11.2 Loop querying

The functions to query the information about loops are declared in cfgloop.h. Some of the information can be taken directly from the structures. loop_father field of each basic block contains the innermost loop to that the block belongs. The most useful fields of loop structure (that are kept up-to-date at all times) are:

• header, latch: Header and latch basic blocks of the loop.
• num_nodes: Number of basic blocks in the loop (including the basic blocks of the sub-loops).
• depth: The depth of the loop in the loops tree, i.e., the number of super-loops of the loop.
• outer, inner, next: The super-loop, the first sub-loop, and the sibling of the loop in the loops tree.
• single_exit: The exit edge of the loop, if the loop has exactly one exit and the loops were analyzed with LOOPS_HAVE_MARKED_SINGLE_EXITS.

There are other fields in the loop structures, many of them used only by some of the passes, or not updated during CFG changes; in general, they should not be accessed directly.

The most important functions to query loop structures are:

• flow_loops_dump: Dumps the information about loops to a file.
• verify_loop_structure: Checks consistency of the loop structures.
• loop_latch_edge: Returns the latch edge of a loop.
• loop_preheader_edge: If loops have preheaders, returns the preheader edge of a loop.
• flow_loop_nested_p: Tests whether loop is a sub-loop of another loop.
• flow_bb_inside_loop_p: Tests whether a basic block belongs to a loop (including its sub-loops).
• find_common_loop: Finds the common super-loop of two loops.
• superloop_at_depth: Returns the super-loop of a loop with the given depth.
• tree_num_loop_insns, num_loop_insns: Estimates the number of insns in the loop, on GIMPLE and on RTL.
• loop_exit_edge_p: Tests whether edge is an exit from a loop.
• mark_loop_exit_edges: Marks all exit edges of all loops with EDGE_LOOP_EXIT flag.
• get_loop_body, get_loop_body_in_dom_order, get_loop_body_in_bfs_order: Enumerates the basic blocks in the loop in depth-first search order in reversed CFG, ordered by dominance relation, and breath-first search order, respectively.
• get_loop_exit_edges: Enumerates the exit edges of a loop.
• just_once_each_iteration_p: Returns true if the basic block is executed exactly once during each iteration of a loop (that is, it does not belong to a sub-loop, and it dominates the latch of the loop).