gcc/doc/gccint/control-flow-graph.rst

   1 ..
   2   Copyright 1988-2022 Free Software Foundation, Inc.
   3   This is part of the GCC manual.
   4   For copying conditions, see the copyright.rst file.
   5
   6 .. index:: CFG, Control Flow Graph, basic-block.h
   7
   8 .. _control-flow:
   9
  10 Control Flow Graph
  11 ------------------
  12
  13 A control flow graph (CFG) is a data structure built on top of the
  14 intermediate code representation (the RTL or ``GIMPLE`` instruction
  15 stream) abstracting the control flow behavior of a function that is
  16 being compiled.  The CFG is a directed graph where the vertices
  17 represent basic blocks and edges represent possible transfer of
  18 control flow from one basic block to another.  The data structures
  19 used to represent the control flow graph are defined in
  20 :samp:`basic-block.h`.
  21
  22 In GCC, the representation of control flow is maintained throughout
  23 the compilation process, from constructing the CFG early in
  24 ``pass_build_cfg`` to ``pass_free_cfg`` (see :samp:`passes.def`).
  25 The CFG takes various different modes and may undergo extensive
  26 manipulations, but the graph is always valid between its construction
  27 and its release.  This way, transfer of information such as data flow,
  28 a measured profile, or the loop tree, can be propagated through the
  29 passes pipeline, and even from ``GIMPLE`` to ``RTL``.
  30
  31 Often the CFG may be better viewed as integral part of instruction
  32 chain, than structure built on the top of it.  Updating the compiler's
  33 intermediate representation for instructions cannot be easily done
  34 without proper maintenance of the CFG simultaneously.
  35
  36 .. toctree::
  37   :maxdepth: 2
  38
  39   control-flow-graph/basic-blocks
  40   control-flow-graph/edges
  41   control-flow-graph/profile-information
  42   control-flow-graph/maintaining-the-cfg
  43   control-flow-graph/liveness-information