c syntax

[ranjith kumar]

Hi,
  I am trying to understand contents of basicblock.h
file.
In that basic block is defined as a structure.
My question is why its definition is followd by
GTY(..........) at 214. What kind of syntax is that?
I am unable to understand  lines 220 and 221.
Are they defining pointers to functions?

Thanks in advance.
 
  214 struct basic_block_def GTY((chain_next
("%h.next_bb"), chain_prev ("%h.prev_bb")))
    215 {
    216   /* Pointers to the first and last trees of
the block.  */
    217   tree stmt_list;
    218
    219   /* The edges into and out of the block.  */
    220   VEC(edge,gc) *preds;
    221   VEC(edge,gc) *succs;
    222
    223   /* Auxiliary info specific to a pass.  */
    224   PTR GTY ((skip (""))) aux;
    225
    226   /* Innermost loop containing the block.  */
    227   struct loop * GTY ((skip (""))) loop_father;
    228
    229   /* The dominance and postdominance
information node.  */
    230   struct et_node * GTY ((skip (""))) dom[2];
    231
    232   /* Previous and next blocks in the chain. 
*/
    233   struct basic_block_def *prev_bb;
    234   struct basic_block_def *next_bb;
    235
    236   union basic_block_il_dependent {
    237       struct rtl_bb_info * GTY ((tag ("1")))
rtl;
    238     } GTY ((desc ("((%1.flags & BB_RTL) !=
0)"))) il;
    239
    240   /* Chain of PHI nodes for this block.  */
    241   tree phi_nodes;
    242
    243   /* A list of predictions.  */
    244   struct edge_prediction *predictions;
    245
    246   /* Expected number of executions: calculated
in profile.c.  */
    247   gcov_type count;
    248



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