Sphinxify the CommandLine document.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161479 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 1615 insertions, 0 deletions

diff --git a/docs/CommandLine.rst b/docs/CommandLine.rst
new file mode 100644
index 0000000..302f5a4
--- /dev/null
+++ b/docs/CommandLine.rst
@@ -0,0 +1,1615 @@
+.. _commandline:
+
+==============================
+CommandLine 2.0 Library Manual
+==============================
+
+Introduction
+============
+
+This document describes the CommandLine argument processing library.  It will
+show you how to use it, and what it can do.  The CommandLine library uses a
+declarative approach to specifying the command line options that your program
+takes.  By default, these options declarations implicitly hold the value parsed
+for the option declared (of course this `can be changed`_).
+
+Although there are a **lot** of command line argument parsing libraries out
+there in many different languages, none of them fit well with what I needed.  By
+looking at the features and problems of other libraries, I designed the
+CommandLine library to have the following features:
+
+#. Speed: The CommandLine library is very quick and uses little resources.  The
+   parsing time of the library is directly proportional to the number of
+   arguments parsed, not the number of options recognized.  Additionally,
+   command line argument values are captured transparently into user defined
+   global variables, which can be accessed like any other variable (and with the
+   same performance).
+
+#. Type Safe: As a user of CommandLine, you don't have to worry about
+   remembering the type of arguments that you want (is it an int?  a string? a
+   bool? an enum?) and keep casting it around.  Not only does this help prevent
+   error prone constructs, it also leads to dramatically cleaner source code.
+
+#. No subclasses required: To use CommandLine, you instantiate variables that
+   correspond to the arguments that you would like to capture, you don't
+   subclass a parser.  This means that you don't have to write **any**
+   boilerplate code.
+
+#. Globally accessible: Libraries can specify command line arguments that are
+   automatically enabled in any tool that links to the library.  This is
+   possible because the application doesn't have to keep a list of arguments to
+   pass to the parser.  This also makes supporting `dynamically loaded options`_
+   trivial.
+
+#. Cleaner: CommandLine supports enum and other types directly, meaning that
+   there is less error and more security built into the library.  You don't have
+   to worry about whether your integral command line argument accidentally got
+   assigned a value that is not valid for your enum type.
+
+#. Powerful: The CommandLine library supports many different types of arguments,
+   from simple `boolean flags`_ to `scalars arguments`_ (`strings`_,
+   `integers`_, `enums`_, `doubles`_), to `lists of arguments`_.  This is
+   possible because CommandLine is...
+
+#. Extensible: It is very simple to add a new argument type to CommandLine.
+   Simply specify the parser that you want to use with the command line option
+   when you declare it. `Custom parsers`_ are no problem.
+
+#. Labor Saving: The CommandLine library cuts down on the amount of grunt work
+   that you, the user, have to do.  For example, it automatically provides a
+   ``-help`` option that shows the available command line options for your tool.
+   Additionally, it does most of the basic correctness checking for you.
+
+#. Capable: The CommandLine library can handle lots of different forms of
+   options often found in real programs.  For example, `positional`_ arguments,
+   ``ls`` style `grouping`_ options (to allow processing '``ls -lad``'
+   naturally), ``ld`` style `prefix`_ options (to parse '``-lmalloc
+   -L/usr/lib``'), and interpreter style options.
+
+This document will hopefully let you jump in and start using CommandLine in your
+utility quickly and painlessly.  Additionally it should be a simple reference
+manual to figure out how stuff works.  If it is failing in some area (or you
+want an extension to the library), nag the author, `Chris
+Lattner <mailto:sabre@nondot.org>`_.
+
+Quick Start Guide
+=================
+
+This section of the manual runs through a simple CommandLine'ification of a
+basic compiler tool.  This is intended to show you how to jump into using the
+CommandLine library in your own program, and show you some of the cool things it
+can do.
+
+To start out, you need to include the CommandLine header file into your program:
+
+.. code-block:: c++
+
+  #include "llvm/Support/CommandLine.h"
+
+Additionally, you need to add this as the first line of your main program:
+
+.. code-block:: c++
+
+  int main(int argc, char **argv) {
+    cl::ParseCommandLineOptions(argc, argv);
+    ...
+  }
+
+... which actually parses the arguments and fills in the variable declarations.
+
+Now that you are ready to support command line arguments, we need to tell the
+system which ones we want, and what type of arguments they are.  The CommandLine
+library uses a declarative syntax to model command line arguments with the
+global variable declarations that capture the parsed values.  This means that
+for every command line option that you would like to support, there should be a
+global variable declaration to capture the result.  For example, in a compiler,
+we would like to support the Unix-standard '``-o <filename>``' option to specify
+where to put the output.  With the CommandLine library, this is represented like
+this:
+
+.. _scalars arguments:
+.. _here:
+
+.. code-block:: c++
+
+  cl::opt<string> OutputFilename("o", cl::desc("Specify output filename"), cl::value_desc("filename"));
+
+This declares a global variable "``OutputFilename``" that is used to capture the
+result of the "``o``" argument (first parameter).  We specify that this is a
+simple scalar option by using the "``cl::opt``" template (as opposed to the
+"``cl::list``" template), and tell the CommandLine library that the data
+type that we are parsing is a string.
+
+The second and third parameters (which are optional) are used to specify what to
+output for the "``-help``" option.  In this case, we get a line that looks like
+this:
+
+::
+
+  USAGE: compiler [options]
+
+  OPTIONS:
+    -help             - display available options (-help-hidden for more)
+    -o <filename>     - Specify output filename
+
+Because we specified that the command line option should parse using the
+``string`` data type, the variable declared is automatically usable as a real
+string in all contexts that a normal C++ string object may be used.  For
+example:
+
+.. code-block:: c++
+
+  ...
+  std::ofstream Output(OutputFilename.c_str());
+  if (Output.good()) ...
+  ...
+
+There are many different options that you can use to customize the command line
+option handling library, but the above example shows the general interface to
+these options.  The options can be specified in any order, and are specified
+with helper functions like `cl::desc(...)`_, so there are no positional
+dependencies to remember.  The available options are discussed in detail in the
+`Reference Guide`_.
+
+Continuing the example, we would like to have our compiler take an input
+filename as well as an output filename, but we do not want the input filename to
+be specified with a hyphen (ie, not ``-filename.c``).  To support this style of
+argument, the CommandLine library allows for `positional`_ arguments to be
+specified for the program.  These positional arguments are filled with command
+line parameters that are not in option form.  We use this feature like this:
+
+.. code-block:: c++
+
+
+  cl::opt<string> InputFilename(cl::Positional, cl::desc("<input file>"), cl::init("-"));
+
+This declaration indicates that the first positional argument should be treated
+as the input filename.  Here we use the `cl::init`_ option to specify an initial
+value for the command line option, which is used if the option is not specified
+(if you do not specify a `cl::init`_ modifier for an option, then the default
+constructor for the data type is used to initialize the value).  Command line
+options default to being optional, so if we would like to require that the user
+always specify an input filename, we would add the `cl::Required`_ flag, and we
+could eliminate the `cl::init`_ modifier, like this:
+
+.. code-block:: c++
+
+  cl::opt<string> InputFilename(cl::Positional, cl::desc("<input file>"), cl::Required);
+
+Again, the CommandLine library does not require the options to be specified in
+any particular order, so the above declaration is equivalent to:
+
+.. code-block:: c++
+
+  cl::opt<string> InputFilename(cl::Positional, cl::Required, cl::desc("<input file>"));
+
+By simply adding the `cl::Required`_ flag, the CommandLine library will
+automatically issue an error if the argument is not specified, which shifts all
+of the command line option verification code out of your application into the
+library.  This is just one example of how using flags can alter the default
+behaviour of the library, on a per-option basis.  By adding one of the
+declarations above, the ``-help`` option synopsis is now extended to:
+
+::
+
+  USAGE: compiler [options] <input file>
+
+  OPTIONS:
+    -help             - display available options (-help-hidden for more)
+    -o <filename>     - Specify output filename
+
+... indicating that an input filename is expected.
+
+Boolean Arguments
+-----------------
+
+In addition to input and output filenames, we would like the compiler example to
+support three boolean flags: "``-f``" to force writing binary output to a
+terminal, "``--quiet``" to enable quiet mode, and "``-q``" for backwards
+compatibility with some of our users.  We can support these by declaring options
+of boolean type like this:
+
+.. code-block:: c++
+
+  cl::opt<bool> Force ("f", cl::desc("Enable binary output on terminals"));
+  cl::opt<bool> Quiet ("quiet", cl::desc("Don't print informational messages"));
+  cl::opt<bool> Quiet2("q", cl::desc("Don't print informational messages"), cl::Hidden);
+
+This does what you would expect: it declares three boolean variables
+("``Force``", "``Quiet``", and "``Quiet2``") to recognize these options.  Note
+that the "``-q``" option is specified with the "`cl::Hidden`_" flag.  This
+modifier prevents it from being shown by the standard "``-help``" output (note
+that it is still shown in the "``-help-hidden``" output).
+
+The CommandLine library uses a `different parser`_ for different data types.
+For example, in the string case, the argument passed to the option is copied
+literally into the content of the string variable... we obviously cannot do that
+in the boolean case, however, so we must use a smarter parser.  In the case of
+the boolean parser, it allows no options (in which case it assigns the value of
+true to the variable), or it allows the values "``true``" or "``false``" to be
+specified, allowing any of the following inputs:
+
+::
+
+  compiler -f          # No value, 'Force' == true
+  compiler -f=true     # Value specified, 'Force' == true
+  compiler -f=TRUE     # Value specified, 'Force' == true
+  compiler -f=FALSE    # Value specified, 'Force' == false
+
+... you get the idea.  The `bool parser`_ just turns the string values into
+boolean values, and rejects things like '``compiler -f=foo``'.  Similarly, the
+`float`_, `double`_, and `int`_ parsers work like you would expect, using the
+'``strtol``' and '``strtod``' C library calls to parse the string value into the
+specified data type.
+
+With the declarations above, "``compiler -help``" emits this:
+
+::
+
+  USAGE: compiler [options] <input file>
+
+  OPTIONS:
+    -f     - Enable binary output on terminals
+    -o     - Override output filename
+    -quiet - Don't print informational messages
+    -help  - display available options (-help-hidden for more)
+
+and "``compiler -help-hidden``" prints this:
+
+::
+
+  USAGE: compiler [options] <input file>
+
+  OPTIONS:
+    -f     - Enable binary output on terminals
+    -o     - Override output filename
+    -q     - Don't print informational messages
+    -quiet - Don't print informational messages
+    -help  - display available options (-help-hidden for more)
+
+This brief example has shown you how to use the '`cl::opt`_' class to parse
+simple scalar command line arguments.  In addition to simple scalar arguments,
+the CommandLine library also provides primitives to support CommandLine option
+`aliases`_, and `lists`_ of options.
+
+.. _aliases:
+
+Argument Aliases
+----------------
+
+So far, the example works well, except for the fact that we need to check the
+quiet condition like this now:
+
+.. code-block:: c++
+
+  ...
+    if (!Quiet && !Quiet2) printInformationalMessage(...);
+  ...
+
+... which is a real pain!  Instead of defining two values for the same
+condition, we can use the "`cl::alias`_" class to make the "``-q``" option an
+**alias** for the "``-quiet``" option, instead of providing a value itself:
+
+.. code-block:: c++
+
+  cl::opt<bool> Force ("f", cl::desc("Overwrite output files"));
+  cl::opt<bool> Quiet ("quiet", cl::desc("Don't print informational messages"));
+  cl::alias     QuietA("q", cl::desc("Alias for -quiet"), cl::aliasopt(Quiet));
+
+The third line (which is the only one we modified from above) defines a "``-q``"
+alias that updates the "``Quiet``" variable (as specified by the `cl::aliasopt`_
+modifier) whenever it is specified.  Because aliases do not hold state, the only
+thing the program has to query is the ``Quiet`` variable now.  Another nice
+feature of aliases is that they automatically hide themselves from the ``-help``
+output (although, again, they are still visible in the ``-help-hidden output``).
+
+Now the application code can simply use:
+
+.. code-block:: c++
+
+  ...
+    if (!Quiet) printInformationalMessage(...);
+  ...
+
+... which is much nicer!  The "`cl::alias`_" can be used to specify an
+alternative name for any variable type, and has many uses.
+
+.. _unnamed alternatives using the generic parser:
+
+Selecting an alternative from a set of possibilities
+----------------------------------------------------
+
+So far we have seen how the CommandLine library handles builtin types like
+``std::string``, ``bool`` and ``int``, but how does it handle things it doesn't
+know about, like enums or '``int*``'s?
+
+The answer is that it uses a table-driven generic parser (unless you specify
+your own parser, as described in the `Extension Guide`_).  This parser maps
+literal strings to whatever type is required, and requires you to tell it what
+this mapping should be.
+
+Let's say that we would like to add four optimization levels to our optimizer,
+using the standard flags "``-g``", "``-O0``", "``-O1``", and "``-O2``".  We
+could easily implement this with boolean options like above, but there are
+several problems with this strategy:
+
+#. A user could specify more than one of the options at a time, for example,
+   "``compiler -O3 -O2``".  The CommandLine library would not be able to catch
+   this erroneous input for us.
+
+#. We would have to test 4 different variables to see which ones are set.
+
+#. This doesn't map to the numeric levels that we want... so we cannot easily
+   see if some level >= "``-O1``" is enabled.
+
+To cope with these problems, we can use an enum value, and have the CommandLine
+library fill it in with the appropriate level directly, which is used like this:
+
+.. code-block:: c++
+
+  enum OptLevel {
+    g, O1, O2, O3
+  };
+
+  cl::opt<OptLevel> OptimizationLevel(cl::desc("Choose optimization level:"),
+    cl::values(
+      clEnumVal(g , "No optimizations, enable debugging"),
+      clEnumVal(O1, "Enable trivial optimizations"),
+      clEnumVal(O2, "Enable default optimizations"),
+      clEnumVal(O3, "Enable expensive optimizations"),
+     clEnumValEnd));
+
+  ...
+    if (OptimizationLevel >= O2) doPartialRedundancyElimination(...);
+  ...
+
+This declaration defines a variable "``OptimizationLevel``" of the
+"``OptLevel``" enum type.  This variable can be assigned any of the values that
+are listed in the declaration (Note that the declaration list must be terminated
+with the "``clEnumValEnd``" argument!).  The CommandLine library enforces that
+the user can only specify one of the options, and it ensure that only valid enum
+values can be specified.  The "``clEnumVal``" macros ensure that the command
+line arguments matched the enum values.  With this option added, our help output
+now is:
+
+::
+
+  USAGE: compiler [options] <input file>
+
+  OPTIONS:
+    Choose optimization level:
+      -g          - No optimizations, enable debugging
+      -O1         - Enable trivial optimizations
+      -O2         - Enable default optimizations
+      -O3         - Enable expensive optimizations
+    -f            - Enable binary output on terminals
+    -help         - display available options (-help-hidden for more)
+    -o <filename> - Specify output filename
+    -quiet        - Don't print informational messages
+
+In this case, it is sort of awkward that flag names correspond directly to enum
+names, because we probably don't want a enum definition named "``g``" in our
+program.  Because of this, we can alternatively write this example like this:
+
+.. code-block:: c++
+
+  enum OptLevel {
+    Debug, O1, O2, O3
+  };
+
+  cl::opt<OptLevel> OptimizationLevel(cl::desc("Choose optimization level:"),
+    cl::values(
+     clEnumValN(Debug, "g", "No optimizations, enable debugging"),
+      clEnumVal(O1        , "Enable trivial optimizations"),
+      clEnumVal(O2        , "Enable default optimizations"),
+      clEnumVal(O3        , "Enable expensive optimizations"),
+     clEnumValEnd));
+
+  ...
+    if (OptimizationLevel == Debug) outputDebugInfo(...);
+  ...
+
+By using the "``clEnumValN``" macro instead of "``clEnumVal``", we can directly
+specify the name that the flag should get.  In general a direct mapping is nice,
+but sometimes you can't or don't want to preserve the mapping, which is when you
+would use it.
+
+Named Alternatives
+------------------
+
+Another useful argument form is a named alternative style.  We shall use this
+style in our compiler to specify different debug levels that can be used.
+Instead of each debug level being its own switch, we want to support the
+following options, of which only one can be specified at a time:
+"``--debug-level=none``", "``--debug-level=quick``",
+"``--debug-level=detailed``".  To do this, we use the exact same format as our
+optimization level flags, but we also specify an option name.  For this case,
+the code looks like this:
+
+.. code-block:: c++
+
+  enum DebugLev {
+    nodebuginfo, quick, detailed
+  };
+
+  // Enable Debug Options to be specified on the command line
+  cl::opt<DebugLev> DebugLevel("debug_level", cl::desc("Set the debugging level:"),
+    cl::values(
+      clEnumValN(nodebuginfo, "none", "disable debug information"),
+       clEnumVal(quick,               "enable quick debug information"),
+       clEnumVal(detailed,            "enable detailed debug information"),
+      clEnumValEnd));
+
+This definition defines an enumerated command line variable of type "``enum
+DebugLev``", which works exactly the same way as before.  The difference here is
+just the interface exposed to the user of your program and the help output by
+the "``-help``" option:
+
+::
+
+  USAGE: compiler [options] <input file>
+
+  OPTIONS:
+    Choose optimization level:
+      -g          - No optimizations, enable debugging
+      -O1         - Enable trivial optimizations
+      -O2         - Enable default optimizations
+      -O3         - Enable expensive optimizations
+    -debug_level  - Set the debugging level:
+      =none       - disable debug information
+      =quick      - enable quick debug information
+      =detailed   - enable detailed debug information
+    -f            - Enable binary output on terminals
+    -help         - display available options (-help-hidden for more)
+    -o <filename> - Specify output filename
+    -quiet        - Don't print informational messages
+
+Again, the only structural difference between the debug level declaration and
+the optimization level declaration is that the debug level declaration includes
+an option name (``"debug_level"``), which automatically changes how the library
+processes the argument.  The CommandLine library supports both forms so that you
+can choose the form most appropriate for your application.
+
+.. _lists:
+
+Parsing a list of options
+-------------------------
+
+Now that we have the standard run-of-the-mill argument types out of the way,
+lets get a little wild and crazy.  Lets say that we want our optimizer to accept
+a **list** of optimizations to perform, allowing duplicates.  For example, we
+might want to run: "``compiler -dce -constprop -inline -dce -strip``".  In this
+case, the order of the arguments and the number of appearances is very
+important.  This is what the "``cl::list``" template is for.  First, start by
+defining an enum of the optimizations that you would like to perform:
+
+.. code-block:: c++
+
+  enum Opts {
+    // 'inline' is a C++ keyword, so name it 'inlining'
+    dce, constprop, inlining, strip
+  };
+
+Then define your "``cl::list``" variable:
+
+.. code-block:: c++
+
+  cl::list<Opts> OptimizationList(cl::desc("Available Optimizations:"),
+    cl::values(
+      clEnumVal(dce               , "Dead Code Elimination"),
+      clEnumVal(constprop         , "Constant Propagation"),
+     clEnumValN(inlining, "inline", "Procedure Integration"),
+      clEnumVal(strip             , "Strip Symbols"),
+    clEnumValEnd));
+
+This defines a variable that is conceptually of the type
+"``std::vector<enum Opts>``".  Thus, you can access it with standard vector
+methods:
+
+.. code-block:: c++
+
+  for (unsigned i = 0; i != OptimizationList.size(); ++i)
+    switch (OptimizationList[i])
+       ...
+
+... to iterate through the list of options specified.
+
+Note that the "``cl::list``" template is completely general and may be used with
+any data types or other arguments that you can use with the "``cl::opt``"
+template.  One especially useful way to use a list is to capture all of the
+positional arguments together if there may be more than one specified.  In the
+case of a linker, for example, the linker takes several '``.o``' files, and
+needs to capture them into a list.  This is naturally specified as:
+
+.. code-block:: c++
+
+  ...
+  cl::list<std::string> InputFilenames(cl::Positional, cl::desc("<Input files>"), cl::OneOrMore);
+  ...
+
+This variable works just like a "``vector<string>``" object.  As such, accessing
+the list is simple, just like above.  In this example, we used the
+`cl::OneOrMore`_ modifier to inform the CommandLine library that it is an error
+if the user does not specify any ``.o`` files on our command line.  Again, this
+just reduces the amount of checking we have to do.
+
+Collecting options as a set of flags
+------------------------------------
+
+Instead of collecting sets of options in a list, it is also possible to gather
+information for enum values in a **bit vector**.  The representation used by the
+`cl::bits`_ class is an ``unsigned`` integer.  An enum value is represented by a
+0/1 in the enum's ordinal value bit position. 1 indicating that the enum was
+specified, 0 otherwise.  As each specified value is parsed, the resulting enum's
+bit is set in the option's bit vector:
+
+.. code-block:: c++
+
+  bits |= 1 << (unsigned)enum;
+
+Options that are specified multiple times are redundant.  Any instances after
+the first are discarded.
+
+Reworking the above list example, we could replace `cl::list`_ with `cl::bits`_:
+
+.. code-block:: c++
+
+  cl::bits<Opts> OptimizationBits(cl::desc("Available Optimizations:"),
+    cl::values(
+      clEnumVal(dce               , "Dead Code Elimination"),
+      clEnumVal(constprop         , "Constant Propagation"),
+     clEnumValN(inlining, "inline", "Procedure Integration"),
+      clEnumVal(strip             , "Strip Symbols"),
+    clEnumValEnd));
+
+To test to see if ``constprop`` was specified, we can use the ``cl:bits::isSet``
+function:
+
+.. code-block:: c++
+
+  if (OptimizationBits.isSet(constprop)) {
+    ...
+  }
+
+It's also possible to get the raw bit vector using the ``cl::bits::getBits``
+function:
+
+.. code-block:: c++
+
+  unsigned bits = OptimizationBits.getBits();
+
+Finally, if external storage is used, then the location specified must be of
+**type** ``unsigned``. In all other ways a `cl::bits`_ option is equivalent to a
+`cl::list`_ option.
+
+.. _additional extra text:
+
+Adding freeform text to help output
+-----------------------------------
+
+As our program grows and becomes more mature, we may decide to put summary
+information about what it does into the help output.  The help output is styled
+to look similar to a Unix ``man`` page, providing concise information about a
+program.  Unix ``man`` pages, however often have a description about what the
+program does.  To add this to your CommandLine program, simply pass a third
+argument to the `cl::ParseCommandLineOptions`_ call in main.  This additional
+argument is then printed as the overview information for your program, allowing
+you to include any additional information that you want.  For example:
+
+.. code-block:: c++
+
+  int main(int argc, char **argv) {
+    cl::ParseCommandLineOptions(argc, argv, " CommandLine compiler example\n\n"
+                                "  This program blah blah blah...\n");
+    ...
+  }
+
+would yield the help output:
+
+::
+
+  **OVERVIEW: CommandLine compiler example
+
+    This program blah blah blah...**
+
+  USAGE: compiler [options] <input file>
+
+  OPTIONS:
+    ...
+    -help             - display available options (-help-hidden for more)
+    -o <filename>     - Specify output filename
+
+.. _Reference Guide:
+
+Reference Guide
+===============
+
+Now that you know the basics of how to use the CommandLine library, this section
+will give you the detailed information you need to tune how command line options
+work, as well as information on more "advanced" command line option processing
+capabilities.
+
+.. _positional:
+.. _positional argument:
+.. _Positional Arguments:
+.. _Positional arguments section:
+.. _positional options:
+
+Positional Arguments
+--------------------
+
+Positional arguments are those arguments that are not named, and are not
+specified with a hyphen.  Positional arguments should be used when an option is
+specified by its position alone.  For example, the standard Unix ``grep`` tool
+takes a regular expression argument, and an optional filename to search through
+(which defaults to standard input if a filename is not specified).  Using the
+CommandLine library, this would be specified as:
+
+.. code-block:: c++
+
+  cl::opt<string> Regex   (cl::Positional, cl::desc("<regular expression>"), cl::Required);
+  cl::opt<string> Filename(cl::Positional, cl::desc("<input file>"), cl::init("-"));
+
+Given these two option declarations, the ``-help`` output for our grep
+replacement would look like this:
+
+::
+
+  USAGE: spiffygrep [options] <regular expression> <input file>
+
+  OPTIONS:
+    -help - display available options (-help-hidden for more)
+
+... and the resultant program could be used just like the standard ``grep``
+tool.
+
+Positional arguments are sorted by their order of construction.  This means that
+command line options will be ordered according to how they are listed in a .cpp
+file, but will not have an ordering defined if the positional arguments are
+defined in multiple .cpp files.  The fix for this problem is simply to define
+all of your positional arguments in one .cpp file.
+
+Specifying positional options with hyphens
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Sometimes you may want to specify a value to your positional argument that
+starts with a hyphen (for example, searching for '``-foo``' in a file).  At
+first, you will have trouble doing this, because it will try to find an argument
+named '``-foo``', and will fail (and single quotes will not save you).  Note
+that the system ``grep`` has the same problem:
+
+::
+
+  $ spiffygrep '-foo' test.txt
+  Unknown command line argument '-foo'.  Try: spiffygrep -help'
+
+  $ grep '-foo' test.txt
+  grep: illegal option -- f
+  grep: illegal option -- o
+  grep: illegal option -- o
+  Usage: grep -hblcnsviw pattern file . . .
+
+The solution for this problem is the same for both your tool and the system
+version: use the '``--``' marker.  When the user specifies '``--``' on the
+command line, it is telling the program that all options after the '``--``'
+should be treated as positional arguments, not options.  Thus, we can use it
+like this:
+
+::
+
+  $ spiffygrep -- -foo test.txt
+    ...output...
+
+Determining absolute position with getPosition()
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Sometimes an option can affect or modify the meaning of another option. For
+example, consider ``gcc``'s ``-x LANG`` option. This tells ``gcc`` to ignore the
+suffix of subsequent positional arguments and force the file to be interpreted
+as if it contained source code in language ``LANG``. In order to handle this
+properly, you need to know the absolute position of each argument, especially
+those in lists, so their interaction(s) can be applied correctly. This is also
+useful for options like ``-llibname`` which is actually a positional argument
+that starts with a dash.
+
+So, generally, the problem is that you have two ``cl::list`` variables that
+interact in some way. To ensure the correct interaction, you can use the
+``cl::list::getPosition(optnum)`` method. This method returns the absolute
+position (as found on the command line) of the ``optnum`` item in the
+``cl::list``.
+
+The idiom for usage is like this:
+
+.. code-block:: c++
+
+  static cl::list<std::string> Files(cl::Positional, cl::OneOrMore);
+  static cl::list<std::string> Libraries("l", cl::ZeroOrMore);
+
+  int main(int argc, char**argv) {
+    // ...
+    std::vector<std::string>::iterator fileIt = Files.begin();
+    std::vector<std::string>::iterator libIt  = Libraries.begin();
+    unsigned libPos = 0, filePos = 0;
+    while ( 1 ) {
+      if ( libIt != Libraries.end() )
+        libPos = Libraries.getPosition( libIt - Libraries.begin() );
+      else
+        libPos = 0;
+      if ( fileIt != Files.end() )
+        filePos = Files.getPosition( fileIt - Files.begin() );
+      else
+        filePos = 0;
+
+      if ( filePos != 0 && (libPos == 0 || filePos < libPos) ) {
+        // Source File Is next
+        ++fileIt;
+      }
+      else if ( libPos != 0 && (filePos == 0 || libPos < filePos) ) {
+        // Library is next
+        ++libIt;
+      }
+      else
+        break; // we're done with the list
+    }
+  }
+
+Note that, for compatibility reasons, the ``cl::opt`` also supports an
+``unsigned getPosition()`` option that will provide the absolute position of
+that option. You can apply the same approach as above with a ``cl::opt`` and a
+``cl::list`` option as you can with two lists.
+
+.. _interpreter style options:
+.. _cl::ConsumeAfter:
+.. _this section for more information:
+
+The ``cl::ConsumeAfter`` modifier
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``cl::ConsumeAfter`` `formatting option`_ is used to construct programs that
+use "interpreter style" option processing.  With this style of option
+processing, all arguments specified after the last positional argument are
+treated as special interpreter arguments that are not interpreted by the command
+line argument.
+
+As a concrete example, lets say we are developing a replacement for the standard
+Unix Bourne shell (``/bin/sh``).  To run ``/bin/sh``, first you specify options
+to the shell itself (like ``-x`` which turns on trace output), then you specify
+the name of the script to run, then you specify arguments to the script.  These
+arguments to the script are parsed by the Bourne shell command line option
+processor, but are not interpreted as options to the shell itself.  Using the
+CommandLine library, we would specify this as:
+
+.. code-block:: c++
+
+  cl::opt<string> Script(cl::Positional, cl::desc("<input script>"), cl::init("-"));
+  cl::list<string>  Argv(cl::ConsumeAfter, cl::desc("<program arguments>..."));
+  cl::opt<bool>    Trace("x", cl::desc("Enable trace output"));
+
+which automatically provides the help output:
+
+::
+
+  USAGE: spiffysh [options] <input script> <program arguments>...
+
+  OPTIONS:
+    -help - display available options (-help-hidden for more)
+    -x    - Enable trace output
+
+At runtime, if we run our new shell replacement as ```spiffysh -x test.sh -a -x
+-y bar``', the ``Trace`` variable will be set to true, the ``Script`` variable
+will be set to "``test.sh``", and the ``Argv`` list will contain ``["-a", "-x",
+"-y", "bar"]``, because they were specified after the last positional argument
+(which is the script name).
+
+There are several limitations to when ``cl::ConsumeAfter`` options can be
+specified.  For example, only one ``cl::ConsumeAfter`` can be specified per
+program, there must be at least one `positional argument`_ specified, there must
+not be any `cl::list`_ positional arguments, and the ``cl::ConsumeAfter`` option
+should be a `cl::list`_ option.
+
+.. _can be changed:
+.. _Internal vs External Storage:
+
+Internal vs External Storage
+----------------------------
+
+By default, all command line options automatically hold the value that they
+parse from the command line.  This is very convenient in the common case,
+especially when combined with the ability to define command line options in the
+files that use them.  This is called the internal storage model.
+
+Sometimes, however, it is nice to separate the command line option processing
+code from the storage of the value parsed.  For example, lets say that we have a
+'``-debug``' option that we would like to use to enable debug information across
+the entire body of our program.  In this case, the boolean value controlling the
+debug code should be globally accessible (in a header file, for example) yet the
+command line option processing code should not be exposed to all of these
+clients (requiring lots of .cpp files to ``#include CommandLine.h``).
+
+To do this, set up your .h file with your option, like this for example:
+
+.. code-block:: c++
+
+  // DebugFlag.h - Get access to the '-debug' command line option
+  //
+
+  // DebugFlag - This boolean is set to true if the '-debug' command line option
+  // is specified.  This should probably not be referenced directly, instead, use
+  // the DEBUG macro below.
+  //
+  extern bool DebugFlag;
+
+  // DEBUG macro - This macro should be used by code to emit debug information.
+  // In the '-debug' option is specified on the command line, and if this is a
+  // debug build, then the code specified as the option to the macro will be
+  // executed.  Otherwise it will not be.
+  #ifdef NDEBUG
+  #define DEBUG(X)
+  #else
+  #define DEBUG(X) do { if (DebugFlag) { X; } } while (0)
+  #endif
+
+This allows clients to blissfully use the ``DEBUG()`` macro, or the
+``DebugFlag`` explicitly if they want to.  Now we just need to be able to set
+the ``DebugFlag`` boolean when the option is set.  To do this, we pass an
+additional argument to our command line argument processor, and we specify where
+to fill in with the `cl::location`_ attribute:
+
+.. code-block:: c++
+
+  bool DebugFlag;                  // the actual value
+  static cl::opt<bool, true>       // The parser
+  Debug("debug", cl::desc("Enable debug output"), cl::Hidden, cl::location(DebugFlag));
+
+In the above example, we specify "``true``" as the second argument to the
+`cl::opt`_ template, indicating that the template should not maintain a copy of
+the value itself.  In addition to this, we specify the `cl::location`_
+attribute, so that ``DebugFlag`` is automatically set.
+
+Option Attributes
+-----------------
+
+This section describes the basic attributes that you can specify on options.
+
+* The option name attribute (which is required for all options, except
+  `positional options`_) specifies what the option name is.  This option is
+  specified in simple double quotes:
+
+  .. code-block:: c++
+
+    cl::opt<**bool**> Quiet("quiet");
+
+.. _cl::desc(...):
+
+* The **cl::desc** attribute specifies a description for the option to be
+  shown in the ``-help`` output for the program.
+
+.. _cl::value_desc:
+
+* The **cl::value_desc** attribute specifies a string that can be used to
+  fine tune the ``-help`` output for a command line option.  Look `here`_ for an
+  example.
+
+.. _cl::init:
+
+* The **cl::init** attribute specifies an initial value for a `scalar`_
+  option.  If this attribute is not specified then the command line option value
+  defaults to the value created by the default constructor for the
+  type.
+
+  .. warning::
+
+    If you specify both **cl::init** and **cl::location** for an option, you
+    must specify **cl::location** first, so that when the command-line parser
+    sees **cl::init**, it knows where to put the initial value. (You will get an
+    error at runtime if you don't put them in the right order.)
+
+.. _cl::location:
+
+* The **cl::location** attribute where to store the value for a parsed command
+  line option if using external storage.  See the section on `Internal vs
+  External Storage`_ for more information.
+
+.. _cl::aliasopt:
+
+* The **cl::aliasopt** attribute specifies which option a `cl::alias`_ option is
+  an alias for.
+
+.. _cl::values:
+
+* The **cl::values** attribute specifies the string-to-value mapping to be used
+  by the generic parser.  It takes a **clEnumValEnd terminated** list of
+  (option, value, description) triplets that specify the option name, the value
+  mapped to, and the description shown in the ``-help`` for the tool.  Because
+  the generic parser is used most frequently with enum values, two macros are
+  often useful:
+
+  #. The **clEnumVal** macro is used as a nice simple way to specify a triplet
+     for an enum.  This macro automatically makes the option name be the same as
+     the enum name.  The first option to the macro is the enum, the second is
+     the description for the command line option.
+
+  #. The **clEnumValN** macro is used to specify macro options where the option
+     name doesn't equal the enum name.  For this macro, the first argument is
+     the enum value, the second is the flag name, and the second is the
+     description.
+
+  You will get a compile time error if you try to use cl::values with a parser
+  that does not support it.
+
+.. _cl::multi_val:
+
+* The **cl::multi_val** attribute specifies that this option takes has multiple
+  values (example: ``-sectalign segname sectname sectvalue``). This attribute
+  takes one unsigned argument - the number of values for the option. This
+  attribute is valid only on ``cl::list`` options (and will fail with compile
+  error if you try to use it with other option types). It is allowed to use all
+  of the usual modifiers on multi-valued options (besides
+  ``cl::ValueDisallowed``, obviously).
+
+Option Modifiers
+----------------
+
+Option modifiers are the flags and expressions that you pass into the
+constructors for `cl::opt`_ and `cl::list`_.  These modifiers give you the
+ability to tweak how options are parsed and how ``-help`` output is generated to
+fit your application well.
+
+These options fall into five main categories:
+
+#. Hiding an option from ``-help`` output
+
+#. Controlling the number of occurrences required and allowed
+
+#. Controlling whether or not a value must be specified
+
+#. Controlling other formatting options
+
+#. Miscellaneous option modifiers
+
+It is not possible to specify two options from the same category (you'll get a
+runtime error) to a single option, except for options in the miscellaneous
+category.  The CommandLine library specifies defaults for all of these settings
+that are the most useful in practice and the most common, which mean that you
+usually shouldn't have to worry about these.
+
+Hiding an option from ``-help`` output
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``cl::NotHidden``, ``cl::Hidden``, and ``cl::ReallyHidden`` modifiers are
+used to control whether or not an option appears in the ``-help`` and
+``-help-hidden`` output for the compiled program:
+
+.. _cl::NotHidden:
+
+* The **cl::NotHidden** modifier (which is the default for `cl::opt`_ and
+  `cl::list`_ options) indicates the option is to appear in both help
+  listings.
+
+.. _cl::Hidden:
+
+* The **cl::Hidden** modifier (which is the default for `cl::alias`_ options)
+  indicates that the option should not appear in the ``-help`` output, but
+  should appear in the ``-help-hidden`` output.
+
+.. _cl::ReallyHidden:
+
+* The **cl::ReallyHidden** modifier indicates that the option should not appear
+  in any help output.
+
+Controlling the number of occurrences required and allowed
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This group of options is used to control how many time an option is allowed (or
+required) to be specified on the command line of your program.  Specifying a
+value for this setting allows the CommandLine library to do error checking for
+you.
+
+The allowed values for this option group are:
+
+.. _cl::Optional:
+
+* The **cl::Optional** modifier (which is the default for the `cl::opt`_ and
+  `cl::alias`_ classes) indicates that your program will allow either zero or
+  one occurrence of the option to be specified.
+
+.. _cl::ZeroOrMore:
+
+* The **cl::ZeroOrMore** modifier (which is the default for the `cl::list`_
+  class) indicates that your program will allow the option to be specified zero
+  or more times.
+
+.. _cl::Required:
+
+* The **cl::Required** modifier indicates that the specified option must be
+  specified exactly one time.
+
+.. _cl::OneOrMore:
+
+* The **cl::OneOrMore** modifier indicates that the option must be specified at
+  least one time.
+
+* The **cl::ConsumeAfter** modifier is described in the `Positional arguments
+  section`_.
+
+If an option is not specified, then the value of the option is equal to the
+value specified by the `cl::init`_ attribute.  If the ``cl::init`` attribute is
+not specified, the option value is initialized with the default constructor for
+the data type.
+
+If an option is specified multiple times for an option of the `cl::opt`_ class,
+only the last value will be retained.
+
+Controlling whether or not a value must be specified
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This group of options is used to control whether or not the option allows a
+value to be present.  In the case of the CommandLine library, a value is either
+specified with an equal sign (e.g. '``-index-depth=17``') or as a trailing
+string (e.g. '``-o a.out``').
+
+The allowed values for this option group are:
+
+.. _cl::ValueOptional:
+
+* The **cl::ValueOptional** modifier (which is the default for ``bool`` typed
+  options) specifies that it is acceptable to have a value, or not.  A boolean
+  argument can be enabled just by appearing on the command line, or it can have
+  an explicit '``-foo=true``'.  If an option is specified with this mode, it is
+  illegal for the value to be provided without the equal sign.  Therefore
+  '``-foo true``' is illegal.  To get this behavior, you must use
+  the `cl::ValueRequired`_ modifier.
+
+.. _cl::ValueRequired:
+
+* The **cl::ValueRequired** modifier (which is the default for all other types
+  except for `unnamed alternatives using the generic parser`_) specifies that a
+  value must be provided.  This mode informs the command line library that if an
+  option is not provides with an equal sign, that the next argument provided
+  must be the value.  This allows things like '``-o a.out``' to work.
+
+.. _cl::ValueDisallowed:
+
+* The **cl::ValueDisallowed** modifier (which is the default for `unnamed
+  alternatives using the generic parser`_) indicates that it is a runtime error
+  for the user to specify a value.  This can be provided to disallow users from
+  providing options to boolean options (like '``-foo=true``').
+
+In general, the default values for this option group work just like you would
+want them to.  As mentioned above, you can specify the `cl::ValueDisallowed`_
+modifier to a boolean argument to restrict your command line parser.  These
+options are mostly useful when `extending the library`_.
+
+.. _formatting option:
+
+Controlling other formatting options
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The formatting option group is used to specify that the command line option has
+special abilities and is otherwise different from other command line arguments.
+As usual, you can only specify one of these arguments at most.
+
+.. _cl::NormalFormatting:
+
+* The **cl::NormalFormatting** modifier (which is the default all options)
+  specifies that this option is "normal".
+
+.. _cl::Positional:
+
+* The **cl::Positional** modifier specifies that this is a positional argument
+  that does not have a command line option associated with it.  See the
+  `Positional Arguments`_ section for more information.
+
+* The **cl::ConsumeAfter** modifier specifies that this option is used to
+  capture "interpreter style" arguments.  See `this section for more
+  information`_.
+
+.. _prefix:
+.. _cl::Prefix:
+
+* The **cl::Prefix** modifier specifies that this option prefixes its value.
+  With 'Prefix' options, the equal sign does not separate the value from the
+  option name specified. Instead, the value is everything after the prefix,
+  including any equal sign if present. This is useful for processing odd
+  arguments like ``-lmalloc`` and ``-L/usr/lib`` in a linker tool or
+  ``-DNAME=value`` in a compiler tool.  Here, the '``l``', '``D``' and '``L``'
+  options are normal string (or list) options, that have the **cl::Prefix**
+  modifier added to allow the CommandLine library to recognize them.  Note that
+  **cl::Prefix** options must not have the **cl::ValueDisallowed** modifier
+  specified.
+
+.. _grouping:
+.. _cl::Grouping:
+
+* The **cl::Grouping** modifier is used to implement Unix-style tools (like
+  ``ls``) that have lots of single letter arguments, but only require a single
+  dash.  For example, the '``ls -labF``' command actually enables four different
+  options, all of which are single letters.  Note that **cl::Grouping** options
+  cannot have values.
+
+The CommandLine library does not restrict how you use the **cl::Prefix** or
+**cl::Grouping** modifiers, but it is possible to specify ambiguous argument
+settings.  Thus, it is possible to have multiple letter options that are prefix
+or grouping options, and they will still work as designed.
+
+To do this, the CommandLine library uses a greedy algorithm to parse the input
+option into (potentially multiple) prefix and grouping options.  The strategy
+basically looks like this:
+
+::
+
+  parse(string OrigInput) {
+
+  1. string input = OrigInput;
+  2. if (isOption(input)) return getOption(input).parse();  // Normal option
+  3. while (!isOption(input) && !input.empty()) input.pop_back();  // Remove the last letter
+  4. if (input.empty()) return error();  // No matching option
+  5. if (getOption(input).isPrefix())
+       return getOption(input).parse(input);
+  6. while (!input.empty()) {  // Must be grouping options
+       getOption(input).parse();
+       OrigInput.erase(OrigInput.begin(), OrigInput.begin()+input.length());
+       input = OrigInput;
+       while (!isOption(input) && !input.empty()) input.pop_back();
+     }
+  7. if (!OrigInput.empty()) error();
+
+  }
+
+Miscellaneous option modifiers
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The miscellaneous option modifiers are the only flags where you can specify more
+than one flag from the set: they are not mutually exclusive.  These flags
+specify boolean properties that modify the option.
+
+.. _cl::CommaSeparated:
+
+* The **cl::CommaSeparated** modifier indicates that any commas specified for an
+  option's value should be used to split the value up into multiple values for
+  the option.  For example, these two options are equivalent when
+  ``cl::CommaSeparated`` is specified: "``-foo=a -foo=b -foo=c``" and
+  "``-foo=a,b,c``".  This option only makes sense to be used in a case where the
+  option is allowed to accept one or more values (i.e. it is a `cl::list`_
+  option).
+
+.. _cl::PositionalEatsArgs:
+
+* The **cl::PositionalEatsArgs** modifier (which only applies to positional
+  arguments, and only makes sense for lists) indicates that positional argument
+  should consume any strings after it (including strings that start with a "-")
+  up until another recognized positional argument.  For example, if you have two
+  "eating" positional arguments, "``pos1``" and "``pos2``", the string "``-pos1
+  -foo -bar baz -pos2 -bork``" would cause the "``-foo -bar -baz``" strings to
+  be applied to the "``-pos1``" option and the "``-bork``" string to be applied
+  to the "``-pos2``" option.
+
+.. _cl::Sink:
+
+* The **cl::Sink** modifier is used to handle unknown options. If there is at
+  least one option with ``cl::Sink`` modifier specified, the parser passes
+  unrecognized option strings to it as values instead of signaling an error. As
+  with ``cl::CommaSeparated``, this modifier only makes sense with a `cl::list`_
+  option.
+
+So far, these are the only three miscellaneous option modifiers.
+
+.. _response files:
+
+Response files
+^^^^^^^^^^^^^^
+
+Some systems, such as certain variants of Microsoft Windows and some older
+Unices have a relatively low limit on command-line length. It is therefore
+customary to use the so-called 'response files' to circumvent this
+restriction. These files are mentioned on the command-line (using the "@file")
+syntax. The program reads these files and inserts the contents into argv,
+thereby working around the command-line length limits. Response files are
+enabled by an optional fourth argument to `cl::ParseEnvironmentOptions`_ and
+`cl::ParseCommandLineOptions`_.
+
+Top-Level Classes and Functions
+-------------------------------
+
+Despite all of the built-in flexibility, the CommandLine option library really
+only consists of one function `cl::ParseCommandLineOptions`_) and three main
+classes: `cl::opt`_, `cl::list`_, and `cl::alias`_.  This section describes
+these three classes in detail.
+
+.. _cl::ParseCommandLineOptions:
+
+The ``cl::ParseCommandLineOptions`` function
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``cl::ParseCommandLineOptions`` function is designed to be called directly
+from ``main``, and is used to fill in the values of all of the command line
+option variables once ``argc`` and ``argv`` are available.
+
+The ``cl::ParseCommandLineOptions`` function requires two parameters (``argc``
+and ``argv``), but may also take an optional third parameter which holds
+`additional extra text`_ to emit when the ``-help`` option is invoked, and a
+fourth boolean parameter that enables `response files`_.
+
+.. _cl::ParseEnvironmentOptions:
+
+The ``cl::ParseEnvironmentOptions`` function
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``cl::ParseEnvironmentOptions`` function has mostly the same effects as
+`cl::ParseCommandLineOptions`_, except that it is designed to take values for
+options from an environment variable, for those cases in which reading the
+command line is not convenient or desired. It fills in the values of all the
+command line option variables just like `cl::ParseCommandLineOptions`_ does.
+
+It takes four parameters: the name of the program (since ``argv`` may not be
+available, it can't just look in ``argv[0]``), the name of the environment
+variable to examine, the optional `additional extra text`_ to emit when the
+``-help`` option is invoked, and the boolean switch that controls whether
+`response files`_ should be read.
+
+``cl::ParseEnvironmentOptions`` will break the environment variable's value up
+into words and then process them using `cl::ParseCommandLineOptions`_.
+**Note:** Currently ``cl::ParseEnvironmentOptions`` does not support quoting, so
+an environment variable containing ``-option "foo bar"`` will be parsed as three
+words, ``-option``, ``"foo``, and ``bar"``, which is different from what you
+would get from the shell with the same input.
+
+The ``cl::SetVersionPrinter`` function
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``cl::SetVersionPrinter`` function is designed to be called directly from
+``main`` and *before* ``cl::ParseCommandLineOptions``. Its use is optional. It
+simply arranges for a function to be called in response to the ``--version``
+option instead of having the ``CommandLine`` library print out the usual version
+string for LLVM. This is useful for programs that are not part of LLVM but wish
+to use the ``CommandLine`` facilities. Such programs should just define a small
+function that takes no arguments and returns ``void`` and that prints out
+whatever version information is appropriate for the program. Pass the address of
+that function to ``cl::SetVersionPrinter`` to arrange for it to be called when
+the ``--version`` option is given by the user.
+
+.. _cl::opt:
+.. _scalar:
+
+The ``cl::opt`` class
+^^^^^^^^^^^^^^^^^^^^^
+
+The ``cl::opt`` class is the class used to represent scalar command line
+options, and is the one used most of the time.  It is a templated class which
+can take up to three arguments (all except for the first have default values
+though):
+
+.. code-block:: c++
+
+  namespace cl {
+    template <class DataType, bool ExternalStorage = false,
+              class ParserClass = parser<DataType> >
+    class opt;
+  }
+
+The first template argument specifies what underlying data type the command line
+argument is, and is used to select a default parser implementation.  The second
+template argument is used to specify whether the option should contain the
+storage for the option (the default) or whether external storage should be used
+to contain the value parsed for the option (see `Internal vs External Storage`_
+for more information).
+
+The third template argument specifies which parser to use.  The default value
+selects an instantiation of the ``parser`` class based on the underlying data
+type of the option.  In general, this default works well for most applications,
+so this option is only used when using a `custom parser`_.
+
+.. _lists of arguments:
+.. _cl::list:
+
+The ``cl::list`` class
+^^^^^^^^^^^^^^^^^^^^^^
+
+The ``cl::list`` class is the class used to represent a list of command line
+options.  It too is a templated class which can take up to three arguments:
+
+.. code-block:: c++
+
+  namespace cl {
+    template <class DataType, class Storage = bool,
+              class ParserClass = parser<DataType> >
+    class list;
+  }
+
+This class works the exact same as the `cl::opt`_ class, except that the second
+argument is the **type** of the external storage, not a boolean value.  For this
+class, the marker type '``bool``' is used to indicate that internal storage
+should be used.
+
+.. _cl::bits:
+
+The ``cl::bits`` class
+^^^^^^^^^^^^^^^^^^^^^^
+
+The ``cl::bits`` class is the class used to represent a list of command line
+options in the form of a bit vector.  It is also a templated class which can
+take up to three arguments:
+
+.. code-block:: c++
+
+  namespace cl {
+    template <class DataType, class Storage = bool,
+              class ParserClass = parser<DataType> >
+    class bits;
+  }
+
+This class works the exact same as the `cl::list`_ class, except that the second
+argument must be of **type** ``unsigned`` if external storage is used.
+
+.. _cl::alias:
+
+The ``cl::alias`` class
+^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``cl::alias`` class is a nontemplated class that is used to form aliases for
+other arguments.
+
+.. code-block:: c++
+
+  namespace cl {
+    class alias;
+  }
+
+The `cl::aliasopt`_ attribute should be used to specify which option this is an
+alias for.  Alias arguments default to being `cl::Hidden`_, and use the aliased
+options parser to do the conversion from string to data.
+
+.. _cl::extrahelp:
+
+The ``cl::extrahelp`` class
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``cl::extrahelp`` class is a nontemplated class that allows extra help text
+to be printed out for the ``-help`` option.
+
+.. code-block:: c++
+
+  namespace cl {
+    struct extrahelp;
+  }
+
+To use the extrahelp, simply construct one with a ``const char*`` parameter to
+the constructor. The text passed to the constructor will be printed at the
+bottom of the help message, verbatim. Note that multiple ``cl::extrahelp``
+**can** be used, but this practice is discouraged. If your tool needs to print
+additional help information, put all that help into a single ``cl::extrahelp``
+instance.
+
+For example:
+
+.. code-block:: c++
+
+  cl::extrahelp("\nADDITIONAL HELP:\n\n  This is the extra help\n");
+
+.. _different parser:
+.. _discussed previously:
+
+Builtin parsers
+---------------
+
+Parsers control how the string value taken from the command line is translated
+into a typed value, suitable for use in a C++ program.  By default, the
+CommandLine library uses an instance of ``parser<type>`` if the command line
+option specifies that it uses values of type '``type``'.  Because of this,
+custom option processing is specified with specializations of the '``parser``'
+class.
+
+The CommandLine library provides the following builtin parser specializations,
+which are sufficient for most applications. It can, however, also be extended to
+work with new data types and new ways of interpreting the same data.  See the
+`Writing a Custom Parser`_ for more details on this type of library extension.
+
+.. _enums:
+.. _cl::parser:
+
+* The generic ``parser<t>`` parser can be used to map strings values to any data
+  type, through the use of the `cl::values`_ property, which specifies the
+  mapping information.  The most common use of this parser is for parsing enum
+  values, which allows you to use the CommandLine library for all of the error
+  checking to make sure that only valid enum values are specified (as opposed to
+  accepting arbitrary strings).  Despite this, however, the generic parser class
+  can be used for any data type.
+
+.. _boolean flags:
+.. _bool parser:
+
+* The **parser<bool> specialization** is used to convert boolean strings to a
+  boolean value.  Currently accepted strings are "``true``", "``TRUE``",
+  "``True``", "``1``", "``false``", "``FALSE``", "``False``", and "``0``".
+
+* The **parser<boolOrDefault> specialization** is used for cases where the value
+  is boolean, but we also need to know whether the option was specified at all.
+  boolOrDefault is an enum with 3 values, BOU_UNSET, BOU_TRUE and BOU_FALSE.
+  This parser accepts the same strings as **``parser<bool>``**.
+
+.. _strings:
+
+* The **parser<string> specialization** simply stores the parsed string into the
+  string value specified.  No conversion or modification of the data is
+  performed.
+
+.. _integers:
+.. _int:
+
+* The **parser<int> specialization** uses the C ``strtol`` function to parse the
+  string input.  As such, it will accept a decimal number (with an optional '+'
+  or '-' prefix) which must start with a non-zero digit.  It accepts octal
+  numbers, which are identified with a '``0``' prefix digit, and hexadecimal
+  numbers with a prefix of '``0x``' or '``0X``'.
+
+.. _doubles:
+.. _float:
+.. _double:
+
+* The **parser<double>** and **parser<float> specializations** use the standard
+  C ``strtod`` function to convert floating point strings into floating point
+  values.  As such, a broad range of string formats is supported, including
+  exponential notation (ex: ``1.7e15``) and properly supports locales.
+
+.. _Extension Guide:
+.. _extending the library:
+
+Extension Guide
+===============
+
+Although the CommandLine library has a lot of functionality built into it
+already (as discussed previously), one of its true strengths lie in its
+extensibility.  This section discusses how the CommandLine library works under
+the covers and illustrates how to do some simple, common, extensions.
+
+.. _Custom parsers:
+.. _custom parser:
+.. _Writing a Custom Parser:
+
+Writing a custom parser
+-----------------------
+
+One of the simplest and most common extensions is the use of a custom parser.
+As `discussed previously`_, parsers are the portion of the CommandLine library
+that turns string input from the user into a particular parsed data type,
+validating the input in the process.
+
+There are two ways to use a new parser:
+
+#. Specialize the `cl::parser`_ template for your custom data type.
+
+   This approach has the advantage that users of your custom data type will
+   automatically use your custom parser whenever they define an option with a
+   value type of your data type.  The disadvantage of this approach is that it
+   doesn't work if your fundamental data type is something that is already
+   supported.
+
+#. Write an independent class, using it explicitly from options that need it.
+
+   This approach works well in situations where you would line to parse an
+   option using special syntax for a not-very-special data-type.  The drawback
+   of this approach is that users of your parser have to be aware that they are
+   using your parser instead of the builtin ones.
+
+To guide the discussion, we will discuss a custom parser that accepts file
+sizes, specified with an optional unit after the numeric size.  For example, we
+would like to parse "102kb", "41M", "1G" into the appropriate integer value.  In
+this case, the underlying data type we want to parse into is '``unsigned``'.  We
+choose approach #2 above because we don't want to make this the default for all
+``unsigned`` options.
+
+To start out, we declare our new ``FileSizeParser`` class:
+
+.. code-block:: c++
+
+  struct FileSizeParser : public cl::basic_parser<unsigned> {
+    // parse - Return true on error.
+    bool parse(cl::Option &O, const char *ArgName, const std::string &ArgValue,
+               unsigned &Val);
+  };
+
+Our new class inherits from the ``cl::basic_parser`` template class to fill in
+the default, boiler plate code for us.  We give it the data type that we parse
+into, the last argument to the ``parse`` method, so that clients of our custom
+parser know what object type to pass in to the parse method.  (Here we declare
+that we parse into '``unsigned``' variables.)
+
+For most purposes, the only method that must be implemented in a custom parser
+is the ``parse`` method.  The ``parse`` method is called whenever the option is
+invoked, passing in the option itself, the option name, the string to parse, and
+a reference to a return value.  If the string to parse is not well-formed, the
+parser should output an error message and return true.  Otherwise it should
+return false and set '``Val``' to the parsed value.  In our example, we
+implement ``parse`` as:
+
+.. code-block:: c++
+
+  bool FileSizeParser::parse(cl::Option &O, const char *ArgName,
+                             const std::string &Arg, unsigned &Val) {
+    const char *ArgStart = Arg.c_str();
+    char *End;
+
+    // Parse integer part, leaving 'End' pointing to the first non-integer char
+    Val = (unsigned)strtol(ArgStart, &End, 0);
+
+    while (1) {
+      switch (*End++) {
+      case 0: return false;   // No error
+      case 'i':               // Ignore the 'i' in KiB if people use that
+      case 'b': case 'B':     // Ignore B suffix
+        break;
+
+      case 'g': case 'G': Val *= 1024*1024*1024; break;
+      case 'm': case 'M': Val *= 1024*1024;      break;
+      case 'k': case 'K': Val *= 1024;           break;
+
+      default:
+        // Print an error message if unrecognized character!
+        return O.error("'" + Arg + "' value invalid for file size argument!");
+      }
+    }
+  }
+
+This function implements a very simple parser for the kinds of strings we are
+interested in.  Although it has some holes (it allows "``123KKK``" for example),
+it is good enough for this example.  Note that we use the option itself to print
+out the error message (the ``error`` method always returns true) in order to get
+a nice error message (shown below).  Now that we have our parser class, we can
+use it like this:
+
+.. code-block:: c++
+
+  static cl::opt<unsigned, false, FileSizeParser>
+  MFS("max-file-size", cl::desc("Maximum file size to accept"),
+      cl::value_desc("size"));
+
+Which adds this to the output of our program:
+
+::
+
+  OPTIONS:
+    -help                 - display available options (-help-hidden for more)
+    ...
+   -max-file-size=<size> - Maximum file size to accept
+
+And we can test that our parse works correctly now (the test program just prints
+out the max-file-size argument value):
+
+::
+
+  $ ./test
+  MFS: 0
+  $ ./test -max-file-size=123MB
+  MFS: 128974848
+  $ ./test -max-file-size=3G
+  MFS: 3221225472
+  $ ./test -max-file-size=dog
+  -max-file-size option: 'dog' value invalid for file size argument!
+
+It looks like it works.  The error message that we get is nice and helpful, and
+we seem to accept reasonable file sizes.  This wraps up the "custom parser"
+tutorial.
+
+Exploiting external storage
+---------------------------
+
+Several of the LLVM libraries define static ``cl::opt`` instances that will
+automatically be included in any program that links with that library.  This is
+a feature. However, sometimes it is necessary to know the value of the command
+line option outside of the library. In these cases the library does or should
+provide an external storage location that is accessible to users of the
+library. Examples of this include the ``llvm::DebugFlag`` exported by the
+``lib/Support/Debug.cpp`` file and the ``llvm::TimePassesIsEnabled`` flag
+exported by the ``lib/VMCore/PassManager.cpp`` file.
+
+.. todo::
+
+  TODO: complete this section
+
+.. _dynamically loaded options:
+
+Dynamically adding command line options
+
+.. todo::
+
+  TODO: fill in this section