GNU Compiler Collection (GCC) Internals: Maintaining the CFG

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### 15.4 Maintaining the CFG

An important task of each compiler pass is to keep both the control flow graph and all profile information up-to-date. Reconstruction of the control flow graph after each pass is not an option, since it may be very expensive and lost profile information cannot be reconstructed at all.

GCC has two major intermediate representations, and both use the `basic_block` and `edge` data types to represent control flow. Both representations share as much of the CFG maintenance code as possible. For each representation, a set of hooks is defined so that each representation can provide its own implementation of CFG manipulation routines when necessary. These hooks are defined in `cfghooks.h`. There are hooks for almost all common CFG manipulations, including block splitting and merging, edge redirection and creating and deleting basic blocks. These hooks should provide everything you need to maintain and manipulate the CFG in both the RTL and `GIMPLE` representation.

At the moment, the basic block boundaries are maintained transparently when modifying instructions, so there rarely is a need to move them manually (such as in case someone wants to output instruction outside basic block explicitly).

In the RTL representation, each instruction has a `BLOCK_FOR_INSN` value that represents pointer to the basic block that contains the instruction. In the `GIMPLE` representation, the function `gimple_bb` returns a pointer to the basic block containing the queried statement.

When changes need to be applied to a function in its `GIMPLE` representation, GIMPLE statement iterators should be used. These iterators provide an integrated abstraction of the flow graph and the instruction stream. Block statement iterators are constructed using the `gimple_stmt_iterator` data structure and several modifiers are available, including the following:

`gsi_start`

  This function initializes a `gimple_stmt_iterator` that points to the first non-empty statement in a basic block.

`gsi_last`

  This function initializes a `gimple_stmt_iterator` that points to the last statement in a basic block.

`gsi_end_p`

  This predicate is `true` if a `gimple_stmt_iterator` represents the end of a basic block.

`gsi_next`

  This function takes a `gimple_stmt_iterator` and makes it point to its successor.

`gsi_prev`

  This function takes a `gimple_stmt_iterator` and makes it point to its predecessor.

`gsi_insert_after`

  This function inserts a statement after the `gimple_stmt_iterator` passed in. The final parameter determines whether the statement iterator is updated to point to the newly inserted statement, or left pointing to the original statement.

`gsi_insert_before`

  This function inserts a statement before the `gimple_stmt_iterator` passed in. The final parameter determines whether the statement iterator is updated to point to the newly inserted statement, or left pointing to the original statement.

`gsi_remove`

  This function removes the `gimple_stmt_iterator` passed in and rechains the remaining statements in a basic block, if any.

In the RTL representation, the macros `BB_HEAD` and `BB_END` may be used to get the head and end `rtx` of a basic block. No abstract iterators are defined for traversing the insn chain, but you can just use `NEXT_INSN` and `PREV_INSN` instead. See Insns.

Usually a code manipulating pass simplifies the instruction stream and the flow of control, possibly eliminating some edges. This may for example happen when a conditional jump is replaced with an unconditional jump. Updating of edges is not transparent and each optimization pass is required to do so manually. However only few cases occur in practice. The pass may call `purge_dead_edges` on a given basic block to remove superfluous edges, if any.

Another common scenario is redirection of branch instructions, but this is best modeled as redirection of edges in the control flow graph and thus use of `redirect_edge_and_branch` is preferred over more low level functions, such as `redirect_jump` that operate on RTL chain only. The CFG hooks defined in `cfghooks.h` should provide the complete API required for manipulating and maintaining the CFG.

It is also possible that a pass has to insert control flow instruction into the middle of a basic block, thus creating an entry point in the middle of the basic block, which is impossible by definition: The block must be split to make sure it only has one entry point, i.e. the head of the basic block. The CFG hook `split_block` may be used when an instruction in the middle of a basic block has to become the target of a jump or branch instruction.

For a global optimizer, a common operation is to split edges in the flow graph and insert instructions on them. In the RTL representation, this can be easily done using the `insert_insn_on_edge` function that emits an instruction “on the edge”, caching it for a later `commit_edge_insertions` call that will take care of moving the inserted instructions off the edge into the instruction stream contained in a basic block. This includes the creation of new basic blocks where needed. In the `GIMPLE` representation, the equivalent functions are `gsi_insert_on_edge` which inserts a block statement iterator on an edge, and `gsi_commit_edge_inserts` which flushes the instruction to actual instruction stream.

While debugging the optimization pass, the `verify_flow_info` function may be useful to find bugs in the control flow graph updating code.

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