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.

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. 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.

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.

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, and are not elements of the BASIC_BLOCK array. Therefore they have been assigned unique, negative index numbers.

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 (tree and RTL), there are pointers to the head and end of a basic block for both representations.

For RTL, these pointers are rtx head, end. In the RTL function representation, the head pointer always points either to a NOTE_INSN_BASIC_BLOCK or to a CODE_LABEL, if present. In the RTL representation of a function, the instruction stream contains not only the “real” instructions, but also 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, making such updates difficult. The NOTE_INSN_BASIC_BLOCK note is the only kind of note that may appear in the instruction stream contained in a basic block. The instruction stream of a basic block always follows a NOTE_INSN_BASIC_BLOCK, but zero or more CODE_LABEL nodes can precede the block note. A basic block ends by control flow instruction or last instruction before following CODE_LABEL or NOTE_INSN_BASIC_BLOCK. A CODE_LABEL cannot appear in 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 tree representation, the head and end of the basic block are being pointed to by the stmt_list field, but this special tree should never be referenced directly. Instead, at the tree level abstract containers and iterators are used to access statements and expressions in basic blocks. These iterators are called block statement iterators (BSIs). Grep for ^bsi in the various tree-* files. The following snippet will pretty-print all the statements of the program in the GIMPLE representation.

     FOR_EACH_BB (bb)
       {
          block_stmt_iterator si;
     
          for (si = bsi_start (bb); !bsi_end_p (si); bsi_next (&si))
            {
               tree stmt = bsi_stmt (si);
               print_generic_stmt (stderr, stmt, 0);
            }
       }