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### 24.3 Interacting with the pass manager

There needs to be a way to add/reorder/remove passes dynamically. This is useful for both analysis plugins (plugging in after a certain pass such as CFG or an IPA pass) and optimization plugins.

Basic support for inserting new passes or replacing existing passes is provided. A plugin registers a new pass with GCC by calling `register_callback` with the `PLUGIN_PASS_MANAGER_SETUP` event and a pointer to a `struct register_pass_info` object defined as follows

    enum pass_positioning_ops
    {
      PASS_POS_INSERT_AFTER,  // Insert after the reference pass.
      PASS_POS_INSERT_BEFORE, // Insert before the reference pass.
      PASS_POS_REPLACE        // Replace the reference pass.
    };

    struct register_pass_info
    {
      struct opt_pass *pass;            /* New pass provided by the plugin.  */
      const char *reference_pass_name;  /* Name of the reference pass for hooking
                                           up the new pass.  */
      int ref_pass_instance_number;     /* Insert the pass at the specified
                                           instance number of the reference pass.  */
                                        /* Do it for every instance if it is 0.  */
      enum pass_positioning_ops pos_op; /* how to insert the new pass.  */
    };

    /* Sample plugin code that registers a new pass.  */
    int
    plugin_init (struct plugin_name_args *plugin_info,
                 struct plugin_gcc_version *version)
    {
      struct register_pass_info pass_info;

      ...

      /* Code to fill in the pass_info object with new pass information.  */

      ...

      /* Register the new pass.  */
      register_callback (plugin_info->base_name, PLUGIN_PASS_MANAGER_SETUP, NULL, &pass_info);

      ...
    }