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11.1 Loop representation

This chapter describes the representation of loops in GCC, and functions that can be used to build, modify and analyze this representation. Most of the interfaces and data structures are declared in cfgloop.h. At the moment, loop structures are analyzed and this information is updated only by the optimization passes that deal with loops, but some efforts are being made to make it available throughout most of the optimization passes.

In general, a natural loop has one entry block (header) and possibly several back edges (latches) leading to the header from the inside of the loop. Loops with several latches may appear if several loops share a single header, or if there is a branching in the middle of the loop. The representation of loops in GCC however allows only loops with a single latch. During loop analysis, headers of such loops are split and forwarder blocks are created in order to disambiguate their structures. A heuristic based on profile information is used to determine whether the latches correspond to sub-loops or to control flow in a single loop. This means that the analysis sometimes changes the CFG, and if you run it in the middle of an optimization pass, you must be able to deal with the new blocks.

Body of the loop is the set of blocks that are dominated by its header, and reachable from its latch against the direction of edges in CFG. The loops are organized in a containment hierarchy (tree) such that all the loops immediately contained inside loop L are the children of L in the tree. This tree is represented by the struct loops structure. The root of this tree is a fake loop that contains all blocks in the function. Each of the loops is represented in a struct loop structure. Each loop is assigned an index (num field of the struct loop structure), and the pointer to the loop is stored in the corresponding field of the parray field of the loops structure. Index of a sub-loop is always greater than the index of its super-loop. The indices do not have to be continuous, there may be empty (NULL) entries in the parray created by deleting loops. The index of a loop never changes. The first unused index is stored in the num field of the loops structure.

Each basic block contains the reference to the innermost loop it belongs to (loop_father). For this reason, it is only possible to have one struct loops structure initialized at the same time for each CFG. It is recommended to use the global variable current_loops to contain the struct loops structure, especially if the loop structures are updated throughout several passes. Many of the loop manipulation functions assume that dominance information is up-to-date.

The loops are analyzed through loop_optimizer_init function. The argument of this function is a set of flags represented in an integer bitmask. These flags specify what other properties of the loop structures should be calculated/enforced and preserved later:

These properties may also be computed/enforced later, using functions create_preheaders, force_single_succ_latches, mark_irreducible_loops and mark_single_exit_loops.

The memory occupied by the loops structures should be freed with loop_optimizer_finalize function.

The CFG manipulation functions in general do not update loop structures. Specialized versions that additionally do so are provided for the most common tasks. On GIMPLE, cleanup_tree_cfg_loop function can be used to cleanup CFG while updating the loops structures if current_loops is set.