Customizing LLVMC: Reference Manual

- - -

Contents

Introduction
Compiling with llvmc
Predefined options
Compiling LLVMC-based drivers
Customizing LLVMC: the compilation graph
Describing options
Conditional evaluation
Writing a tool description
- Actions
-
Language map
Option preprocessor
More advanced topics
-

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 described in -high-level TableGen code, from which an efficient C++ representation is -automatically derived. This makes it possible to 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 TableGen code:

-o FILE - Output file name.
-x LANGUAGE - Specify the language of the following input files -until the next -x option.
-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. Exit with code -zero if no errors were found, and return the number of found errors -otherwise. Hidden option, useful for debugging.
--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.
--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.
--help, --help-hidden, --version - These options have -their standard meaning.

Compiling LLVMC-based drivers

It's easiest to start working on your own LLVMC driver by copying the skeleton -project which lives under $LLVMC_DIR/examples/Skeleton:

-$ cd $LLVMC_DIR/examples
-$ cp -r Skeleton MyDriver
-$ cd MyDriver
-$ ls
-AutoGenerated.td  Hooks.cpp  Main.cpp  Makefile
-

As you can see, our basic driver consists of only three files (not counting the -build script). AutoGenerated.td contains TableGen description of the -compilation graph; its format is documented in the following -sections. Hooks.cpp is an empty file that should be used for hook -definitions (see below). Main.cpp is just a helper used to compile the -auto-generated C++ code produced from TableGen source.

The first thing that you should do is to change the LLVMC_BASED_DRIVER -variable in the Makefile:

-LLVMC_BASED_DRIVER=MyDriver
-

It can also be a good idea to put your TableGen code into a file with a less -generic name:

-$ touch MyDriver.td
-$ vim AutoGenerated.td
-[...]
-include "MyDriver.td"
-

If you have more than one TableGen source file, they all should be included from -AutoGenerated.td, since this file is used by the build system to generate -C++ code.

To build your driver, just cd to its source directory and run make. The -resulting executable will be put into $LLVM_OBJ_DIR/$(BuildMode)/bin.

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 = $LLVM_OBJ_DIR/tools/llvmc/
-$ mkdir $LLVMC_OBJ_DIR/examples/MyDriver/
-$ cp $LLVMC_SRC_DIR/examples/MyDriver/Makefile \
-  $LLVMC_OBJ_DIR/examples/MyDriver/
-$ cd $LLVMC_OBJ_DIR/examples/MyDriver
-$ make
-

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 llvmc/src/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 -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. Default weight of an -optional edge can be changed 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 compilation graphs are defined, they are merged together. Multiple -edges with the same end nodes are not allowed (i.e. the graph is not a -multigraph), and will lead to a compile-time error.

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 supported by the driver 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. More than -one option list can be defined (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 by default.
- 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 -occurrence 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 occurrence 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")
- switch_list_option - like switch_option with the zero_or_more -property, but remembers how many times the switch was turned on. Useful -mostly for forwarding. Example: when -foo is a switch option (with the -zero_or_more property), the command driver -foo -foo is forwarded -as some-tool -foo, but when -foo is a switch list, the same command -is forwarded as some-tool -foo -foo.
-
-
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 optional and one_or_more.
- optional - the option can be specified either zero times or exactly -once. The default for switch options. Useful only for list options in -conjunction with multi_val. Incompatible with required, -zero_or_more and one_or_more.
- one_or_more - the option must be specified at least once. Can be useful -to allow switch options be both obligatory and be specified multiple -times. For list options is useful only in conjunction with multi_val; -for ordinary it is synonymous with required. Incompatible with -required, optional and zero_or_more.
- zero_or_more - the option can be specified zero or more times. Useful -to allow a single switch option to be specified more than -once. Incompatible with required, optional 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")).
-
-

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 multiple arguments, 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 number 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 (first -argument) equals a given value (second argument). -Example: (parameter_equals "W", "all").
- element_in_list - Returns true if a command-line parameter list (first -argument) contains a given value (second argument). -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 command 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 multiple arguments. -Examples: (not_empty "o"), (not_empty "o", "l").
- any_not_empty - Returns true if not_empty returns true for any of -the provided options. -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)). Can be given multiple arguments.
- any_not_empty - Returns true if not_empty returns true for any of -the provided options. -Example: (any_empty "foo", "bar", "baz") is equivalent to (or -(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 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 logical combinator that returns true iff any of its arguments -return 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 -llvmc/src/Base.td file):

-def llvm_gcc_cpp : Tool<[
-    (in_language "c++"),
-    (out_language "llvm-assembler"),
-    (output_suffix "bc"),
-    (command "llvm-g++ -c -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 given multiple arguments, in -case the tool supports multiple input languages. Used for typechecking and -mapping file extensions to tools.
- out_language - output language name. Multiple output languages are -allowed. Used for typechecking the compilation graph.
- output_suffix - output file suffix. Can also be changed dynamically, see -documentation on actions.
-

-
-
command - the actual command used to run the tool. You can use 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).
-
-

-
-
out_file_option, in_file_option - Options appended to the -command string to designate output and input files. Default values are -"-o" and "", respectively.
-
-

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"),
-    (command "llvm-gcc"),
-    (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. A tool can have multiple output languages.

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 llvm/src/Base.td.in):

-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 driver is started. 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 with multiple -arguments. That is, instead of [(unset_option "A"), (unset_option "B")] you -can use (unset_option "A", "B"). Obviously, (set_option "A", "B") is -only valid if both A and B are switches.

More advanced topics

Hooks and environment variables

Normally, LLVMC searches for programs in 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 achieved via the hooks mechanism. To write your -own hooks, add their definitions to the Hooks.cpp or drop a .cpp file -into your driver 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 command tool property:

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

To pass arguments to hooks, use the following syntax:

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

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

-(command "$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):

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

Debugging

When writing LLVMC-based drivers, 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 (which 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. When invoked with --check-graph, llvmc doesn't -perform any compilation tasks and returns the number of encountered errors as -its status code. In the future, these checks will be performed at compile-time -and this option will disappear.

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 llvmc program behaves when it needs to choose the correct linker options -(think g++ vs. gcc).

- -

- -

- -Mikhail Glushenkov
-LLVM Compiler Infrastructure
- -Last modified: $Date$ -

Currently, there isn't much. LLVM supports an intermediate representation which is useful for code representation but will not support the high level (abstract syntax tree) representation needed by most compilers. There are no - facilities for lexical nor semantic analysis. There is, however, a mostly - implemented configuration-driven - compiler driver which simplifies the task - of running optimizations, linking, and executable generation.

diff --git a/docs/index.html b/docs/index.html index fa9f1fae0fc..422dfb2347f 100644 --- a/docs/index.html +++ b/docs/index.html @@ -216,14 +216,6 @@ in LLVM.

Bugpoint - automatic bug finder and test-case reducer description and usage information.

Compiler Driver (llvmc) Tutorial -- This document is a tutorial introduction to the usage and -configuration of the LLVM compiler driver tool, llvmc.

- -

Compiler Driver (llvmc) -Reference - This document describes the design and configuration -of llvmc in more detail.

LLVM Bitcode File Format - This describes the file format and encoding used for LLVM "bc" files.

Customizing LLVMC: Reference Manual

Introduction

Compiling with `llvmc`

Predefined options

Compiling LLVMC-based drivers

Customizing LLVMC: the compilation graph

Describing options

Conditional evaluation

Writing a tool description

Actions

Language map

Option preprocessor

More advanced topics

Hooks and environment variables

Debugging

Conditioning on the executable name

Tutorial - Using LLVMC

Introduction

Using the `llvmc` program

Using LLVMC to generate toolchain drivers