer variables alone.
+       */
+      if (!var || var->data.mode != ir_var_shader_storage)
+         return visit_continue;
+
+      if (var->data.image_write_only) {
+         found = var;
+         return visit_stop;
+      }
+
+      return visit_continue;
+   }
+
+   ir_variable *get_variable() {
+      return found;
+   }
+
+private:
+   ir_variable *found;
+};
 
 void
 _mesa_ast_to_hir(exec_list *instructions, struct _mesa_glsl_parse_state *state)
@@ -162,6 +204,20 @@ _mesa_ast_to_hir(exec_list *instructions, struct _mesa_glsl_parse_state *state)
     */
    remove_per_vertex_blocks(instructions, state, ir_var_shader_in);
    remove_per_vertex_blocks(instructions, state, ir_var_shader_out);
+
+   /* Check that we don't have reads from write-only variables */
+   read_from_write_only_variable_visitor v;
+   v.run(instructions);
+   ir_variable *error_var = v.get_variable();
+   if (error_var) {
+      /* It would be nice to have proper location information, but for that
+       * we would need to check this as we process each kind of AST node
+       */
+      YYLTYPE loc;
+      memset(&loc, 0, sizeof(loc));
+      _mesa_glsl_error(&loc, state, "Read from write-only variable `%s'",
+                       error_var->name);
+   }
 }