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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
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   Permission is granted to copy, distribute and/or modify this document
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Invariant Sections being "GNU General Public License" and "Funding Free
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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: gccint.info,  Node: Back End,  Prev: Front End,  Up: gcc Directory

Anatomy of a Target Back End
----------------------------

   A back end for a target architecture in GCC has the following parts:

   * A directory `MACHINE' under `gcc/config', containing a machine
     description `MACHINE.md' file (*note Machine Descriptions: Machine
     Desc.), header files `MACHINE.h' and `MACHINE-protos.h' and a
     source file `MACHINE.c' (*note Target Description Macros and
     Functions: Target Macros.), possibly a target Makefile fragment
     `t-MACHINE' (*note The Target Makefile Fragment: Target
     Fragment.), and maybe some other files.  The names of these files
     may be changed from the defaults given by explicit specifications
     in `config.gcc'.

   * Entries in `config.gcc' (*note The `config.gcc' File: System
     Config.) for the systems with this target architecture.

   * Documentation in `gcc/doc/invoke.texi' for any command-line
     options supported by this target (*note Run-time Target
     Specification: Run-time Target.).  This means both entries in the
     summary table of options and details of the individual options.

   * Documentation in `gcc/doc/extend.texi' for any target-specific
     attributes supported (*note Defining target-specific uses of
     `__attribute__': Target Attributes.), including where the same
     attribute is already supported on some targets, which are
     enumerated in the manual.

   * Documentation in `gcc/doc/extend.texi' for any target-specific
     pragmas supported.

   * Documentation in `gcc/doc/extend.texi' of any target-specific
     built-in functions supported.

   * Documentation in `gcc/doc/md.texi' of any target-specific
     constraint letters (*note Constraints for Particular Machines:
     Machine Constraints.).

   * A note in `gcc/doc/contrib.texi' under the person or people who
     contributed the target support.

   * Entries in `gcc/doc/install.texi' for all target triplets
     supported with this target architecture, giving details of any
     special notes about installation for this target, or saying that
     there are no special notes if there are none.

   * Possibly other support outside the `gcc' directory for runtime
     libraries.  FIXME: reference docs for this.  The libstdc++ porting
     manual needs to be installed as info for this to work, or to be a
     chapter of this manual.

   If the back end is added to the official GCC CVS repository, the
following are also necessary:

   * An entry for the target architecture in `readings.html' on the GCC
     web site, with any relevant links.

   * A news item about the contribution of support for that target
     architecture, in `index.html' on the GCC web site.

   * Normally, one or more maintainers of that target listed in
     `MAINTAINERS'.  Some existing architectures may be unmaintained,
     but it would be unusual to add support for a target that does not
     have a maintainer when support is added.


File: gccint.info,  Node: Test Suites,  Prev: gcc Directory,  Up: Source Tree

Test Suites
===========

   GCC contains several test suites to help maintain compiler quality.
Most of the runtime libraries and language front ends in GCC have test
suites.  Currently only the C language test suites are documented here;
FIXME: document the others.

* Menu:

* Test Idioms::  Idioms used in test suite code.
* C Tests::      The C language test suites.


File: gccint.info,  Node: Test Idioms,  Next: C Tests,  Up: Test Suites

Idioms Used in Test Suite Code
------------------------------

   In the `gcc.c-torture' test suites, test cases are commonly named
after the date on which they were added.  This allows people to tell at
a glance whether a test failure is because of a recently found bug that
has not yet been fixed, or whether it may be a regression.  In other
test suites, more descriptive names are used.  In general C test cases
have a trailing `-N.c', starting with `-1.c', in case other test cases
with similar names are added later.

   Test cases should use `abort ()' to indicate failure and `exit (0)'
for success; on some targets these may be redefined to indicate failure
and success in other ways.

   In the `gcc.dg' test suite, it is often necessary to test that an
error is indeed a hard error and not just a warning--for example, where
it is a constraint violation in the C standard, which must become an
error with `-pedantic-errors'.  The following idiom, where the first
line shown is line LINE of the file and the line that generates the
error, is used for this:

     /* { dg-bogus "warning" "warning in place of error" } */
     /* { dg-error "REGEXP" "MESSAGE" { target *-*-* } LINE } */

   It may be necessary to check that an expression is an integer
constant expression and has a certain value.  To check that `E' has
value `V', an idiom similar to the following is used:

     char x[((E) == (V) ? 1 : -1)];

   In `gcc.dg' tests, `__typeof__' is sometimes used to make assertions
about the types of expressions.  See, for example,
`gcc.dg/c99-condexpr-1.c'.  The more subtle uses depend on the exact
rules for the types of conditional expressions in the C standard; see,
for example, `gcc.dg/c99-intconst-1.c'.

   It is useful to be able to test that optimizations are being made
properly.  This cannot be done in all cases, but it can be done where
the optimization will lead to code being optimized away (for example,
where flow analysis or alias analysis should show that certain code
cannot be called) or to functions not being called because they have
been expanded as built-in functions.  Such tests go in
`gcc.c-torture/execute'.  Where code should be optimized away, a call
to a nonexistent function such as `link_failure ()' may be inserted; a
definition

     #ifndef __OPTIMIZE__
     void
     link_failure (void)
     {
       abort ();
     }
     #endif

will also be needed so that linking still succeeds when the test is run
without optimization.  When all calls to a built-in function should
have been optimized and no calls to the non-built-in version of the
function should remain, that function may be defined as `static' to
call `abort ()' (although redeclaring a function as static may not work
on all targets).

   FIXME: discuss non-C test suites here.


File: gccint.info,  Node: C Tests,  Prev: Test Idioms,  Up: Test Suites

C Language Test Suites
----------------------

   GCC contains the following C language test suites, in the
`gcc/testsuite' directory:

`gcc.c-torture/compat'
     FIXME: describe this.

     This directory should probably not be used for new tests.

`gcc.c-torture/compile'
     This test suite contains test cases that should compile, but do not
     need to link or run.  These test cases are compiled with several
     different combinations of optimization options.  All warnings are
     disabled for these test cases, so this directory is not suitable if
     you wish to test for the presence or absence of compiler warnings.
     While special options can be set, and tests disabled on specific
     platforms, by the use of `.x' files, mostly these test cases
     should not contain platform dependencies.  FIXME: discuss how
     defines such as `NO_LABEL_VALUES' and `STACK_SIZE' are used.

`gcc.c-torture/execute'
     This test suite contains test cases that should compile, link and
     run; otherwise the same comments as for `gcc.c-torture/compile'
     apply.

`gcc.c-torture/unsorted'
     FIXME: describe this.

     This directory should probably not be used for new tests.

`gcc.dg'
     This test suite contains tests using the more modern `dg' harness.
     Magic comments determine whether the file is preprocessed,
     compiled, linked or run.  In these tests, error and warning
     message texts are compared against expected texts or regular
     expressions given in comments.  These tests are run with the
     options `-ansi -pedantic' unless other options are given in the
     test.  Except as noted below they are not run with multiple
     optimization options.

`gcc.dg/cpp'
     This subdirectory contains tests of the preprocessor.

`gcc.dg/debug'
     This subdirectory contains tests for debug formats.  Tests in this
     subdirectory are run for each debug format that the compiler
     supports.

`gcc.dg/format'
     This subdirectory contains tests of the `-Wformat' format
     checking.  Tests in this directory are run with and without
     `-DWIDE'.

`gcc.dg/noncompile'
     This subdirectory contains tests of code that should not compile
     and does not need any special compilation options.  They are run
     with multiple optimization options, since sometimes invalid code
     crashes the compiler with optimization.

`gcc.dg/special'
     FIXME: describe this.

`gcc.c-torture/misc-tests'
     FIXME: describe this, when it should be used for new tests and
     when it shouldn't.

   FIXME: merge in `testsuite/README.gcc' and discuss the format of
test cases and magic comments more.


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.

   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 `c-convert.c', `c-decl.c',
     `c-errors.c', `c-lang.c', `c-objc-common.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-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 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.

   * 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, 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.  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 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: 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.

