# Customizing LLVMC: Reference Manual

Contents

- Introduction
- Compiling with LLVMC
- Predefined options
- Compiling LLVMC plugins
- Compiling standalone LLVMC-based drivers
- Customizing LLVMC: the compilation graph
- Describing options
  - External options
- Conditional evaluation
- Writing a tool description
  - Actions
- Language map
- Option preprocessor
- More advanced topics
  - Hooks and environment variables
  - How plugins are loaded
  - Debugging
  - Conditioning on the executable name

Written by Mikhail Glushenkov

# Introduction

LLVMC is a generic compiler driver, designed to be customizable and extensible. It plays the same role for LLVM as the `gcc` program does for GCC - LLVMC's job is essentially to transform a set of input files into a set of targets depending on configuration rules and user options. What makes LLVMC different is that these transformation rules are completely customizable - in fact, LLVMC knows nothing about the specifics of transformation (even the command-line options are mostly not hard-coded) and regards the transformation structure as an abstract graph. The structure of this graph is completely determined by plugins, which can be either statically or dynamically linked. This makes it possible to easily adapt LLVMC for other purposes - for example, as a build tool for game resources.

Because LLVMC employs TableGen as its configuration language, you need to be familiar with it to customize LLVMC.

# Compiling with LLVMC

LLVMC tries hard to be as compatible with `gcc` as possible, although there are some small differences. Most of the time, however, you shouldn't be able to notice them:

    $ # This works as expected:
    $ llvmc -O3 -Wall hello.cpp
    $ ./a.out
    hello

One nice feature of LLVMC is that one doesn't have to distinguish between different compilers for different languages (think `g++` vs. `gcc`) - the right toolchain is chosen automatically based on input language names (which are, in turn, determined from file extensions). If you want to force files ending with ".c" to compile as C++, use the `-x` option, just like you would do it with `gcc`:

    $ # hello.c is really a C++ file
    $ llvmc -x c++ hello.c
    $ ./a.out
    hello

On the other hand, when using LLVMC as a linker to combine several C++ object files you should provide the `--linker` option since it's impossible for LLVMC to choose the right linker in that case:

    $ llvmc -c hello.cpp
    $ llvmc hello.o
    [A lot of link-time errors skipped]
    $ llvmc --linker=c++ hello.o
    $ ./a.out
    hello

By default, LLVMC uses `llvm-gcc` to compile the source code. It is also possible to choose the `clang` compiler with the `-clang` option.

# Predefined options

LLVMC has some built-in options that can't be overridden in the configuration libraries:

- `-o FILE` - Output file name.
- `-x LANGUAGE` - Specify the language of the following input files until the next -x option.
- `-load PLUGIN_NAME` - Load the specified plugin DLL. Example: `-load $LLVM_DIR/Release/lib/LLVMCSimple.so`.
- `-v` - Enable verbose mode, i.e. print out all executed commands.
- `--save-temps` - Write temporary files to the current directory and do not delete them on exit. This option can also take an argument: the `--save-temps=obj` switch will write files into the directory specified with the `-o` option. The `--save-temps=cwd` and `--save-temps` switches are both synonyms for the default behaviour.
- `--temp-dir DIRECTORY` - Store temporary files in the given directory. This directory is deleted on exit unless `--save-temps` is specified. If `--save-temps=obj` is also specified, `--temp-dir` is given the precedence.
- `--check-graph` - Check the compilation for common errors like mismatched output/input language names, multiple default edges and cycles. Because of plugins, these checks can't be performed at compile-time. Exit with code zero if no errors were found, and return the number of found errors otherwise. Hidden option, useful for debugging LLVMC plugins.
- `--view-graph` - Show a graphical representation of the compilation graph and exit. Requires that you have `dot` and `gv` programs installed. Hidden option, useful for debugging LLVMC plugins.
- `--write-graph` - Write a `compilation-graph.dot` file in the current directory with the compilation graph description in Graphviz format (identical to the file used by the `--view-graph` option). The `-o` option can be used to set the output file name. Hidden option, useful for debugging LLVMC plugins.
- `-help`, `-help-hidden`, `--version` - These options have their standard meaning.

# Compiling LLVMC plugins

It's easiest to start working on your own LLVMC plugin by copying the skeleton project which lives under `$LLVMC_DIR/plugins/Simple`:

    $ cd $LLVMC_DIR/plugins
    $ cp -r Simple MyPlugin
    $ cd MyPlugin
    $ ls
    Makefile PluginMain.cpp Simple.td

As you can see, our basic plugin consists of only two files (not counting the build script). `Simple.td` contains TableGen description of the compilation graph; its format is documented in the following sections. `PluginMain.cpp` is just a helper file used to compile the auto-generated C++ code produced from TableGen source. It can also contain hook definitions (see below).

The first thing that you should do is to change the `LLVMC_PLUGIN` variable in the `Makefile` to avoid conflicts (since this variable is used to name the resulting library):

    LLVMC_PLUGIN=MyPlugin

It is also a good idea to rename `Simple.td` to something less generic:

    $ mv Simple.td MyPlugin.td

To build your plugin as a dynamic library, just `cd` to its source directory and run `make`. The resulting file will be called `plugin_llvmc_$(LLVMC_PLUGIN).$(DLL_EXTENSION)` (in our case, `plugin_llvmc_MyPlugin.so`). This library can be then loaded in with the `-load` option. Example:

    $ cd $LLVMC_DIR/plugins/Simple
    $ make
    $ llvmc -load $LLVM_DIR/Release/lib/plugin_llvmc_Simple.so

# Compiling standalone LLVMC-based drivers

By default, the `llvmc` executable consists of a driver core plus several statically linked plugins (`Base` and `Clang` at the moment). You can produce a standalone LLVMC-based driver executable by linking the core with your own plugins. The recommended way to do this is by starting with the provided `Skeleton` example (`$LLVMC_DIR/example/Skeleton`):

    $ cd $LLVMC_DIR/example/
    $ cp -r Skeleton mydriver
    $ cd mydriver
    $ vim Makefile
    [...]
    $ make

If you're compiling LLVM with different source and object directories, then you must perform the following additional steps before running `make`:

    # LLVMC_SRC_DIR = $LLVM_SRC_DIR/tools/llvmc/
    # LLVMC_OBJ_DIR = $LLVM_OBJ_DIR/tools/llvmc/
    $ cp $LLVMC_SRC_DIR/example/mydriver/Makefile \
      $LLVMC_OBJ_DIR/example/mydriver/
    $ cd $LLVMC_OBJ_DIR/example/mydriver
    $ make

Another way to do the same thing is by using the following command:

    $ cd $LLVMC_DIR
    $ make LLVMC_BUILTIN_PLUGINS=MyPlugin LLVMC_BASED_DRIVER_NAME=mydriver

This works with both srcdir == objdir and srcdir != objdir, but assumes that the plugin source directory was placed under `$LLVMC_DIR/plugins`.

Sometimes, you will want a 'bare-bones' version of LLVMC that has no built-in plugins. It can be compiled with the following command:

    $ cd $LLVMC_DIR
    $ make LLVMC_BUILTIN_PLUGINS=""

# Customizing LLVMC: the compilation graph

Each TableGen configuration file should include the common definitions:

    include "llvm/CompilerDriver/Common.td"

Internally, LLVMC stores information about possible source transformations in form of a graph. Nodes in this graph represent tools, and edges between two nodes represent a transformation path. A special "root" node is used to mark entry points for the transformations. LLVMC also assigns a weight to each edge (more on this later) to choose between several alternative edges.

The definition of the compilation graph (see file `plugins/Base/Base.td` for an example) is just a list of edges:

    def CompilationGraph : CompilationGraph<[
        Edge<"root", "llvm_gcc_c">,
        Edge<"root", "llvm_gcc_assembler">,
        ...

        Edge<"llvm_gcc_c", "llc">,
        Edge<"llvm_gcc_cpp", "llc">,
        ...

        OptionalEdge<"llvm_gcc_c", "opt", (case (switch_on "opt"),
                                          (inc_weight))>,
        OptionalEdge<"llvm_gcc_cpp", "opt", (case (switch_on "opt"),
                                                  (inc_weight))>,
        ...

        OptionalEdge<"llvm_gcc_assembler", "llvm_gcc_cpp_linker",
            (case (input_languages_contain "c++"), (inc_weight),
                  (or (parameter_equals "linker", "g++"),
                      (parameter_equals "linker", "c++")), (inc_weight))>,
        ...

        ]>;

As you can see, the edges can be either default or optional, where optional edges are differentiated by an additional `case` expression used to calculate the weight of this edge. Notice also that we refer to tools via their names (as strings). This makes it possible to add edges to an existing compilation graph in plugins without having to know about all tool definitions used in the graph.

The default edges are assigned a weight of 1, and optional edges get a weight of 0 + 2*N where N is the number of tests that evaluated to true in the `case` expression. It is also possible to provide an integer parameter to `inc_weight` and `dec_weight` - in this case, the weight is increased (or decreased) by the provided value instead of the default 2. It is also possible to change the default weight of an optional edge by using the `default` clause of the `case` construct.

When passing an input file through the graph, LLVMC picks the edge with the maximum weight. To avoid ambiguity, there should be only one default edge between two nodes (with the exception of the root node, which gets a special treatment - there you are allowed to specify one default edge per language).

When multiple plugins are loaded, their compilation graphs are merged together. Since multiple edges that have the same end nodes are not allowed (i.e. the graph is not a multigraph), an edge defined in several plugins will be replaced by the definition from the plugin that was loaded last. Plugin load order can be controlled by using the plugin priority feature described above.

To get a visual representation of the compilation graph (useful for debugging), run `llvmc --view-graph`. You will need `dot` and `gsview` installed for this to work properly.

# Describing options

Command-line options that the plugin supports are defined by using an `OptionList`:

    def Options : OptionList<[
    (switch_option "E", (help "Help string")),
    (alias_option "quiet", "q")
    ...
    ]>;

As you can see, the option list is just a list of DAGs, where each DAG is an option description consisting of the option name and some properties. A plugin can define more than one option list (they are all merged together in the end), which can be handy if one wants to separate option groups syntactically.

- Possible option types:

  > - `switch_option` - a simple boolean switch without arguments, for example `-O2` or `-time`. At most one occurrence is allowed.
  > - `parameter_option` - option that takes one argument, for example `-std=c99`. It is also allowed to use spaces instead of the equality sign: `-std c99`. At most one occurrence is allowed.
  > - `parameter_list_option` - same as the above, but more than one option occurence is allowed.
  > - `prefix_option` - same as the parameter_option, but the option name and argument do not have to be separated. Example: `-ofile`. This can be also specified as `-o file`; however, `-o=file` will be parsed incorrectly (`=file` will be interpreted as option value). At most one occurrence is allowed.
  > - `prefix_list_option` - same as the above, but more than one occurence of the option is allowed; example: `-lm -lpthread`.
  > - `alias_option` - a special option type for creating aliases. Unlike other option types, aliases are not allowed to have any properties besides the aliased option name. Usage example: `(alias_option "preprocess", "E")`

- Possible option properties:

  > - `help` - help string associated with this option. Used for `-help` output.
  > - `required` - this option must be specified exactly once (or, in case of the list options without the `multi_val` property, at least once). Incompatible with `zero_or_one` and `one_or_more`.
  > - `one_or_more` - the option must be specified at least one time. Useful only for list options in conjunction with `multi_val`; for ordinary lists it is synonymous with `required`. Incompatible with `required` and `zero_or_one`.
  > - `optional` - the option can be specified zero or one times. Useful only for list options in conjunction with `multi_val`. Incompatible with `required` and `one_or_more`.
  > - `hidden` - the description of this option will not appear in the `-help` output (but will appear in the `-help-hidden` output).
  > - `really_hidden` - the option will not be mentioned in any help output.
  > - `comma_separated` - Indicates that any commas specified for an option's value should be used to split the value up into multiple values for the option. This property is valid only for list options. In conjunction with `forward_value` can be used to implement option forwarding in style of gcc's `-Wa,`.
  > - `multi_val n` - this option takes n arguments (can be useful in some special cases). Usage example: `(parameter_list_option "foo", (multi_val 3))`; the command-line syntax is '-foo a b c'. Only list options can have this attribute; you can, however, use the `one_or_more`, `optional` and `required` properties.
  > - `init` - this option has a default value, either a string (if it is a parameter), or a boolean (if it is a switch; as in C++, boolean constants are called `true` and `false`). List options can't have `init` attribute. Usage examples: `(switch_option "foo", (init true))`; `(prefix_option "bar", (init "baz"))`.
  > - `extern` - this option is defined in some other plugin, see below.

## External options

Sometimes, when linking several plugins together, one plugin needs to access options defined in some other plugin. Because of the way options are implemented, such options must be marked as `extern`. This is what the `extern` option property is for. Example:

    ...
    (switch_option "E", (extern))
    ...

If an external option has additional attributes besides 'extern', they are ignored. See also the section on plugin priorities.

# Conditional evaluation

The 'case' construct is the main means by which programmability is achieved in LLVMC. It can be used to calculate edge weights, program actions and modify the shell commands to be executed. The 'case' expression is designed after the similarly-named construct in functional languages and takes the form `(case (test_1), statement_1, (test_2), statement_2, ... (test_N), statement_N)`. The statements are evaluated only if the corresponding tests evaluate to true.

Examples:

    // Edge weight calculation

    // Increases edge weight by 5 if "-A" is provided on the
    // command-line, and by 5 more if "-B" is also provided.
    (case
        (switch_on "A"), (inc_weight 5),
        (switch_on "B"), (inc_weight 5))

    // Tool command line specification

    // Evaluates to "cmdline1" if the option "-A" is provided on the
    // command line; to "cmdline2" if "-B" is provided;
    // otherwise to "cmdline3".

    (case
        (switch_on "A"), "cmdline1",
        (switch_on "B"), "cmdline2",
        (default), "cmdline3")

Note the slight difference in 'case' expression handling in contexts of edge weights and command line specification - in the second example the value of the `"B"` switch is never checked when switch `"A"` is enabled, and the whole expression always evaluates to `"cmdline1"` in that case.

Case expressions can also be nested, i.e. the following is legal:

    (case (switch_on "E"), (case (switch_on "o"), ..., (default), ...)
          (default), ...)

You should, however, try to avoid doing that because it hurts readability. It is usually better to split tool descriptions and/or use TableGen inheritance instead.

- Possible tests are:
  - `switch_on` - Returns true if a given command-line switch is provided by the user. Can be given a list as argument, in that case `(switch_on ["foo", "bar", "baz"])` is equivalent to `(and (switch_on "foo"), (switch_on "bar"), (switch_on "baz"))`. Example: `(switch_on "opt")`.
  - `any_switch_on` - Given a list of switch options, returns true if any of the switches is turned on. Example: `(any_switch_on ["foo", "bar", "baz"])` is equivalent to `(or (switch_on "foo"), (switch_on "bar"), (switch_on "baz"))`.
  - `parameter_equals` - Returns true if a command-line parameter equals a given value. Example: `(parameter_equals "W", "all")`.
  - `element_in_list` - Returns true if a command-line parameter list contains a given value. Example: `(element_in_list "l", "pthread")`.
  - `input_languages_contain` - Returns true if a given language belongs to the current input language set. Example: `(input_languages_contain "c++")`.
  - `in_language` - Evaluates to true if the input file language is equal to the argument. At the moment works only with `cmd_line` and `actions` (on non-join nodes). Example: `(in_language "c++")`.
  - `not_empty` - Returns true if a given option (which should be either a parameter or a parameter list) is set by the user. Like `switch_on`, can be also given a list as argument. Example: `(not_empty "o")`.
  - `any_not_empty` - Returns true if `not_empty` returns true for any of the options in the list. Example: `(any_not_empty ["foo", "bar", "baz"])` is equivalent to `(or (not_empty "foo"), (not_empty "bar"), (not_empty "baz"))`.
  - `empty` - The opposite of `not_empty`. Equivalent to `(not (not_empty X))`. Provided for convenience. Can be given a list as argument.
  - `any_not_empty` - Returns true if `not_empty` returns true for any of the options in the list. Example: `(any_empty ["foo", "bar", "baz"])` is equivalent to `(not (and (not_empty "foo"), (not_empty "bar"), (not_empty "baz")))`.
  - `single_input_file` - Returns true if there was only one input file provided on the command-line. Used without arguments: `(single_input_file)`.
  - `multiple_input_files` - Equivalent to `(not (single_input_file))` (the case of zero input files is considered an error).
  - `default` - Always evaluates to true. Should always be the last test in the `case` expression.
  - `and` - A standard binary logical combinator that returns true iff all of its arguments return true. Used like this: `(and (test1), (test2), ... (testN))`. Nesting of `and` and `or` is allowed, but not encouraged.
  - `or` - A binary logical combinator that returns true iff any of its arguments returns true. Example: `(or (test1), (test2), ... (testN))`.
  - `not` - Standard unary logical combinator that negates its argument. Example: `(not (or (test1), (test2), ... (testN)))`.

# Writing a tool description

As was said earlier, nodes in the compilation graph represent tools, which are described separately. A tool definition looks like this (taken from the `include/llvm/CompilerDriver/Tools.td` file):

    def llvm_gcc_cpp : Tool<[
        (in_language "c++"),
        (out_language "llvm-assembler"),
        (output_suffix "bc"),
        (cmd_line "llvm-g++ -c $INFILE -o $OUTFILE -emit-llvm"),
        (sink)
        ]>;

This defines a new tool called `llvm_gcc_cpp`, which is an alias for `llvm-g++`. As you can see, a tool definition is just a list of properties; most of them should be self-explanatory. The `sink` property means that this tool should be passed all command-line options that aren't mentioned in the option list.

The complete list of all currently implemented tool properties follows.

- Possible tool properties:
  - `in_language` - input language name. Can be either a string or a list, in case the tool supports multiple input languages.
  - `out_language` - output language name. Multiple output languages are not allowed.
  - `output_suffix` - output file suffix. Can also be changed dynamically, see documentation on actions.
  - `cmd_line` - the actual command used to run the tool. You can use `$INFILE` and `$OUTFILE` variables, output redirection with `>`, hook invocations (`$CALL`), environment variables (via `$ENV`) and the `case` construct.
  - `join` - this tool is a "join node" in the graph, i.e. it gets a list of input files and joins them together. Used for linkers.
  - `sink` - all command-line options that are not handled by other tools are passed to this tool.
  - `actions` - A single big `case` expression that specifies how this tool reacts on command-line options (described in more detail below).

## Actions

A tool often needs to react to command-line options, and this is precisely what the `actions` property is for. The next example illustrates this feature:

    def llvm_gcc_linker : Tool<[
        (in_language "object-code"),
        (out_language "executable"),
        (output_suffix "out"),
        (cmd_line "llvm-gcc $INFILE -o $OUTFILE"),
        (join),
        (actions (case (not_empty "L"), (forward "L"),
                       (not_empty "l"), (forward "l"),
                       (not_empty "dummy"),
                                 [(append_cmd "-dummy1"), (append_cmd "-dummy2")])
        ]>;

The `actions` tool property is implemented on top of the omnipresent `case` expression. It associates one or more different actions with given conditions - in the example, the actions are `forward`, which forwards a given option unchanged, and `append_cmd`, which appends a given string to the tool execution command. Multiple actions can be associated with a single condition by using a list of actions (used in the example to append some dummy options). The same `case` construct can also be used in the `cmd_line` property to modify the tool command line.

The "join" property used in the example means that this tool behaves like a linker.

The list of all possible actions follows.

- Possible actions:

  > - `append_cmd` - Append a string to the tool invocation command. Example: `(case (switch_on "pthread"), (append_cmd "-lpthread"))`.
  > - `error` - Exit with error. Example: `(error "Mixing -c and -S is not allowed!")`.
  > - `warning` - Print a warning. Example: `(warning "Specifying both -O1 and -O2 is meaningless!")`.
  > - `forward` - Forward the option unchanged. Example: `(forward "Wall")`.
  > - `forward_as` - Change the option's name, but forward the argument unchanged. Example: `(forward_as "O0", "--disable-optimization")`.
  > - `forward_value` - Forward only option's value. Cannot be used with switch options (since they don't have values), but works fine with lists. Example: `(forward_value "Wa,")`.
  > - `forward_transformed_value` - As above, but applies a hook to the option's value before forwarding (see below). When `forward_transformed_value` is applied to a list option, the hook must have signature `std::string hooks::HookName (const std::vector<std::string>&)`. Example: `(forward_transformed_value "m", "ConvertToMAttr")`.
  > - `output_suffix` - Modify the output suffix of this tool. Example: `(output_suffix "i")`.
  > - `stop_compilation` - Stop compilation after this tool processes its input. Used without arguments. Example: `(stop_compilation)`.

# Language map

If you are adding support for a new language to LLVMC, you'll need to modify the language map, which defines mappings from file extensions to language names. It is used to choose the proper toolchain(s) for a given input file set. Language map definition looks like this:

    def LanguageMap : LanguageMap<
        [LangToSuffixes<"c++", ["cc", "cp", "cxx", "cpp", "CPP", "c++", "C"]>,
         LangToSuffixes<"c", ["c"]>,
         ...
        ]>;

For example, without those definitions the following command wouldn't work:

    $ llvmc hello.cpp
    llvmc: Unknown suffix: cpp

The language map entries are needed only for the tools that are linked from the root node. Since a tool can't have multiple output languages, for inner nodes of the graph the input and output languages should match. This is enforced at compile-time.

# Option preprocessor

It is sometimes useful to run error-checking code before processing the compilation graph. For example, if optimization options "-O1" and "-O2" are implemented as switches, we might want to output a warning if the user invokes the driver with both of these options enabled.

The `OptionPreprocessor` feature is reserved specially for these occasions. Example (adapted from the built-in Base plugin):

    def Preprocess : OptionPreprocessor<
    (case (not (any_switch_on ["O0", "O1", "O2", "O3"])),
               (set_option "O2"),
          (and (switch_on "O3"), (any_switch_on ["O0", "O1", "O2"])),
               (unset_option ["O0", "O1", "O2"]),
          (and (switch_on "O2"), (any_switch_on ["O0", "O1"])),
               (unset_option ["O0", "O1"]),
          (and (switch_on "O1"), (switch_on "O0")),
               (unset_option "O0"))
    >;

Here, `OptionPreprocessor` is used to unset all spurious `-O` options so that they are not forwarded to the compiler. If no optimization options are specified, `-O2` is enabled.

`OptionPreprocessor` is basically a single big `case` expression, which is evaluated only once right after the plugin is loaded. The only allowed actions in `OptionPreprocessor` are `error`, `warning`, and two special actions: `unset_option` and `set_option`. As their names suggest, they can be used to set or unset a given option. To set an option with `set_option`, use the two-argument form: `(set_option "parameter", VALUE)`. Here, `VALUE` can be either a string, a string list, or a boolean constant.

For convenience, `set_option` and `unset_option` also work on lists. That is, instead of `[(unset_option "A"), (unset_option "B")]` you can use `(unset_option ["A", "B"])`. Obviously, `(set_option ["A", "B"])` is valid only if both `A` and `B` are switches.

# More advanced topics

## Hooks and environment variables

Normally, LLVMC executes programs from the system `PATH`. Sometimes, this is not sufficient: for example, we may want to specify tool paths or names in the configuration file. This can be easily achieved via the hooks mechanism. To write your own hooks, just add their definitions to the `PluginMain.cpp` or drop a `.cpp` file into the your plugin directory. Hooks should live in the `hooks` namespace and have the signature `std::string hooks::MyHookName ([const char* Arg0 [ const char* Arg2 [, ...]]])`. They can be used from the `cmd_line` tool property:

    (cmd_line "$CALL(MyHook)/path/to/file -o $CALL(AnotherHook)")

To pass arguments to hooks, use the following syntax:

    (cmd_line "$CALL(MyHook, 'Arg1', 'Arg2', 'Arg # 3')/path/to/file -o1 -o2")

It is also possible to use environment variables in the same manner:

    (cmd_line "$ENV(VAR1)/path/to/file -o $ENV(VAR2)")

To change the command line string based on user-provided options use the `case` expression (documented above):

    (cmd_line
      (case
        (switch_on "E"),
           "llvm-g++ -E -x c $INFILE -o $OUTFILE",
        (default),
           "llvm-g++ -c -x c $INFILE -o $OUTFILE -emit-llvm"))

## How plugins are loaded

It is possible for LLVMC plugins to depend on each other. For example, one can create edges between nodes defined in some other plugin. To make this work, however, that plugin should be loaded first. To achieve this, the concept of plugin priority was introduced. By default, every plugin has priority zero; to specify the priority explicitly, put the following line in your plugin's TableGen file:

    def Priority : PluginPriority<$PRIORITY_VALUE>;
    # Where PRIORITY_VALUE is some integer > 0

Plugins are loaded in order of their (increasing) priority, starting with 0. Therefore, the plugin with the highest priority value will be loaded last.

## Debugging

When writing LLVMC plugins, it can be useful to get a visual view of the resulting compilation graph. This can be achieved via the command line option `--view-graph`. This command assumes that Graphviz and Ghostview are installed. There is also a `--write-graph` option that creates a Graphviz source file (`compilation-graph.dot`) in the current directory.

Another useful `llvmc` option is `--check-graph`. It checks the compilation graph for common errors like mismatched output/input language names, multiple default edges and cycles. These checks can't be performed at compile-time because the plugins can load code dynamically. When invoked with `--check-graph`, `llvmc` doesn't perform any compilation tasks and returns the number of encountered errors as its status code.

## Conditioning on the executable name

For now, the executable name (the value passed to the driver in `argv[0]`) is accessible only in the C++ code (i.e. hooks). Use the following code:

    namespace llvmc {
    extern const char* ProgramName;
    }

    namespace hooks {

    std::string MyHook() {
    //...
    if (strcmp(ProgramName, "mydriver") == 0) {
       //...

    }

    } // end namespace hooks

In general, you're encouraged not to make the behaviour dependent on the executable file name, and use command-line switches instead. See for example how the `Base` plugin behaves when it needs to choose the correct linker options (think `g++` vs. `gcc`).