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<h3 class="section">15.2 Loop querying</h3>
<p><a name="index-Loop-querying-3231"></a>
The functions to query the information about loops are declared in
<samp><span class="file">cfgloop.h</span></samp>. Some of the information can be taken directly from
the structures. <code>loop_father</code> 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:
<ul>
<li><code>header</code>, <code>latch</code>: Header and latch basic blocks of the
loop.
<li><code>num_nodes</code>: Number of basic blocks in the loop (including
the basic blocks of the sub-loops).
<li><code>depth</code>: The depth of the loop in the loops tree, i.e., the
number of super-loops of the loop.
<li><code>outer</code>, <code>inner</code>, <code>next</code>: The super-loop, the first
sub-loop, and the sibling of the loop in the loops tree.
</ul>
<p>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.
<p>The most important functions to query loop structures are:
<ul>
<li><code>flow_loops_dump</code>: Dumps the information about loops to a
file.
<li><code>verify_loop_structure</code>: Checks consistency of the loop
structures.
<li><code>loop_latch_edge</code>: Returns the latch edge of a loop.
<li><code>loop_preheader_edge</code>: If loops have preheaders, returns
the preheader edge of a loop.
<li><code>flow_loop_nested_p</code>: Tests whether loop is a sub-loop of
another loop.
<li><code>flow_bb_inside_loop_p</code>: Tests whether a basic block belongs
to a loop (including its sub-loops).
<li><code>find_common_loop</code>: Finds the common super-loop of two loops.
<li><code>superloop_at_depth</code>: Returns the super-loop of a loop with
the given depth.
<li><code>tree_num_loop_insns</code>, <code>num_loop_insns</code>: Estimates the
number of insns in the loop, on GIMPLE and on RTL.
<li><code>loop_exit_edge_p</code>: Tests whether edge is an exit from a
loop.
<li><code>mark_loop_exit_edges</code>: Marks all exit edges of all loops
with <code>EDGE_LOOP_EXIT</code> flag.
<li><code>get_loop_body</code>, <code>get_loop_body_in_dom_order</code>,
<code>get_loop_body_in_bfs_order</code>: 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.
<li><code>single_exit</code>: Returns the single exit edge of the loop, or
<code>NULL</code> if the loop has more than one exit. You can only use this
function if LOOPS_HAVE_MARKED_SINGLE_EXITS property is used.
<li><code>get_loop_exit_edges</code>: Enumerates the exit edges of a loop.
<li><code>just_once_each_iteration_p</code>: 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).
</ul>
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