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

   Published by the Free Software Foundation
59 Temple Place - Suite 330
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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'.

   * If necessary, a file `MACHINE-modes.def' in the `MACHINE'
     directory, containing additional machine modes to represent
     condition codes.  *Note Condition Code::, for further details.

   * 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.
* libgcj Tests::    The Java library test suites.
* gcov Testing::    Support for testing gcov.
* profopt Testing:: Support for testing profile-directed optimizations.
* compat Testing::  Support for testing binary compatibility.


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).

   All testcases must be portable.  Target-specific testcases must have
appropriate code to avoid causing failures on unsupported systems;
unfortunately, the mechanisms for this differ by directory.

   FIXME: discuss non-C test suites here.


File: gccint.info,  Node: C Tests,  Next: libgcj 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.dg'
     This contains tests of particular features of the C compiler,
     using the more modern `dg' harness.  Correctness tests for various
     compiler features should go here if possible.

     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'
     This contains particular code fragments which have historically
     broken easily.  These tests are run with multiple optimization
     options, so tests for features which only break at some
     optimization levels belong here.  This also contains tests to
     check that certain optimizations occur.  It might be worthwhile to
     separate the correctness tests cleanly from the code quality
     tests, but it hasn't been done yet.

`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/execute/ieee'
     This contains tests which are specific to IEEE floating point.

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

     This directory should probably not be used for new tests.

`gcc.c-torture/misc-tests'
     This directory contains C tests that require special handling.
     Some of these tests have individual expect files, and others share
     special-purpose expect files:

    ``bprob*.c''
          Test `-fbranch-probabilities' using `bprob.exp', which in
          turn uses the generic, language-independent framework (*note
          Support for testing profile-directed optimizations: profopt
          Testing.).

    ``dg-*.c''
          Test the testsuite itself using `dg-test.exp'.

    ``gcov*.c''
          Test `gcov' output using `gcov.exp', which in turn uses the
          language-independent support (*note Support for testing gcov:
          gcov Testing.).

    ``i386-pf-*.c''
          Test i386-specific support for data prefetch using
          `i386-prefetch.exp'.

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


File: gccint.info,  Node: libgcj Tests,  Next: gcov Testing,  Prev: C Tests,  Up: Test Suites

The Java library test suites.
-----------------------------

   Runtime tests are executed via `make check' in the
`TARGET/libjava/testsuite' directory in the build tree.  Additional
runtime tests can be checked into this testsuite.

   Regression testing of the core packages in libgcj is also covered by
the Mauve test suite.  The Mauve Project develops tests for the Java
Class Libraries.  These tests are run as part of libgcj testing by
placing the Mauve tree within the libjava testsuite sources at
`libjava/testsuite/libjava.mauve/mauve', or by specifying the location
of that tree when invoking `make', as in `make MAUVEDIR=~/mauve check'.

   To detect regressions, a mechanism in `mauve.exp' compares the
failures for a test run against the list of expected failures in
`libjava/testsuite/libjava.mauve/xfails' from the source hierarchy.
Update this file when adding new failing tests to Mauve, or when fixing
bugs in libgcj that had caused Mauve test failures.

   The Jacks project provides a test suite for Java compilers that can
be used to test changes that affect the GCJ front end.  This test suite
is run as part of Java testing by placing the Jacks tree within the the
libjava testsuite sources at `libjava/testsuite/libjava.jacks/jacks'.

   We encourage developers to contribute test cases to Mauve and Jacks.


File: gccint.info,  Node: gcov Testing,  Next: profopt Testing,  Prev: libgcj Tests,  Up: Test Suites

Support for testing `gcov'
--------------------------

   Language-independent support for testing `gcov', and for checking
that branch profiling produces expected values, is provided by the
expect file `gcov.exp'.  `gcov' tests also rely on procedures in
`gcc.dg.exp' to compile and run the test program.  A typical `gcov'
test contains the following DejaGNU commands within comments:

     { dg-options "-fprofile-arcs -ftest-coverage" }
     { dg-do run { target native } }
     { dg-final { run-gcov sourcefile } }

   Checks of `gcov' output can include line counts, branch percentages,
and call return percentages.  All of these checks are requested via
commands that appear in comments in the test's source file.  Commands
to check line counts are processed by default.  Commands to check
branch percentages and call return percentages are processed if there
is a file with the same basename as the source file and a suffix `.x'
that contains a line `set gcov_verify_branches 1' or `set
gcov_verify_calls 1', respectively.

   A line count command appears within a comment on the source line
that is expected to get the specified count and has the form
`count(CNT)'.  A test should only check line counts for lines that will
get the same count for any architecture.

   Commands to check branch percentages (`branch') and call return
percentages (`returns') are very similar to each other.  A beginning
command appears on or before the first of a range of lines that will
report the percentage, and the ending command follows that range of
lines.  The beginning command can include a list of percentages, all of
which are expected to be found within the range.  A range is terminated
by the next command of the same kind.  A command `branch(end)' or
`returns(end)' marks the end of a range without starting a new one.
For example:

     if (i > 10 && j > i && j < 20)  /* branch(27 50 75) */
                                     /* branch(end) */
       foo (i, j);

   For a call return percentage, the value specified is the percentage
of calls reported to return.  For a branch percentage, the value is
either the expected percentage or 100 minus that value, since the
direction of a branch can differ depending on the target or the
optimization level.

   Not all branches and calls need to be checked.  A test should not
check for branches that might be optimized away or replaced with
predicated instructions.  Don't check for calls inserted by the
compiler or ones that might be inlined or optimized away.

   A single test can check for combinations of line counts, branch
percentages, and call return percentages.  The command to check a line
count must appear on the line that will report that count, but commands
to check branch percentages and call return percentages can bracket the
lines that report them.


File: gccint.info,  Node: profopt Testing,  Next: compat Testing,  Prev: gcov Testing,  Up: Test Suites

Support for testing profile-directed optimizations
--------------------------------------------------

   The file `profopt.exp' provides language-independent support for
checking correct execution of a test built with profile-directed
optimization.  This testing requires that a test program be built and
executed twice.  The first time it is compiled to generate profile
data, and the second time it is compiled to use the data that was
generated during the first execution.  The second execution is to
verify that the test produces the expected results.

   To check that the optimization actually generated better code, a
test can be built and run a third time with normal optimizations to
verify that the performance is better with the profile-directed
optimizations.  `profopt.exp' has the beginnings of this kind of
support.

   `profopt.exp' provides generic support for profile-directed
optimizations.  Each set of tests that uses it provides information
about a specific optimization:

`tool'
     tool being tested, e.g., gcc

`profile_option'
     options used to generate profile data

`feedback_option'
     options used to optimize using that profile data

`prof_ext'
     suffix of profile data files

`PROFOPT_OPTIONS'
     list of options with which to run each test, similar to the lists
     for torture tests


File: gccint.info,  Node: compat Testing,  Prev: profopt Testing,  Up: Test Suites

Support for testing binary compatibility
----------------------------------------

   The file `compat.exp' provides language-independent support for
binary compatibility testing.  It supports testing interoperability of
two compilers that follow the same ABI, or of multiple sets of compiler
options that should not affect binary compatibility.  It is intended to
be used for test suites that complement ABI test suites.

   A test supported by this framework has three parts, each in a
separate source file: a main program and two pieces that interact with
each other to split up the functionality being tested.

`TESTNAME_main.SUFFIX'
     Contains the main program, which calls a function in file
     `TESTNAME_x.SUFFIX'.

`TESTNAME_x.SUFFIX'
     Contains at least one call to a function in `TESTNAME_y.SUFFIX'.

`TESTNAME_y.SUFFIX'
     Shares data with, or gets arguments from, `TESTNAME_x.SUFFIX'.

   Within each test, the main program and one functional piece are
compiled by the GCC under test.  The other piece can be compiled by an
alternate compiler.  If no alternate compiler is specified, then all
three source files are all compiled by the GCC under test.  It's also
possible to specify a pair of lists of compiler options, one list for
each compiler, so that each test will be compiled with each pair of
options.

   `compat.exp' defines default pairs of compiler options.  These can
be overridden by defining the environment variable `COMPAT_OPTIONS' as:

     COMPAT_OPTIONS="[list [list {TST1} {ALT1}]
       ...[list {TSTN} {ALTN}]]"

   where TSTI and ALTI are lists of options, with TSTI used by the
compiler under test and ALTI used by the alternate compiler.  For
example, with `[list [list {-g -O0} {-O3}] [list {-fpic} {-fPIC -O2}]]',
the test is first built with `-g -O0' by the compiler under test and
with `-O3' by the alternate compiler.  The test is built a second time
using `-fpic' by the compiler under test and `-fPIC -O2' by the
alternate compiler.

   An alternate compiler is specified by defining an environment
variable; for C++ define `ALT_CXX_UNDER_TEST' to be the full pathname
of an installed compiler.  That will be written to the `site.exp' file
used by DejaGNU.  The default is to build each test with the compiler
under test using the first of each pair of compiler options from
`COMPAT_OPTIONS'.  When `ALT_CXX_UNDER_TEST' is `same', each test is
built using the compiler under test but with combinations of the
options from `COMPAT_OPTIONS'.

   To run only the C++ compatibility suite using the compiler under test
and another version of GCC using specific compiler options, do the
following from `OBJDIR/gcc':

     rm site.exp
     make -k \
       ALT_CXX_UNDER_TEST=${alt_prefix}/bin/g++ \
       COMPAT_OPTIONS="lists as shown above" \
       check-c++ \
       RUNTESTFLAGS="compat.exp"

   A test that fails when the source files are compiled with different
compilers, but passes when the files are compiled with the same
compiler, demonstrates incompatibility of the generated code or runtime
support.  A test that fails for the alternate compiler but passes for
the compiler under test probably tests for a bug that was fixed in the
compiler under test but is present in the alternate compiler.


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-opts.c',
     `c-pragma.c', `c-semantics.c', and `c-lex.c', along with header
     files `c-common.h', `c-dump.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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   Some additional files are used by all or many passes:

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

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

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

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

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

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

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

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

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


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

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

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

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

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

* Menu:

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


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

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

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

