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INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* gccint: (gccint).            Internals of the GNU Compiler Collection.
END-INFO-DIR-ENTRY
   This file documents the internals of the GNU compilers.

   Published by the Free Software Foundation
59 Temple Place - Suite 330
Boston, MA 02111-1307 USA

File: gccint.info,  Node: Passes,  Next: Trees,  Prev: Source Tree,  Up: Top

Passes and Files of the Compiler
********************************

   The overall control structure of the compiler is in `toplev.c'.  This
file is responsible for initialization, decoding arguments, opening and
closing files, and sequencing the passes.  Routines for emitting
diagnostic messages are defined in `diagnostic.c'.  The files
`pretty-print.h' and `pretty-print.c' provide basic support for
language-independent pretty-printing.

   The parsing pass is invoked only once, to parse the entire input.  A
high level tree representation is then generated from the input, one
function at a time.  This tree code is then transformed into RTL
intermediate code, and processed.  The files involved in transforming
the trees into RTL are `expr.c', `expmed.c', and `stmt.c'.  The order
of trees that are processed, is not necessarily the same order they are
generated from the input, due to deferred inlining, and other
considerations.

   Each time the parsing pass reads a complete function definition or
top-level declaration, it calls either the function
`rest_of_compilation', or the function `rest_of_decl_compilation' in
`toplev.c', which are responsible for all further processing necessary,
ending with output of the assembler language.  All other compiler
passes run, in sequence, within `rest_of_compilation'.  When that
function returns from compiling a function definition, the storage used
for that function definition's compilation is entirely freed, unless it
is an inline function, or was deferred for some reason (this can occur
in templates, for example).  (*note An Inline Function is As Fast As a
Macro: (gcc)Inline.).

   Here is a list of all the passes of the compiler and their source
files.  Also included is a description of where debugging dumps can be
requested with `-d' options.

   * Parsing.  This pass reads the entire text of a function definition,
     constructing a high level tree representation.  (Because of the
     semantic analysis that takes place during this pass, it does more
     than is formally considered to be parsing.)

     The tree representation does not entirely follow C syntax, because
     it is intended to support other languages as well.

     Language-specific data type analysis is also done in this pass,
     and every tree node that represents an expression has a data type
     attached.  Variables are represented as declaration nodes.

     The language-independent source files for parsing are `tree.c',
     `fold-const.c', and `stor-layout.c'.  There are also header files
     `tree.h' and `tree.def' which define the format of the tree
     representation.

     C preprocessing, for language front ends, that want or require it,
     is performed by cpplib, which is covered in separate
     documentation.  In particular, the internals are covered in *Note
     Cpplib internals: (cppinternals)Top.

     The source files to parse C are found in the toplevel directory,
     and by convention are named `c-*'.  Some of these are also used by
     the other C-like languages: `c-common.c', `c-common.def',
     `c-format.c', `c-opts.c', `c-pragma.c', `c-semantics.c', `c-lex.c',
     `c-incpath.c', `c-ppoutput.c', `c-cppbuiltin.c', `c-common.h',
     `c-dump.h', `c.opt', `c-incpath.h' and `c-pragma.h',

     Files specific to each language are in subdirectories named after
     the language in question, like `ada', `objc', `cp' (for C++).

   * Tree optimization.   This is the optimization of the tree
     representation, before converting into RTL code.

     Currently, the main optimization performed here is tree-based
     inlining.  This is implemented in `tree-inline.c' and used by both
     C and C++.  Note that tree based inlining turns off rtx based
     inlining (since it's more powerful, it would be a waste of time to
     do rtx based inlining in addition).

     Constant folding and some arithmetic simplifications are also done
     during this pass, on the tree representation.  The routines that
     perform these tasks are located in `fold-const.c'.

   * RTL generation.  This is the conversion of syntax tree into RTL
     code.

     This is where the bulk of target-parameter-dependent code is found,
     since often it is necessary for strategies to apply only when
     certain standard kinds of instructions are available.  The purpose
     of named instruction patterns is to provide this information to
     the RTL generation pass.

     Optimization is done in this pass for `if'-conditions that are
     comparisons, boolean operations or conditional expressions.  Tail
     recursion is detected at this time also.  Decisions are made about
     how best to arrange loops and how to output `switch' statements.

     The source files for RTL generation include `stmt.c', `calls.c',
     `expr.c', `explow.c', `expmed.c', `function.c', `optabs.c' and
     `emit-rtl.c'.  Also, the file `insn-emit.c', generated from the
     machine description by the program `genemit', is used in this
     pass.  The header file `expr.h' is used for communication within
     this pass.

     The header files `insn-flags.h' and `insn-codes.h', generated from
     the machine description by the programs `genflags' and `gencodes',
     tell this pass which standard names are available for use and
     which patterns correspond to them.

     Aside from debugging information output, none of the following
     passes refers to the tree structure representation of the function
     (only part of which is saved).

     The decision of whether the function can and should be expanded
     inline in its subsequent callers is made at the end of rtl
     generation.  The function must meet certain criteria, currently
     related to the size of the function and the types and number of
     parameters it has.  Note that this function may contain loops,
     recursive calls to itself (tail-recursive functions can be
     inlined!), gotos, in short, all constructs supported by GCC.  The
     file `integrate.c' contains the code to save a function's rtl for
     later inlining and to inline that rtl when the function is called.
     The header file `integrate.h' is also used for this purpose.

     The option `-dr' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.rtl' to
     the input file name.

   * Sibling call optimization.   This pass performs tail recursion
     elimination, and tail and sibling call optimizations.  The purpose
     of these optimizations is to reduce the overhead of function calls,
     whenever possible.

     The source file of this pass is `sibcall.c'

     The option `-di' causes a debugging dump of the RTL code after
     this pass is run.  This dump file's name is made by appending
     `.sibling' to the input file name.

   * Jump optimization.  This pass simplifies jumps to the following
     instruction, jumps across jumps, and jumps to jumps.  It deletes
     unreferenced labels and unreachable code, except that unreachable
     code that contains a loop is not recognized as unreachable in this
     pass.  (Such loops are deleted later in the basic block analysis.)
     It also converts some code originally written with jumps into
     sequences of instructions that directly set values from the
     results of comparisons, if the machine has such instructions.

     Jump optimization is performed two or three times.  The first time
     is immediately following RTL generation.  The second time is after
     CSE, but only if CSE says repeated jump optimization is needed.
     The last time is right before the final pass.  That time,
     cross-jumping and deletion of no-op move instructions are done
     together with the optimizations described above.

     The source file of this pass is `jump.c'.

     The option `-dj' causes a debugging dump of the RTL code after
     this pass is run for the first time.  This dump file's name is
     made by appending `.jump' to the input file name.

   * Register scan.  This pass finds the first and last use of each
     register, as a guide for common subexpression elimination.  Its
     source is in `regclass.c'.

   * Jump threading.  This pass detects a condition jump that branches
     to an identical or inverse test.  Such jumps can be `threaded'
     through the second conditional test.  The source code for this
     pass is in `jump.c'.  This optimization is only performed if
     `-fthread-jumps' is enabled.

   * Static Single Assignment (SSA) based optimization passes.  The SSA
     conversion passes (to/from) are turned on by the `-fssa' option
     (it is also done automatically if you enable an SSA optimization
     pass).  These passes utilize a form called Static Single
     Assignment.  In SSA form, each variable (pseudo register) is only
     set once, giving you def-use and use-def chains for free, and
     enabling a lot more optimization passes to be run in linear time.
     Conversion to and from SSA form is handled by functions in `ssa.c'.

     The option `-de' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.ssa' to
     the input file name.
        * SSA Conditional Constant Propagation.  Turned on by the
          `-fssa-ccp' option.  This pass performs conditional constant
          propagation to simplify instructions including conditional
          branches.  This pass is more aggressive than the constant
          propagation done by the CSE and GCSE passes, but operates in
          linear time.

          The option `-dW' causes a debugging dump of the RTL code after
          this pass.  This dump file's name is made by appending
          `.ssaccp' to the input file name.

        * SSA Aggressive Dead Code Elimination.  Turned on by the
          `-fssa-dce' option.  This pass performs elimination of code
          considered unnecessary because it has no externally visible
          effects on the program.  It operates in linear time.

          The option `-dX' causes a debugging dump of the RTL code after
          this pass.  This dump file's name is made by appending
          `.ssadce' to the input file name.

   * Common subexpression elimination.  This pass also does constant
     propagation.  Its source files are `cse.c', and `cselib.c'.  If
     constant  propagation causes conditional jumps to become
     unconditional or to become no-ops, jump optimization is run again
     when CSE is finished.

     The option `-ds' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.cse' to
     the input file name.

   * Global common subexpression elimination.  This pass performs two
     different types of GCSE  depending on whether you are optimizing
     for size or not (LCM based GCSE tends to increase code size for a
     gain in speed, while Morel-Renvoise based GCSE does not).  When
     optimizing for size, GCSE is done using Morel-Renvoise Partial
     Redundancy Elimination, with the exception that it does not try to
     move invariants out of loops--that is left to  the loop
     optimization pass.  If MR PRE GCSE is done, code hoisting (aka
     unification) is also done, as well as load motion.  If you are
     optimizing for speed, LCM (lazy code motion) based GCSE is done.
     LCM is based on the work of Knoop, Ruthing, and Steffen.  LCM
     based GCSE also does loop invariant code motion.  We also perform
     load and store motion when optimizing for speed.  Regardless of
     which type of GCSE is used, the GCSE pass also performs global
     constant and  copy propagation.

     The source file for this pass is `gcse.c', and the LCM routines
     are in `lcm.c'.

     The option `-dG' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.gcse' to
     the input file name.

   * Loop optimization.  This pass moves constant expressions out of
     loops, and optionally does strength-reduction and loop unrolling
     as well.  Its source files are `loop.c' and `unroll.c', plus the
     header `loop.h' used for communication between them.  Loop
     unrolling uses some functions in `integrate.c' and the header
     `integrate.h'.  Loop dependency analysis routines are contained in
     `dependence.c'.

     Second loop optimization pass takes care of basic block level
     optimizations - unrolling, peeling and unswitching loops. The
     source files are `cfgloopanal.c' and `cfgloopmanip.c' containing
     generic loop analysis and manipulation code, `loop-init.c' with
     initialization and finalization code, `loop-unswitch.c' for loop
     unswitching and `loop-unroll.c' for loop unrolling and peeling.

     The option `-dL' causes a debugging dump of the RTL code after
     these passes.  The dump file names are made by appending `.loop'
     and `.loop2' to the input file name.

   * Jump bypassing.  This pass is an aggressive form of GCSE that
     transforms the control flow graph of a function by propagating
     constants into conditional branch instructions.

     The source file for this pass is `gcse.c'.

     The option `-dG' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.bypass'
     to the input file name.

   * Simple optimization pass that splits independent uses of each
     pseudo increasing effect of other optimizations.  This can improve
     effect of the other transformation, such as CSE or register
     allocation.  Its source files are `web.c'.

     The option `-dZ' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.web' to
     the input file name.

   * If `-frerun-cse-after-loop' was enabled, a second common
     subexpression elimination pass is performed after the loop
     optimization pass.  Jump threading is also done again at this time
     if it was specified.

     The option `-dt' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.cse2' to
     the input file name.

   * Data flow analysis (`flow.c').  This pass divides the program into
     basic blocks (and in the process deletes unreachable loops); then
     it computes which pseudo-registers are live at each point in the
     program, and makes the first instruction that uses a value point at
     the instruction that computed the value.

     This pass also deletes computations whose results are never used,
     and combines memory references with add or subtract instructions
     to make autoincrement or autodecrement addressing.

     The option `-df' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.flow' to
     the input file name.  If stupid register allocation is in use, this
     dump file reflects the full results of such allocation.

   * Instruction combination (`combine.c').  This pass attempts to
     combine groups of two or three instructions that are related by
     data flow into single instructions.  It combines the RTL
     expressions for the instructions by substitution, simplifies the
     result using algebra, and then attempts to match the result
     against the machine description.

     The option `-dc' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.combine'
     to the input file name.

   * If-conversion is a transformation that transforms control
     dependencies into data dependencies (IE it transforms conditional
     code into a single control stream).  It is implemented in the file
     `ifcvt.c'.

     The option `-dE' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.ce' to
     the input file name.

   * Register movement (`regmove.c').  This pass looks for cases where
     matching constraints would force an instruction to need a reload,
     and this reload would be a register-to-register move.  It then
     attempts to change the registers used by the instruction to avoid
     the move instruction.

     The option `-dN' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.regmove'
     to the input file name.

   * Instruction scheduling (`sched.c').  This pass looks for
     instructions whose output will not be available by the time that
     it is used in subsequent instructions.  (Memory loads and floating
     point instructions often have this behavior on RISC machines).  It
     re-orders instructions within a basic block to try to separate the
     definition and use of items that otherwise would cause pipeline
     stalls.

     Instruction scheduling is performed twice.  The first time is
     immediately after instruction combination and the second is
     immediately after reload.

     The option `-dS' causes a debugging dump of the RTL code after this
     pass is run for the first time.  The dump file's name is made by
     appending `.sched' to the input file name.

   * Register allocation.  These passes make sure that all occurrences
     of pseudo registers are eliminated, either by allocating them to a
     hard register, replacing them by an equivalent expression (e.g. a
     constant) or by placing them on the stack.  This is done in
     several subpasses:

        * Register class preferencing.  The RTL code is scanned to find
          out which register class is best for each pseudo register.
          The source file is `regclass.c'.

        * Local register allocation (`local-alloc.c').  This pass
          allocates hard registers to pseudo registers that are used
          only within one basic block.  Because the basic block is
          linear, it can use fast and powerful techniques to do a very
          good job.

          The option `-dl' causes a debugging dump of the RTL code after
          this pass.  This dump file's name is made by appending
          `.lreg' to the input file name.

        * Global register allocation (`global.c').  This pass allocates
          hard registers for the remaining pseudo registers (those
          whose life spans are not contained in one basic block).

        * Graph coloring register allocator.  The files `ra.c',
          `ra-build.c', `ra-colorize.c', `ra-debug.c', `ra-rewrite.c'
          together with the header `ra.h' contain another register
          allocator, which is used when the option `-fnew-ra' is given.
          In that case it is run instead of the above mentioned local
          and global register allocation passes, and the option `-dl'
          causes a debugging dump of its work.

        * Reloading.  This pass renumbers pseudo registers with the
          hardware registers numbers they were allocated.  Pseudo
          registers that did not get hard registers are replaced with
          stack slots.  Then it finds instructions that are invalid
          because a value has failed to end up in a register, or has
          ended up in a register of the wrong kind.  It fixes up these
          instructions by reloading the problematical values
          temporarily into registers.  Additional instructions are
          generated to do the copying.

          The reload pass also optionally eliminates the frame pointer
          and inserts instructions to save and restore call-clobbered
          registers around calls.

          Source files are `reload.c' and `reload1.c', plus the header
          `reload.h' used for communication between them.

          The option `-dg' causes a debugging dump of the RTL code after
          this pass.  This dump file's name is made by appending
          `.greg' to the input file name.

   * Instruction scheduling is repeated here to try to avoid pipeline
     stalls due to memory loads generated for spilled pseudo registers.

     The option `-dR' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.sched2'
     to the input file name.

   * Basic block reordering.  This pass implements profile guided code
     positioning.  If profile information is not available, various
     types of static analysis are performed to make the predictions
     normally coming from the profile feedback (IE execution frequency,
     branch probability, etc).  It is implemented in the file
     `bb-reorder.c', and the various prediction routines are in
     `predict.c'.

     The option `-dB' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.bbro' to
     the input file name.

   * Delayed branch scheduling.  This optional pass attempts to find
     instructions that can go into the delay slots of other
     instructions, usually jumps and calls.  The source file name is
     `reorg.c'.

     The option `-dd' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.dbr' to
     the input file name.

   * Branch shortening.  On many RISC machines, branch instructions
     have a limited range.  Thus, longer sequences of instructions must
     be used for long branches.  In this pass, the compiler figures out
     what how far each instruction will be from each other instruction,
     and therefore whether the usual instructions, or the longer
     sequences, must be used for each branch.

   * Conversion from usage of some hard registers to usage of a register
     stack may be done at this point.  Currently, this is supported only
     for the floating-point registers of the Intel 80387 coprocessor.
     The source file name is `reg-stack.c'.

     The options `-dk' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.stack' to
     the input file name.

   * Final.  This pass outputs the assembler code for the function.  It
     is also responsible for identifying spurious test and compare
     instructions.  Machine-specific peephole optimizations are
     performed at the same time.  The function entry and exit sequences
     are generated directly as assembler code in this pass; they never
     exist as RTL.

     The source files are `final.c' plus `insn-output.c'; the latter is
     generated automatically from the machine description by the tool
     `genoutput'.  The header file `conditions.h' is used for
     communication between these files.

   * Debugging information output.  This is run after final because it
     must output the stack slot offsets for pseudo registers that did
     not get hard registers.  Source files are `dbxout.c' for DBX
     symbol table format, `sdbout.c' for SDB symbol table format,
     `dwarfout.c' for DWARF symbol table format, files `dwarf2out.c' and
     `dwarf2asm.c' for DWARF2 symbol table format, and `vmsdbgout.c'
     for VMS debug symbol table format.

   Some additional files are used by all or many passes:

   * Every pass uses `machmode.def' and `machmode.h' which define the
     machine modes.

   * Several passes use `real.h', which defines the default
     representation of floating point constants and how to operate on
     them.

   * All the passes that work with RTL use the header files `rtl.h' and
     `rtl.def', and subroutines in file `rtl.c'.  The tools `gen*' also
     use these files to read and work with the machine description RTL.

   * All the tools that read the machine description use support
     routines found in `gensupport.c', `errors.c', and `read-rtl.c'.

   * Several passes refer to the header file `insn-config.h' which
     contains a few parameters (C macro definitions) generated
     automatically from the machine description RTL by the tool
     `genconfig'.

   * Several passes use the instruction recognizer, which consists of
     `recog.c' and `recog.h', plus the files `insn-recog.c' and
     `insn-extract.c' that are generated automatically from the machine
     description by the tools `genrecog' and `genextract'.

   * Several passes use the header files `regs.h' which defines the
     information recorded about pseudo register usage, and
     `basic-block.h' which defines the information recorded about basic
     blocks.

   * `hard-reg-set.h' defines the type `HARD_REG_SET', a bit-vector
     with a bit for each hard register, and some macros to manipulate
     it.  This type is just `int' if the machine has few enough hard
     registers; otherwise it is an array of `int' and some of the
     macros expand into loops.

   * Several passes use instruction attributes.  A definition of the
     attributes defined for a particular machine is in file
     `insn-attr.h', which is generated from the machine description by
     the program `genattr'.  The file `insn-attrtab.c' contains
     subroutines to obtain the attribute values for insns and
     information about processor pipeline characteristics for the
     instruction scheduler.  It is generated from the machine
     description by the program `genattrtab'.


File: gccint.info,  Node: Trees,  Next: RTL,  Prev: Passes,  Up: Top

Trees: The intermediate representation used by the C and C++ front ends
***********************************************************************

   This chapter documents the internal representation used by GCC to
represent C and C++ source programs.  When presented with a C or C++
source program, GCC parses the program, performs semantic analysis
(including the generation of error messages), and then produces the
internal representation described here.  This representation contains a
complete representation for the entire translation unit provided as
input to the front end.  This representation is then typically processed
by a code-generator in order to produce machine code, but could also be
used in the creation of source browsers, intelligent editors, automatic
documentation generators, interpreters, and any other programs needing
the ability to process C or C++ code.

   This chapter explains the internal representation.  In particular, it
documents the internal representation for C and C++ source constructs,
and the macros, functions, and variables that can be used to access
these constructs.  The C++ representation is largely a superset of the
representation used in the C front end.  There is only one construct
used in C that does not appear in the C++ front end and that is the GNU
"nested function" extension.  Many of the macros documented here do not
apply in C because the corresponding language constructs do not appear
in C.

   If you are developing a "back end", be it is a code-generator or some
other tool, that uses this representation, you may occasionally find
that you need to ask questions not easily answered by the functions and
macros available here.  If that situation occurs, it is quite likely
that GCC already supports the functionality you desire, but that the
interface is simply not documented here.  In that case, you should ask
the GCC maintainers (via mail to <gcc@gcc.gnu.org>) about documenting
the functionality you require.  Similarly, if you find yourself writing
functions that do not deal directly with your back end, but instead
might be useful to other people using the GCC front end, you should
submit your patches for inclusion in GCC.

* Menu:

* Deficiencies::        Topics net yet covered in this document.
* Tree overview::       All about `tree's.
* Types::               Fundamental and aggregate types.
* Scopes::              Namespaces and classes.
* Functions::           Overloading, function bodies, and linkage.
* Declarations::        Type declarations and variables.
* Attributes::          Declaration and type attributes.
* Expression trees::    From `typeid' to `throw'.


File: gccint.info,  Node: Deficiencies,  Next: Tree overview,  Up: Trees

Deficiencies
============

   There are many places in which this document is incomplet and
incorrekt.  It is, as of yet, only _preliminary_ documentation.


File: gccint.info,  Node: Tree overview,  Next: Types,  Prev: Deficiencies,  Up: Trees

Overview
========

   The central data structure used by the internal representation is the
`tree'.  These nodes, while all of the C type `tree', are of many
varieties.  A `tree' is a pointer type, but the object to which it
points may be of a variety of types.  From this point forward, we will
refer to trees in ordinary type, rather than in `this font', except
when talking about the actual C type `tree'.

   You can tell what kind of node a particular tree is by using the
`TREE_CODE' macro.  Many, many macros take trees as input and return
trees as output.  However, most macros require a certain kind of tree
node as input.  In other words, there is a type-system for trees, but
it is not reflected in the C type-system.

   For safety, it is useful to configure GCC with `--enable-checking'.
Although this results in a significant performance penalty (since all
tree types are checked at run-time), and is therefore inappropriate in a
release version, it is extremely helpful during the development process.

   Many macros behave as predicates.  Many, although not all, of these
predicates end in `_P'.  Do not rely on the result type of these macros
being of any particular type.  You may, however, rely on the fact that
the type can be compared to `0', so that statements like
     if (TEST_P (t) && !TEST_P (y))
       x = 1;

and
     int i = (TEST_P (t) != 0);

are legal.  Macros that return `int' values now may be changed to
return `tree' values, or other pointers in the future.  Even those that
continue to return `int' may return multiple nonzero codes where
previously they returned only zero and one.  Therefore, you should not
write code like
     if (TEST_P (t) == 1)

as this code is not guaranteed to work correctly in the future.

   You should not take the address of values returned by the macros or
functions described here.  In particular, no guarantee is given that the
values are lvalues.

   In general, the names of macros are all in uppercase, while the
names of functions are entirely in lowercase.  There are rare
exceptions to this rule.  You should assume that any macro or function
whose name is made up entirely of uppercase letters may evaluate its
arguments more than once.  You may assume that a macro or function
whose name is made up entirely of lowercase letters will evaluate its
arguments only once.

   The `error_mark_node' is a special tree.  Its tree code is
`ERROR_MARK', but since there is only ever one node with that code, the
usual practice is to compare the tree against `error_mark_node'.  (This
test is just a test for pointer equality.)  If an error has occurred
during front-end processing the flag `errorcount' will be set.  If the
front end has encountered code it cannot handle, it will issue a
message to the user and set `sorrycount'.  When these flags are set,
any macro or function which normally returns a tree of a particular
kind may instead return the `error_mark_node'.  Thus, if you intend to
do any processing of erroneous code, you must be prepared to deal with
the `error_mark_node'.

   Occasionally, a particular tree slot (like an operand to an
expression, or a particular field in a declaration) will be referred to
as "reserved for the back end."  These slots are used to store RTL when
the tree is converted to RTL for use by the GCC back end.  However, if
that process is not taking place (e.g., if the front end is being hooked
up to an intelligent editor), then those slots may be used by the back
end presently in use.

   If you encounter situations that do not match this documentation,
such as tree nodes of types not mentioned here, or macros documented to
return entities of a particular kind that instead return entities of
some different kind, you have found a bug, either in the front end or in
the documentation.  Please report these bugs as you would any other bug.

* Menu:

* Macros and Functions::Macros and functions that can be used with all trees.
* Identifiers::         The names of things.
* Containers::          Lists and vectors.


File: gccint.info,  Node: Macros and Functions,  Next: Identifiers,  Up: Tree overview

Trees
-----

   This section is not here yet.


File: gccint.info,  Node: Identifiers,  Next: Containers,  Prev: Macros and Functions,  Up: Tree overview

Identifiers
-----------

   An `IDENTIFIER_NODE' represents a slightly more general concept that
the standard C or C++ concept of identifier.  In particular, an
`IDENTIFIER_NODE' may contain a `$', or other extraordinary characters.

   There are never two distinct `IDENTIFIER_NODE's representing the
same identifier.  Therefore, you may use pointer equality to compare
`IDENTIFIER_NODE's, rather than using a routine like `strcmp'.

   You can use the following macros to access identifiers:
`IDENTIFIER_POINTER'
     The string represented by the identifier, represented as a
     `char*'.  This string is always `NUL'-terminated, and contains no
     embedded `NUL' characters.

`IDENTIFIER_LENGTH'
     The length of the string returned by `IDENTIFIER_POINTER', not
     including the trailing `NUL'.  This value of `IDENTIFIER_LENGTH
     (x)' is always the same as `strlen (IDENTIFIER_POINTER (x))'.

`IDENTIFIER_OPNAME_P'
     This predicate holds if the identifier represents the name of an
     overloaded operator.  In this case, you should not depend on the
     contents of either the `IDENTIFIER_POINTER' or the
     `IDENTIFIER_LENGTH'.

`IDENTIFIER_TYPENAME_P'
     This predicate holds if the identifier represents the name of a
     user-defined conversion operator.  In this case, the `TREE_TYPE' of
     the `IDENTIFIER_NODE' holds the type to which the conversion
     operator converts.


File: gccint.info,  Node: Containers,  Prev: Identifiers,  Up: Tree overview

Containers
----------

   Two common container data structures can be represented directly with
tree nodes.  A `TREE_LIST' is a singly linked list containing two trees
per node.  These are the `TREE_PURPOSE' and `TREE_VALUE' of each node.
(Often, the `TREE_PURPOSE' contains some kind of tag, or additional
information, while the `TREE_VALUE' contains the majority of the
payload.  In other cases, the `TREE_PURPOSE' is simply `NULL_TREE',
while in still others both the `TREE_PURPOSE' and `TREE_VALUE' are of
equal stature.)  Given one `TREE_LIST' node, the next node is found by
following the `TREE_CHAIN'.  If the `TREE_CHAIN' is `NULL_TREE', then
you have reached the end of the list.

   A `TREE_VEC' is a simple vector.  The `TREE_VEC_LENGTH' is an
integer (not a tree) giving the number of nodes in the vector.  The
nodes themselves are accessed using the `TREE_VEC_ELT' macro, which
takes two arguments.  The first is the `TREE_VEC' in question; the
second is an integer indicating which element in the vector is desired.
The elements are indexed from zero.


File: gccint.info,  Node: Types,  Next: Scopes,  Prev: Tree overview,  Up: Trees

Types
=====

   All types have corresponding tree nodes.  However, you should not
assume that there is exactly one tree node corresponding to each type.
There are often several nodes each of which correspond to the same type.

   For the most part, different kinds of types have different tree
codes.  (For example, pointer types use a `POINTER_TYPE' code while
arrays use an `ARRAY_TYPE' code.)  However, pointers to member functions
use the `RECORD_TYPE' code.  Therefore, when writing a `switch'
statement that depends on the code associated with a particular type,
you should take care to handle pointers to member functions under the
`RECORD_TYPE' case label.

   In C++, an array type is not qualified; rather the type of the array
elements is qualified.  This situation is reflected in the intermediate
representation.  The macros described here will always examine the
qualification of the underlying element type when applied to an array
type.  (If the element type is itself an array, then the recursion
continues until a non-array type is found, and the qualification of this
type is examined.)  So, for example, `CP_TYPE_CONST_P' will hold of the
type `const int ()[7]', denoting an array of seven `int's.

   The following functions and macros deal with cv-qualification of
types:
`CP_TYPE_QUALS'
     This macro returns the set of type qualifiers applied to this type.
     This value is `TYPE_UNQUALIFIED' if no qualifiers have been
     applied.  The `TYPE_QUAL_CONST' bit is set if the type is
     `const'-qualified.  The `TYPE_QUAL_VOLATILE' bit is set if the
     type is `volatile'-qualified.  The `TYPE_QUAL_RESTRICT' bit is set
     if the type is `restrict'-qualified.

`CP_TYPE_CONST_P'
     This macro holds if the type is `const'-qualified.

`CP_TYPE_VOLATILE_P'
     This macro holds if the type is `volatile'-qualified.

`CP_TYPE_RESTRICT_P'
     This macro holds if the type is `restrict'-qualified.

`CP_TYPE_CONST_NON_VOLATILE_P'
     This predicate holds for a type that is `const'-qualified, but
     _not_ `volatile'-qualified; other cv-qualifiers are ignored as
     well: only the `const'-ness is tested.

`TYPE_MAIN_VARIANT'
     This macro returns the unqualified version of a type.  It may be
     applied to an unqualified type, but it is not always the identity
     function in that case.

   A few other macros and functions are usable with all types:
`TYPE_SIZE'
     The number of bits required to represent the type, represented as
     an `INTEGER_CST'.  For an incomplete type, `TYPE_SIZE' will be
     `NULL_TREE'.

`TYPE_ALIGN'
     The alignment of the type, in bits, represented as an `int'.

`TYPE_NAME'
     This macro returns a declaration (in the form of a `TYPE_DECL') for
     the type.  (Note this macro does _not_ return a `IDENTIFIER_NODE',
     as you might expect, given its name!)  You can look at the
     `DECL_NAME' of the `TYPE_DECL' to obtain the actual name of the
     type.  The `TYPE_NAME' will be `NULL_TREE' for a type that is not
     a built-in type, the result of a typedef, or a named class type.

`CP_INTEGRAL_TYPE'
     This predicate holds if the type is an integral type.  Notice that
     in C++, enumerations are _not_ integral types.

`ARITHMETIC_TYPE_P'
     This predicate holds if the type is an integral type (in the C++
     sense) or a floating point type.

`CLASS_TYPE_P'
     This predicate holds for a class-type.

`TYPE_BUILT_IN'
     This predicate holds for a built-in type.

`TYPE_PTRMEM_P'
     This predicate holds if the type is a pointer to data member.

`TYPE_PTR_P'
     This predicate holds if the type is a pointer type, and the
     pointee is not a data member.

`TYPE_PTRFN_P'
     This predicate holds for a pointer to function type.

`TYPE_PTROB_P'
     This predicate holds for a pointer to object type.  Note however
     that it does not hold for the generic pointer to object type `void
     *'.  You may use `TYPE_PTROBV_P' to test for a pointer to object
     type as well as `void *'.

`same_type_p'
     This predicate takes two types as input, and holds if they are the
     same type.  For example, if one type is a `typedef' for the other,
     or both are `typedef's for the same type.  This predicate also
     holds if the two trees given as input are simply copies of one
     another; i.e., there is no difference between them at the source
     level, but, for whatever reason, a duplicate has been made in the
     representation.  You should never use `==' (pointer equality) to
     compare types; always use `same_type_p' instead.

   Detailed below are the various kinds of types, and the macros that
can be used to access them.  Although other kinds of types are used
elsewhere in G++, the types described here are the only ones that you
will encounter while examining the intermediate representation.

`VOID_TYPE'
     Used to represent the `void' type.

`INTEGER_TYPE'
     Used to represent the various integral types, including `char',
     `short', `int', `long', and `long long'.  This code is not used
     for enumeration types, nor for the `bool' type.  Note that GCC's
     `CHAR_TYPE' node is _not_ used to represent `char'.  The
     `TYPE_PRECISION' is the number of bits used in the representation,
     represented as an `unsigned int'.  (Note that in the general case
     this is not the same value as `TYPE_SIZE'; suppose that there were
     a 24-bit integer type, but that alignment requirements for the ABI
     required 32-bit alignment.  Then, `TYPE_SIZE' would be an
     `INTEGER_CST' for 32, while `TYPE_PRECISION' would be 24.)  The
     integer type is unsigned if `TREE_UNSIGNED' holds; otherwise, it
     is signed.

     The `TYPE_MIN_VALUE' is an `INTEGER_CST' for the smallest integer
     that may be represented by this type.  Similarly, the
     `TYPE_MAX_VALUE' is an `INTEGER_CST' for the largest integer that
     may be represented by this type.

`REAL_TYPE'
     Used to represent the `float', `double', and `long double' types.
     The number of bits in the floating-point representation is given
     by `TYPE_PRECISION', as in the `INTEGER_TYPE' case.

`COMPLEX_TYPE'
     Used to represent GCC built-in `__complex__' data types.  The
     `TREE_TYPE' is the type of the real and imaginary parts.

`ENUMERAL_TYPE'
     Used to represent an enumeration type.  The `TYPE_PRECISION' gives
     (as an `int'), the number of bits used to represent the type.  If
     there are no negative enumeration constants, `TREE_UNSIGNED' will
     hold.  The minimum and maximum enumeration constants may be
     obtained with `TYPE_MIN_VALUE' and `TYPE_MAX_VALUE', respectively;
     each of these macros returns an `INTEGER_CST'.

     The actual enumeration constants themselves may be obtained by
     looking at the `TYPE_VALUES'.  This macro will return a
     `TREE_LIST', containing the constants.  The `TREE_PURPOSE' of each
     node will be an `IDENTIFIER_NODE' giving the name of the constant;
     the `TREE_VALUE' will be an `INTEGER_CST' giving the value
     assigned to that constant.  These constants will appear in the
     order in which they were declared.  The `TREE_TYPE' of each of
     these constants will be the type of enumeration type itself.

`BOOLEAN_TYPE'
     Used to represent the `bool' type.

`POINTER_TYPE'
     Used to represent pointer types, and pointer to data member types.
     The `TREE_TYPE' gives the type to which this type points.  If the
     type is a pointer to data member type, then `TYPE_PTRMEM_P' will
     hold.  For a pointer to data member type of the form `T X::*',
     `TYPE_PTRMEM_CLASS_TYPE' will be the type `X', while
     `TYPE_PTRMEM_POINTED_TO_TYPE' will be the type `T'.

`REFERENCE_TYPE'
     Used to represent reference types.  The `TREE_TYPE' gives the type
     to which this type refers.

`FUNCTION_TYPE'
     Used to represent the type of non-member functions and of static
     member functions.  The `TREE_TYPE' gives the return type of the
     function.  The `TYPE_ARG_TYPES' are a `TREE_LIST' of the argument
     types.  The `TREE_VALUE' of each node in this list is the type of
     the corresponding argument; the `TREE_PURPOSE' is an expression
     for the default argument value, if any.  If the last node in the
     list is `void_list_node' (a `TREE_LIST' node whose `TREE_VALUE' is
     the `void_type_node'), then functions of this type do not take
     variable arguments.  Otherwise, they do take a variable number of
     arguments.

     Note that in C (but not in C++) a function declared like `void f()'
     is an unprototyped function taking a variable number of arguments;
     the `TYPE_ARG_TYPES' of such a function will be `NULL'.

`METHOD_TYPE'
     Used to represent the type of a non-static member function.  Like a
     `FUNCTION_TYPE', the return type is given by the `TREE_TYPE'.  The
     type of `*this', i.e., the class of which functions of this type
     are a member, is given by the `TYPE_METHOD_BASETYPE'.  The
     `TYPE_ARG_TYPES' is the parameter list, as for a `FUNCTION_TYPE',
     and includes the `this' argument.

`ARRAY_TYPE'
     Used to represent array types.  The `TREE_TYPE' gives the type of
     the elements in the array.  If the array-bound is present in the
     type, the `TYPE_DOMAIN' is an `INTEGER_TYPE' whose
     `TYPE_MIN_VALUE' and `TYPE_MAX_VALUE' will be the lower and upper
     bounds of the array, respectively.  The `TYPE_MIN_VALUE' will
     always be an `INTEGER_CST' for zero, while the `TYPE_MAX_VALUE'
     will be one less than the number of elements in the array, i.e.,
     the highest value which may be used to index an element in the
     array.

`RECORD_TYPE'
     Used to represent `struct' and `class' types, as well as pointers
     to member functions and similar constructs in other languages.
     `TYPE_FIELDS' contains the items contained in this type, each of
     which can be a `FIELD_DECL', `VAR_DECL', `CONST_DECL', or
     `TYPE_DECL'.  You may not make any assumptions about the ordering
     of the fields in the type or whether one or more of them overlap.
     If `TYPE_PTRMEMFUNC_P' holds, then this type is a pointer-to-member
     type.  In that case, the `TYPE_PTRMEMFUNC_FN_TYPE' is a
     `POINTER_TYPE' pointing to a `METHOD_TYPE'.  The `METHOD_TYPE' is
     the type of a function pointed to by the pointer-to-member
     function.  If `TYPE_PTRMEMFUNC_P' does not hold, this type is a
     class type.  For more information, see *note Classes::.

`UNION_TYPE'
     Used to represent `union' types.  Similar to `RECORD_TYPE' except
     that all `FIELD_DECL' nodes in `TYPE_FIELD' start at bit position
     zero.

`QUAL_UNION_TYPE'
     Used to represent part of a variant record in Ada.  Similar to
     `UNION_TYPE' except that each `FIELD_DECL' has a `DECL_QUALIFIER'
     field, which contains a boolean expression that indicates whether
     the field is present in the object.  The type will only have one
     field, so each field's `DECL_QUALIFIER' is only evaluated if none
     of the expressions in the previous fields in `TYPE_FIELDS' are
     nonzero.  Normally these expressions will reference a field in the
     outer object using a `PLACEHOLDER_EXPR'.

`UNKNOWN_TYPE'
     This node is used to represent a type the knowledge of which is
     insufficient for a sound processing.

`OFFSET_TYPE'
     This node is used to represent a pointer-to-data member.  For a
     data member `X::m' the `TYPE_OFFSET_BASETYPE' is `X' and the
     `TREE_TYPE' is the type of `m'.

`TYPENAME_TYPE'
     Used to represent a construct of the form `typename T::A'.  The
     `TYPE_CONTEXT' is `T'; the `TYPE_NAME' is an `IDENTIFIER_NODE' for
     `A'.  If the type is specified via a template-id, then
     `TYPENAME_TYPE_FULLNAME' yields a `TEMPLATE_ID_EXPR'.  The
     `TREE_TYPE' is non-`NULL' if the node is implicitly generated in
     support for the implicit typename extension; in which case the
     `TREE_TYPE' is a type node for the base-class.

`TYPEOF_TYPE'
     Used to represent the `__typeof__' extension.  The `TYPE_FIELDS'
     is the expression the type of which is being represented.

   There are variables whose values represent some of the basic types.
These include:
`void_type_node'
     A node for `void'.

`integer_type_node'
     A node for `int'.

`unsigned_type_node.'
     A node for `unsigned int'.

`char_type_node.'
     A node for `char'.

It may sometimes be useful to compare one of these variables with a type
in hand, using `same_type_p'.


File: gccint.info,  Node: Scopes,  Next: Functions,  Prev: Types,  Up: Trees

Scopes
======

   The root of the entire intermediate representation is the variable
`global_namespace'.  This is the namespace specified with `::' in C++
source code.  All other namespaces, types, variables, functions, and so
forth can be found starting with this namespace.

   Besides namespaces, the other high-level scoping construct in C++ is
the class.  (Throughout this manual the term "class" is used to mean the
types referred to in the ANSI/ISO C++ Standard as classes; these include
types defined with the `class', `struct', and `union' keywords.)

* Menu:

* Namespaces::          Member functions, types, etc.
* Classes::             Members, bases, friends, etc.

