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15.1 Basic Blocks

A basic block is a straight-line sequence of code with only one entry point and only one exit. In GCC, basic blocks are represented using the basic_block data type.

Special basic blocks represent possible entry and exit points of a function. These blocks are called ENTRY_BLOCK_PTR and EXIT_BLOCK_PTR. These blocks do not contain any code.

The BASIC_BLOCK array contains all basic blocks in an unspecified order. Each basic_block structure has a field that holds a unique integer identifier index that is the index of the block in the BASIC_BLOCK array. The total number of basic blocks in the function is n_basic_blocks. Both the basic block indices and the total number of basic blocks may vary during the compilation process, as passes reorder, create, duplicate, and destroy basic blocks. The index for any block should never be greater than last_basic_block. The indices 0 and 1 are special codes reserved for ENTRY_BLOCK and EXIT_BLOCK, the indices of ENTRY_BLOCK_PTR and EXIT_BLOCK_PTR.

Two pointer members of the basic_block structure are the pointers next_bb and prev_bb. These are used to keep doubly linked chain of basic blocks in the same order as the underlying instruction stream. The chain of basic blocks is updated transparently by the provided API for manipulating the CFG. The macro FOR_EACH_BB can be used to visit all the basic blocks in lexicographical order, except ENTRY_BLOCK and EXIT_BLOCK. The macro FOR_ALL_BB also visits all basic blocks in lexicographical order, including ENTRY_BLOCK and EXIT_BLOCK.

The functions post_order_compute and inverted_post_order_compute can be used to compute topological orders of the CFG. The orders are stored as vectors of basic block indices. The BASIC_BLOCK array can be used to iterate each basic block by index. Dominator traversals are also possible using walk_dominator_tree. Given two basic blocks A and B, block A dominates block B if A is always executed before B.

Each basic_block also contains pointers to the first instruction (the head) and the last instruction (the tail) or end of the instruction stream contained in a basic block. In fact, since the basic_block data type is used to represent blocks in both major intermediate representations of GCC (GIMPLE and RTL), there are pointers to the head and end of a basic block for both representations, stored in intermediate representation specific data in the il field of struct basic_block_def.

For RTL, these pointers are BB_HEAD and BB_END.

In the RTL representation of a function, the instruction stream contains not only the “real” instructions, but also notes or insn notes (to distinguish them from reg notes). Any function that moves or duplicates the basic blocks needs to take care of updating of these notes. Many of these notes expect that the instruction stream consists of linear regions, so updating can sometimes be tedious. All types of insn notes are defined in insn-notes.def.

In the RTL function representation, the instructions contained in a basic block always follow a NOTE_INSN_BASIC_BLOCK, but zero or more CODE_LABEL nodes can precede the block note. A basic block ends with a control flow instruction or with the last instruction before the next CODE_LABEL or NOTE_INSN_BASIC_BLOCK. By definition, a CODE_LABEL cannot appear in the middle of the instruction stream of a basic block.

In addition to notes, the jump table vectors are also represented as “pseudo-instructions” inside the insn stream. These vectors never appear in the basic block and should always be placed just after the table jump instructions referencing them. After removing the table-jump it is often difficult to eliminate the code computing the address and referencing the vector, so cleaning up these vectors is postponed until after liveness analysis. Thus the jump table vectors may appear in the insn stream unreferenced and without any purpose. Before any edge is made fall-thru, the existence of such construct in the way needs to be checked by calling can_fallthru function.

For the GIMPLE representation, the PHI nodes and statements contained in a basic block are in a gimple_seq pointed to by the basic block intermediate language specific pointers. Abstract containers and iterators are used to access the PHI nodes and statements in a basic blocks. These iterators are called GIMPLE statement iterators (GSIs). Grep for ^gsi in the various gimple-* and tree-* files. There is a gimple_stmt_iterator type for iterating over all kinds of statement, and a gphi_iterator subclass for iterating over PHI nodes. The following snippet will pretty-print all PHI nodes the statements of the current function in the GIMPLE representation.

basic_block bb;

   gphi_iterator pi;
   gimple_stmt_iterator si;

   for (pi = gsi_start_phis (bb); !gsi_end_p (pi); gsi_next (&pi))
       gphi *phi = pi.phi ();
       print_gimple_stmt (dump_file, phi, 0, TDF_SLIM);
   for (si = gsi_start_bb (bb); !gsi_end_p (si); gsi_next (&si))
       gimple stmt = gsi_stmt (si);
       print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);

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