GNU Compiler Collection (GCC) Internals: Maintaining the CFG

---

14.4 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.

---