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);

  ...
}