This is doc/gcc.info, produced by makeinfo version 4.0b from
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INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* gcc: (gcc).                  The GNU Compiler Collection.
END-INFO-DIR-ENTRY
   This file documents the use and the internals of the GNU compiler.


   Published by the Free Software Foundation
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Boston, MA 02111-1307 USA


   Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
1999, 2000, 2001 Free Software Foundation, Inc.


   Permission is granted to copy, distribute and/or modify this document
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any later version published by the Free Software Foundation; with the
Invariant Sections being "GNU General Public License" and "Funding Free
Software", the Front-Cover texts being (a) (see below), and with the
Back-Cover Texts being (b) (see below).  A copy of the license is
included in the section entitled "GNU Free Documentation License".

   (a) The FSF's Front-Cover Text is:

   A GNU Manual

   (b) The FSF's Back-Cover Text is:

   You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.


File: gcc.info,  Node: VMS Misc,  Prev: Global Declarations,  Up: VMS

Other VMS Issues
================

   GCC automatically arranges for `main' to return 1 by default if you
fail to specify an explicit return value.  This will be interpreted by
VMS as a status code indicating a normal successful completion.
Version 1 of GCC did not provide this default.

   GCC on VMS works only with the GNU assembler, GAS.  You need version
1.37 or later of GAS in order to produce value debugging information for
the VMS debugger.  Use the ordinary VMS linker with the object files
produced by GAS.

   Under previous versions of GCC, the generated code would occasionally
give strange results when linked to the sharable `VAXCRTL' library.
Now this should work.

   A caveat for use of `const' global variables: the `const' modifier
must be specified in every external declaration of the variable in all
of the source files that use that variable.  Otherwise the linker will
issue warnings about conflicting attributes for the variable.  Your
program will still work despite the warnings, but the variable will be
placed in writable storage.

   Although the VMS linker does distinguish between upper and lower case
letters in global symbols, most VMS compilers convert all such symbols
into upper case and most run-time library routines also have upper case
names.  To be able to reliably call such routines, GCC (by means of the
assembler GAS) converts global symbols into upper case like other VMS
compilers.  However, since the usual practice in C is to distinguish
case, GCC (via GAS) tries to preserve usual C behavior by augmenting
each name that is not all lower case.  This means truncating the name
to at most 23 characters and then adding more characters at the end
which encode the case pattern of those 23.   Names which contain at
least one dollar sign are an exception; they are converted directly into
upper case without augmentation.

   Name augmentation yields bad results for programs that use
precompiled libraries (such as Xlib) which were generated by another
compiler.  You can use the compiler option `/NOCASE_HACK' to inhibit
augmentation; it makes external C functions and variables
case-independent as is usual on VMS.  Alternatively, you could write
all references to the functions and variables in such libraries using
lower case; this will work on VMS, but is not portable to other
systems.  The compiler option `/NAMES' also provides control over
global name handling.

   Function and variable names are handled somewhat differently with
G++.  The GNU C++ compiler performs "name mangling" on function names,
which means that it adds information to the function name to describe
the data types of the arguments that the function takes.  One result of
this is that the name of a function can become very long.  Since the
VMS linker only recognizes the first 31 characters in a name, special
action is taken to ensure that each function and variable has a unique
name that can be represented in 31 characters.

   If the name (plus a name augmentation, if required) is less than 32
characters in length, then no special action is performed.  If the name
is longer than 31 characters, the assembler (GAS) will generate a hash
string based upon the function name, truncate the function name to 23
characters, and append the hash string to the truncated name.  If the
`/VERBOSE' compiler option is used, the assembler will print both the
full and truncated names of each symbol that is truncated.

   The `/NOCASE_HACK' compiler option should not be used when you are
compiling programs that use libg++.  libg++ has several instances of
objects (i.e.  `Filebuf' and `filebuf') which become indistinguishable
in a case-insensitive environment.  This leads to cases where you need
to inhibit augmentation selectively (if you were using libg++ and Xlib
in the same program, for example).  There is no special feature for
doing this, but you can get the result by defining a macro for each
mixed case symbol for which you wish to inhibit augmentation.  The
macro should expand into the lower case equivalent of itself.  For
example:

     #define StuDlyCapS studlycaps

   These macro definitions can be placed in a header file to minimize
the number of changes to your source code.


File: gcc.info,  Node: Makefile,  Next: Portability,  Prev: VMS,  Up: Top

Additional Makefile and configure information.
**********************************************

Makefile Targets
================

`all'
     This is the default target.  Depending on what your
     build/host/target configuration is, it coordinates all the things
     that need to be built.

`doc'
     Produce info-formatted documentation.  Also, `make dvi' is
     available for DVI-formatted documentation, and `make
     generated-manpages' to generate man pages.

`mostlyclean'
     Delete the files made while building the compiler.

`clean'
     That, and all the other files built by `make all'.

`distclean'
     That, and all the files created by `configure'.

`extraclean'
     That, and any temporary or intermediate files, like emacs backup
     files.

`maintainer-clean'
     Distclean plus any file that can be generated from other files.
     Note that additional tools may be required beyond what is normally
     needed to build gcc.

`install'
     Installs gcc.

`uninstall'
     Deletes installed files.

`check'
     Run the testsuite.  This creates a `testsuite' subdirectory that
     has various `.sum' and `.log' files containing the results of the
     testing.  You can run subsets with, for example, `make check-gcc'.
     You can specify specific tests by setting RUNTESTFLAGS to be the
     name of the `.exp' file, optionally followed by (for some tests)
     an equals and a file wildcard, like:

          make check-gcc RUNTESTFLAGS="execute.exp=19980413-*"

     Note that running the testsuite may require additional tools be
     installed, such as TCL or dejagnu.

`bootstrap'
     Builds gcc three times--once with the native compiler, once with
     the native-built compiler it just built, and once with the
     compiler it built the second time.  In theory, the last two should
     produce the same results, which `make compare' can check.  Each
     step of this process is called a "stage", and the results of each
     stage N (N = 1...3) are copied to a subdirectory `stageN/'.

`bootstrap-lean'
     Like `bootstrap', except that the various stages are removed once
     they're no longer needed.  This saves disk space.

`bubblestrap'
     Once bootstrapped, this incrementally rebuilds each of the three
     stages, one at a time.  It does this by "bubbling" the stages up
     from their subdirectories, rebuilding them, and copying them back
     to their subdirectories.  This will allow you to, for example,
     quickly rebuild a bootstrapped compiler after changing the
     sources, without having to do a full bootstrap.

`quickstrap'
     Rebuilds the most recently built stage.  Since each stage requires
     special invocation, using this target means you don't have to keep
     track of which stage you're on or what invocation that stage needs.

`cleanstrap'
     Removed everything (`make clean') and rebuilds (`make bootstrap').

`stageN (N = 1...4)'
     For each stage, moves the appropriate files to the `stageN'
     subdirectory.

`unstageN (N = 1...4)'
     Undoes the corresponding `stageN'.

`restageN (N = 1...4)'
     Undoes the corresponding `stageN' and rebuilds it with the
     appropriate flags.

`compare'
     Compares the results of stages 2 and 3.  This ensures that the
     compiler is running properly, since it should produce the same
     object files regardless of how it itself was compiled.

Configure Terms and History
===========================

   This section is not instructions for building GCC.  If you are
trying to do a build, you should first read
`http://gcc.gnu.org/install/' or whatever installation instructions
came with your source package.

   The configure and build process has a long and colorful history, and
can be confusing to anyone who doesn't know why things are the way they
are.  While there are other documents which describe the configuration
process in detail, here are a few things that everyone working on GCC
should know.

   There are three system names that the build knows about: the machine
you are building on ("build"), the machine that you are building for
("host"), and the machine that GCC will produce code for ("target").
When you configure GCC, you specify these with `--build=', `--host=',
and `--target='.

   Specifying the host without specifying the build should be avoided,
as `configure' may (and once did) assume that the host you specify is
also the build, which may not be true.

   If build, host, and target are all the same, this is called a
"native".  If build and host are the same but target is different, this
is called a "cross".  If build, host, and target are all different this
is called a "canadian" (for obscure reasons dealing with Canada's
political party and the background of the person working on the build
at that time).  If host and target are the same, but build is
different, you are using a cross-compiler to build a native for a
different system.  Some people call this a "host-x-host", "crossed
native", or "cross-built native".  If build and target are the same,
but host is different, you are using a cross compiler to build a cross
compiler that produces code for the machine you're building on.  This
is rare, so there is no common say of describing it (although I propose
calling it a "crossback").

   If build and host are the same, the GCC you are building will also be
used to build the target libraries (like `libstdc++').  If build and
host are different, you must have already build and installed a cross
compiler that will be used to build the target libraries (if you
configured with `--target=foo-bar', this compiler will be called
`foo-bar-gcc').

   In the case of target libraries, the machine you're building for is
the machine you specified with `--target'.  So, build is the machine
you're building on (no change there), host is the machine you're
building for (the target libraries are built for the target, so host is
the target you specified), and target doesn't apply (because you're not
building a compiler, you're building libraries).  The configure/make
process will adjust these variables as needed.  It also sets
`$with_cross_host' to the original `--host' value in case you need it.

   Libiberty, for example, is built twice.  The first time, host comes
from `--host' and the second time host comes from `--target'.
Historically, libiberty has not been built for the build machine,
though, which causes some interesting issues with programs used to
generate sources for the build.  Fixing this, so that libiberty is built
three times, has long been on the to-do list.


File: gcc.info,  Node: Portability,  Next: Interface,  Prev: Makefile,  Up: Top

GCC and Portability
*******************

   The main goal of GCC was to make a good, fast compiler for machines
in the class that the GNU system aims to run on: 32-bit machines that
address 8-bit bytes and have several general registers.  Elegance,
theoretical power and simplicity are only secondary.

   GCC gets most of the information about the target machine from a
machine description which gives an algebraic formula for each of the
machine's instructions.  This is a very clean way to describe the
target.  But when the compiler needs information that is difficult to
express in this fashion, I have not hesitated to define an ad-hoc
parameter to the machine description.  The purpose of portability is to
reduce the total work needed on the compiler; it was not of interest
for its own sake.

   GCC does not contain machine dependent code, but it does contain code
that depends on machine parameters such as endianness (whether the most
significant byte has the highest or lowest address of the bytes in a
word) and the availability of autoincrement addressing.  In the
RTL-generation pass, it is often necessary to have multiple strategies
for generating code for a particular kind of syntax tree, strategies
that are usable for different combinations of parameters.  Often I have
not tried to address all possible cases, but only the common ones or
only the ones that I have encountered.  As a result, a new target may
require additional strategies.  You will know if this happens because
the compiler will call `abort'.  Fortunately, the new strategies can be
added in a machine-independent fashion, and will affect only the target
machines that need them.


File: gcc.info,  Node: Interface,  Next: Passes,  Prev: Portability,  Up: Top

Interfacing to GCC Output
*************************

   GCC is normally configured to use the same function calling
convention normally in use on the target system.  This is done with the
machine-description macros described (*note Target Macros::).

   However, returning of structure and union values is done differently
on some target machines.  As a result, functions compiled with PCC
returning such types cannot be called from code compiled with GCC, and
vice versa.  This does not cause trouble often because few Unix library
routines return structures or unions.

   GCC code returns structures and unions that are 1, 2, 4 or 8 bytes
long in the same registers used for `int' or `double' return values.
(GCC typically allocates variables of such types in registers also.)
Structures and unions of other sizes are returned by storing them into
an address passed by the caller (usually in a register).  The
machine-description macros `STRUCT_VALUE' and `STRUCT_INCOMING_VALUE'
tell GCC where to pass this address.

   By contrast, PCC on most target machines returns structures and
unions of any size by copying the data into an area of static storage,
and then returning the address of that storage as if it were a pointer
value.  The caller must copy the data from that memory area to the
place where the value is wanted.  This is slower than the method used
by GCC, and fails to be reentrant.

   On some target machines, such as RISC machines and the 80386, the
standard system convention is to pass to the subroutine the address of
where to return the value.  On these machines, GCC has been configured
to be compatible with the standard compiler, when this method is used.
It may not be compatible for structures of 1, 2, 4 or 8 bytes.

   GCC uses the system's standard convention for passing arguments.  On
some machines, the first few arguments are passed in registers; in
others, all are passed on the stack.  It would be possible to use
registers for argument passing on any machine, and this would probably
result in a significant speedup.  But the result would be complete
incompatibility with code that follows the standard convention.  So this
change is practical only if you are switching to GCC as the sole C
compiler for the system.  We may implement register argument passing on
certain machines once we have a complete GNU system so that we can
compile the libraries with GCC.

   On some machines (particularly the Sparc), certain types of arguments
are passed "by invisible reference".  This means that the value is
stored in memory, and the address of the memory location is passed to
the subroutine.

   If you use `longjmp', beware of automatic variables.  ISO C says that
automatic variables that are not declared `volatile' have undefined
values after a `longjmp'.  And this is all GCC promises to do, because
it is very difficult to restore register variables correctly, and one
of GCC's features is that it can put variables in registers without
your asking it to.

   If you want a variable to be unaltered by `longjmp', and you don't
want to write `volatile' because old C compilers don't accept it, just
take the address of the variable.  If a variable's address is ever
taken, even if just to compute it and ignore it, then the variable
cannot go in a register:

     {
       int careful;
       &careful;
       ...
     }

   Code compiled with GCC may call certain library routines.  Most of
them handle arithmetic for which there are no instructions.  This
includes multiply and divide on some machines, and floating point
operations on any machine for which floating point support is disabled
with `-msoft-float'.  Some standard parts of the C library, such as
`bcopy' or `memcpy', are also called automatically.  The usual function
call interface is used for calling the library routines.

   Some of these routines can be defined in mostly machine-independent
C; they appear in `libgcc2.c'.  Others must be hand-written in assembly
language for each processor.  Wherever they are defined, they are
compiled into the support library, `libgcc.a', which is automatically
searched when you link programs with GCC.


File: gcc.info,  Node: Passes,  Next: Trees,  Prev: Interface,  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.

   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: 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 `c-convert.c', `c-decl.c',
     `c-errors.c', `c-lang.c', `c-parse.in', `c-aux-info.c', and
     `c-typeck.c', along with a header file `c-tree.h' and some files
     shared with Objective-C and C++.

     The source files for parsing C++ are in `cp/'.  They are `parse.y',
     `class.c', `cvt.c', `decl.c', `decl2.c', `except.c', `expr.c',
     `init.c', `lex.c', `method.c', `ptree.c', `search.c', `spew.c',
     `semantics.c', `tree.c', `typeck2.c', and `typeck.c', along with
     header files `cp-tree.def', `cp-tree.h', and `decl.h'.

     The special source files for parsing Objective-C are in `objc/'.
     They are `objc-act.c', `objc-tree.def', and `objc-act.h'.  Certain
     C-specific files are used for this as well.

     The files `c-common.c', `c-common.def', `c-dump.c', `c-format.c',
     `c-pragma.c', `c-semantics.c', and `c-lex.c', along with header
     files `c-common.h', `c-dump.h', `c-lex.h', and `c-pragma.h', are
     also used for all of the above languages.

   * 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 for C++ in `cp/optimize.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).  The C front end currently does not perform
     tree based inlining.

     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.

   * Sibiling 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' SSA Aggressive Dead Code Elimination.  Turned on
          by the `-fssa-dce' option.  This pass performs conditional
          constant propagation to simplify instructions including
          conditional branches.  This pass is more aggressive than the
          constant propgation done by the CSE and GCSE pases, 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'.

     The option `-dL' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.loop' 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 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).

   * 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.

   * Jump optimization is repeated, this time including cross-jumping
     and deletion of no-op move instructions.

     The option `-dJ' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.jump2' 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, and the files
     `dwarf2out.c' and `dwarf2asm.c' for DWARF2 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.  It is
     generated from the machine description by the program `genattrtab'.


File: gcc.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 and
C++ 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 which 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: gcc.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: gcc.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 a trees as input and return
trees as output.  However, most macros require a certain kinds 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 lower case.  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: gcc.info,  Node: Macros and Functions,  Next: Identifiers,  Up: Tree overview

Trees
-----

   This section is not here yet.


File: gcc.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.

