11.4 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 tree 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). Often the CFG may be better viewed as integral part of instruction chain, than structure built on the top of it. However, in principle the control flow graph for the tree representation is not an integral part of the representation, in that a function tree may be expanded without first building a flow graph for the tree representation at all. This happens when compiling without any tree optimization enabled. When the tree optimizations are enabled and the instruction stream is rewritten in SSA form, the CFG is very tightly coupled with the instruction stream. In particular, statement insertion and removal has to be done with care. In fact, the whole tree representation can not be easily used or maintained without proper maintenance of the CFG simultaneously.

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 tree representation, the function bb_for_stmt returns a pointer to the basic block containing the queried statement.

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

bsi_start

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

bsi_last

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

bsi_end_p

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

bsi_next

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

bsi_prev

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

bsi_insert_after

This function inserts a statement after the block_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.

bsi_insert_before

This function inserts a statement before the block_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.

bsi_remove

This function removes the block_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 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, but also when simplifying possibly trapping instruction to non-trapping while compiling Java. 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. In the RTL representation, the find_sub_basic_blocks may be used to split existing basic block and add necessary edges. 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 tree representation, the equivalent functions are bsi_insert_on_edge which inserts a block statement iterator on an edge, and bsi_commit_edge_inserts which flushes the instruction to actual instruction stream.

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

Note that at present, the representation of control flow in the tree representation is discarded before expanding to RTL. Long term the CFG should be maintained and “expanded” to the RTL representation along with the function tree itself.