GNU Compiler Collection (GCC) 4.2 Internals

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:

• LOOPS_HAVE_PREHEADERS: Forwarder blocks are created in such a way that each loop has only one entry edge, and additionally, the source block of this entry edge has only one successor. This creates a natural place where the code can be moved out of the loop, and ensures that the entry edge of the loop leads from its immediate super-loop.
• LOOPS_HAVE_SIMPLE_LATCHES: Forwarder blocks are created to force the latch block of each loop to have only one successor. This ensures that the latch of the loop does not belong to any of its sub-loops, and makes manipulation with the loops significantly easier. Most of the loop manipulation functions assume that the loops are in this shape. Note that with this flag, the “normal” loop without any control flow inside and with one exit consists of two basic blocks.
• LOOPS_HAVE_MARKED_IRREDUCIBLE_REGIONS: Basic blocks and edges in the strongly connected components that are not natural loops (have more than one entry block) are marked with BB_IRREDUCIBLE_LOOP and EDGE_IRREDUCIBLE_LOOP flags. The flag is not set for blocks and edges that belong to natural loops that are in such an irreducible region (but it is set for the entry and exit edges of such a loop, if they lead to/from this region).
• LOOPS_HAVE_MARKED_SINGLE_EXITS: If a loop has exactly one exit edge, this edge is stored in single_exit field of the loop structure. NULL is stored there otherwise.

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.