11.3 Loop manipulation

The loops tree can be manipulated using the following functions:

• flow_loop_tree_node_add: Adds a node to the tree.
• flow_loop_tree_node_remove: Removes a node from the tree.
• add_bb_to_loop: Adds a basic block to a loop.
• remove_bb_from_loops: Removes a basic block from loops.

The specialized versions of several low-level CFG functions that also update loop structures are provided:

• loop_split_edge_with: Splits an edge, and places a specified RTL code on it. On GIMPLE, the function can still be used, but the code must be NULL.
• bsi_insert_on_edge_immediate_loop: Inserts code on edge, splitting it if necessary. Only works on GIMPLE.
• remove_path: Removes an edge and all blocks it dominates.
• loop_commit_inserts: Commits insertions scheduled on edges, and sets loops for the new blocks. This function can only be used on GIMPLE.
• split_loop_exit_edge: Splits exit edge of the loop, ensuring that PHI node arguments remain in the loop (this ensures that loop-closed SSA form is preserved). Only useful on GIMPLE.

Finally, there are some higher-level loop transformations implemented. While some of them are written so that they should work on non-innermost loops, they are mostly untested in that case, and at the moment, they are only reliable for the innermost loops:

• create_iv: Creates a new induction variable. Only works on GIMPLE. standard_iv_increment_position can be used to find a suitable place for the iv increment.
• duplicate_loop_to_header_edge, tree_duplicate_loop_to_header_edge: These functions (on RTL and on GIMPLE) duplicate the body of the loop prescribed number of times on one of the edges entering loop header, thus performing either loop unrolling or loop peeling. can_duplicate_loop_p (can_unroll_loop_p on GIMPLE) must be true for the duplicated loop.
• loop_version, tree_ssa_loop_version: These function create a copy of a loop, and a branch before them that selects one of them depending on the prescribed condition. This is useful for optimizations that need to verify some assumptions in runtime (one of the copies of the loop is usually left unchanged, while the other one is transformed in some way).
• tree_unroll_loop: Unrolls the loop, including peeling the extra iterations to make the number of iterations divisible by unroll factor, updating the exit condition, and removing the exits that now cannot be taken. Works only on GIMPLE.