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<a name="Loop-querying-1"></a>
<h3 class="section">15.2 Loop querying</h3>
<a name="index-Loop-querying"></a>

<p>The functions to query the information about loops are declared in
<samp>cfgloop.h</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:
</p>
<ul>
<li> <code>header</code>, <code>latch</code>: Header and latch basic blocks of the
loop.
</li><li> <code>num_nodes</code>: Number of basic blocks in the loop (including
the basic blocks of the sub-loops).
</li><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.
</li></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>
<p>The most important functions to query loop structures are:
</p>
<ul>
<li> <code>loop_depth</code>: The depth of the loop in the loops tree, i.e., the
number of super-loops of the loop.
</li><li> <code>flow_loops_dump</code>: Dumps the information about loops to a
file.
</li><li> <code>verify_loop_structure</code>: Checks consistency of the loop
structures.
</li><li> <code>loop_latch_edge</code>: Returns the latch edge of a loop.
</li><li> <code>loop_preheader_edge</code>: If loops have preheaders, returns
the preheader edge of a loop.
</li><li> <code>flow_loop_nested_p</code>: Tests whether loop is a sub-loop of
another loop.
</li><li> <code>flow_bb_inside_loop_p</code>: Tests whether a basic block belongs
to a loop (including its sub-loops).
</li><li> <code>find_common_loop</code>: Finds the common super-loop of two loops.
</li><li> <code>superloop_at_depth</code>: Returns the super-loop of a loop with
the given depth.
</li><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><li> <code>loop_exit_edge_p</code>: Tests whether edge is an exit from a
loop.
</li><li> <code>mark_loop_exit_edges</code>: Marks all exit edges of all loops
with <code>EDGE_LOOP_EXIT</code> flag.
</li><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><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><li> <code>get_loop_exit_edges</code>: Enumerates the exit edges of a loop.
</li><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).
</li></ul>

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