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).
outer, inner, next: The super-loop, the first
sub-loop, and the sibling of the loop in the loops tree.
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:
loop_depth: The depth of the loop in the loops tree, i.e., the
number of super-loops of the loop.
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.
single_exit: Returns the single exit edge of the loop, or
NULL if the loop has more than one exit. You can only use this
function if LOOPS_HAVE_MARKED_SINGLE_EXITS property is used.
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).