### 15.2 Edges

Edges represent possible control flow transfers from the end of some basic block A to the head of another basic block B. We say that A is a predecessor of B, and B is a successor of A. Edges are represented in GCC with the `edge` data type. Each `edge` acts as a link between two basic blocks: The `src` member of an edge points to the predecessor basic block of the `dest` basic block. The members `preds` and `succs` of the `basic_block` data type point to type-safe vectors of edges to the predecessors and successors of the block.

When walking the edges in an edge vector, edge iterators should be used. Edge iterators are constructed using the `edge_iterator` data structure and several methods are available to operate on them:

`ei_start`

  This function initializes an `edge_iterator` that points to the first edge in a vector of edges.

`ei_last`

  This function initializes an `edge_iterator` that points to the last edge in a vector of edges.

`ei_end_p`

  This predicate is `true` if an `edge_iterator` represents the last edge in an edge vector.

`ei_one_before_end_p`

  This predicate is `true` if an `edge_iterator` represents the second last edge in an edge vector.

`ei_next`

  This function takes a pointer to an `edge_iterator` and makes it point to the next edge in the sequence.

`ei_prev`

  This function takes a pointer to an `edge_iterator` and makes it point to the previous edge in the sequence.

`ei_edge`

  This function returns the `edge` currently pointed to by an `edge_iterator`.

`ei_safe_safe`

  This function returns the `edge` currently pointed to by an `edge_iterator`, but returns `NULL` if the iterator is pointing at the end of the sequence. This function has been provided for existing code makes the assumption that a `NULL` edge indicates the end of the sequence.

The convenience macro `FOR_EACH_EDGE` can be used to visit all of the edges in a sequence of predecessor or successor edges. It must not be used when an element might be removed during the traversal, otherwise elements will be missed. Here is an example of how to use the macro:

    edge e;
    edge_iterator ei;

    FOR_EACH_EDGE (e, ei, bb->succs)
      {
         if (e->flags & EDGE_FALLTHRU)
           break;
      }

There are various reasons why control flow may transfer from one block to another. One possibility is that some instruction, for example a `CODE_LABEL`, in a linearized instruction stream just always starts a new basic block. In this case a fall-thru edge links the basic block to the first following basic block. But there are several other reasons why edges may be created. The `flags` field of the `edge` data type is used to store information about the type of edge we are dealing with. Each edge is of one of the following types:

jump

  No type flags are set for edges corresponding to jump instructions. These edges are used for unconditional or conditional jumps and in RTL also for table jumps. They are the easiest to manipulate as they may be freely redirected when the flow graph is not in SSA form.

fall-thru

  Fall-thru edges are present in case where the basic block may continue execution to the following one without branching. These edges have the `EDGE_FALLTHRU` flag set. Unlike other types of edges, these edges must come into the basic block immediately following in the instruction stream. The function `force_nonfallthru` is available to insert an unconditional jump in the case that redirection is needed. Note that this may require creation of a new basic block.

exception handling

  Exception handling edges represent possible control transfers from a trapping instruction to an exception handler. The definition of “trapping” varies. In C++, only function calls can throw, but for Ada exceptions like division by zero or segmentation fault are defined and thus each instruction possibly throwing this kind of exception needs to be handled as control flow instruction. Exception edges have the `EDGE_ABNORMAL` and `EDGE_EH` flags set.

  When updating the instruction stream it is easy to change possibly trapping instruction to non-trapping, by simply removing the exception edge. The opposite conversion is difficult, but should not happen anyway. The edges can be eliminated via `purge_dead_edges` call.

  In the RTL representation, the destination of an exception edge is specified by `REG_EH_REGION` note attached to the insn. In case of a trapping call the `EDGE_ABNORMAL_CALL` flag is set too. In the `GIMPLE` representation, this extra flag is not set.

  In the RTL representation, the predicate `may_trap_p` may be used to check whether instruction still may trap or not. For the tree representation, the `tree_could_trap_p` predicate is available, but this predicate only checks for possible memory traps, as in dereferencing an invalid pointer location.

sibling calls

  Sibling calls or tail calls terminate the function in a non-standard way and thus an edge to the exit must be present. `EDGE_SIBCALL` and `EDGE_ABNORMAL` are set in such case. These edges only exist in the RTL representation.

computed jumps

  Computed jumps contain edges to all labels in the function referenced from the code. All those edges have `EDGE_ABNORMAL` flag set. The edges used to represent computed jumps often cause compile time performance problems, since functions consisting of many taken labels and many computed jumps may have very dense flow graphs, so these edges need to be handled with special care. During the earlier stages of the compilation process, GCC tries to avoid such dense flow graphs by factoring computed jumps. For example, given the following series of jumps,

        goto *x;
        [ … ]

        goto *x;
        [ … ]

        goto *x;
        [ … ]

  factoring the computed jumps results in the following code sequence which has a much simpler flow graph:

        goto y;
        [ … ]

        goto y;
        [ … ]

        goto y;
        [ … ]

      y:
        goto *x;

  However, the classic problem with this transformation is that it has a runtime cost in there resulting code: An extra jump. Therefore, the computed jumps are un-factored in the later passes of the compiler (in the pass called `pass_duplicate_computed_gotos`). Be aware of that when you work on passes in that area. There have been numerous examples already where the compile time for code with unfactored computed jumps caused some serious headaches.

nonlocal goto handlers

  GCC allows nested functions to return into caller using a `goto` to a label passed to as an argument to the callee. The labels passed to nested functions contain special code to cleanup after function call. Such sections of code are referred to as “nonlocal goto receivers”. If a function contains such nonlocal goto receivers, an edge from the call to the label is created with the `EDGE_ABNORMAL` and `EDGE_ABNORMAL_CALL` flags set.

function entry points

  By definition, execution of function starts at basic block 0, so there is always an edge from the `ENTRY_BLOCK_PTR` to basic block 0. There is no `GIMPLE` representation for alternate entry points at this moment. In RTL, alternate entry points are specified by `CODE_LABEL` with `LABEL_ALTERNATE_NAME` defined. This feature is currently used for multiple entry point prologues and is limited to post-reload passes only. This can be used by back-ends to emit alternate prologues for functions called from different contexts. In future full support for multiple entry functions defined by Fortran 90 needs to be implemented.

function exits

  In the pre-reload representation a function terminates after the last instruction in the insn chain and no explicit return instructions are used. This corresponds to the fall-thru edge into exit block. After reload, optimal RTL epilogues are used that use explicit (conditional) return instructions that are represented by edges with no flags set.