Add first proof-of-concept universal compiler driver framework based

on ideas mentioned in PR686.
Written by Mikhail Glushenkov and contributed by Codedgers, Inc.

Old llvmc will be removed soon after new one will have all its properties.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48699 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 270 insertions, 0 deletions

diff --git a/tools/llvmc2/doc/LLVMC-Enhancements.rst b/tools/llvmc2/doc/LLVMC-Enhancements.rst
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+Introduction
+============
+
+A complete rewrite of the LLVMC compiler driver is proposed, aimed at
+making it more configurable and useful.
+
+Motivation
+==========
+
+As it stands, current version of LLVMC does not meet its stated goals
+of configurability and extensibility and is therefore not used
+much. The need for enhancements in LLVMC is also reflected in [1]_. The
+proposed rewrite will fix the aforementioned deficiences and provide
+an extensible, future-proof solution.
+
+Design
+======
+
+A compiler driver's job is essentially to find a way how to transform
+a set of input files into a set of targets, depending on the
+user-provided options. Since several ways of transformation can exist
+potentially, it's natural to use a directed graph to represent all of
+them. In this graph, nodes are tools (for example, ``gcc -S`` is a tool
+that generates assembly from C language files) and edges between them
+mean that the output of one tool can be given as input to another (as
+in ``gcc -S -o - file.c | as``). We'll call this graph the compilation
+graph.
+
+The proposed design revolves around the compilation graph and the
+following core abstractions:
+
+- Target - An (intermediate) compilation target.
+
+- Action - A shell command template that represents basic compilation
+  transformation(example: ``gcc -S $INPUT_FILE -o $OUTPUT_FILE``).
+
+- Tool - Encapsulates information about a concrete tool used in the
+  compilation process, produces Actions. Operation depends on
+  command-line options provided by the user.
+
+- GraphBuilder - Constructs the compilation graph, operation depends
+  on command-line options.
+
+- GraphTraverser - Traverses the compilation graph and constructs a
+  sequence of Actions needed to build the target file, operation
+  depends on command-line options.
+
+A high-level view of the compilation process:
+
+  1. Configuration libraries (see below) are loaded in and the
+  compilation graph is constructed from the tool descriptions.
+
+  2. Information about possible options is gathered from (the nodes of)
+  the compilation graph.
+
+  3. Options are parsed based on data gathered in step 2.
+
+  4. A sequence of Actions needed to build the target is constructed
+  using the compilation graph and provided options.
+
+  5. The resulting action sequence is executed.
+
+Extensibility
+==============
+
+To make this design extensible, TableGen [2]_ will be used for
+automatic generation of the Tool classes. Users wanting to customize
+LLVMC will need to write a configuration library consisting of a set
+of TableGen descriptions of compilation tools plus a number of hooks
+that influence compilation graph construction and traversal. LLVMC
+will have the ability to load user configuration libraries at runtime;
+in fact, its own basic functionality will be implemented as a
+configuration library.
+
+TableGen specification example
+------------------------------
+
+This small example specifies a Tool that converts C source to object
+files. Note that it is only a mock-up of inteded functionality, not a
+final specification::
+
+    def GCC : Tool<
+     GCCProperties, // Properties of this tool
+     GCCOptions     // Options description for this tool
+    >;
+
+    def GCCProperties : ToolProperties<[
+     ToolName<"GCC">,
+     InputLanguageName<"C">,
+     OutputLanguageName<"Object-Code">
+     InputFileExtension<"c">,
+     OutputFileExtension<"o">,
+     CommandFormat<"gcc -c $OPTIONS $FILES">
+    ]>;
+
+    def GCCOptions : ToolOptions<[
+     Option<
+       "-Wall",                 // Option name
+       [None],                  // Allowed values
+       [AddOption<"-Wall">]>,   // Action
+
+     Option<
+       "-Wextra",               // Option name
+       [None],                  // Allowed values
+       [AddOption<"-Wextra">]>, // Action
+
+     Option<
+       "-W",                 // Option name
+       [None],               // Allowed values
+       [AddOption<"-W">]>,   // Action
+
+     Option<
+       "-D",        // Option name
+       [AnyString], // Allowed values
+
+       [AddOptionWithArgument<"-D",GetOptionArgument<"-D">>]
+       // Action:
+       // If the driver was given option "-D<argument>", add
+       // option "-D" with the same argument to the invocation string of
+       // this tool.
+       >
+
+     ]>;
+
+Example of generated code
+-------------------------
+
+The specification above compiles to the following code (again, it's a
+mock-up)::
+
+    class GCC : public Tool {
+
+    public:
+
+      GCC() { //... }
+
+     // Properties
+
+      static const char* ToolName = "GCC";
+      static const char* InputLanguageName = "C";
+      static const char* OutputLanguageName = "Object-Code";
+      static const char* InputFileExtension = "c";
+      static const char* OutputFileExtension = "o";
+      static const char* CommandFormat = "gcc -c $OPTIONS $FILES";
+
+     // Options
+
+     OptionsDescription SupportedOptions() {
+       OptionsDescription supportedOptions;
+
+       supportedOptions.Add(Option("-Wall"));
+       supportedOptions.Add(Option("-Wextra"));
+       supportedOptions.Add(Option("-W"));
+       supportedOptions.Add(Option("-D", AllowedArgs::ANY_STRING));
+
+       return supportedOptions;
+     }
+
+     Action GenerateAction(Options providedOptions) {
+       Action generatedAction(CommandFormat); Option curOpt;
+
+       curOpt = providedOptions.Get("-D");
+       if (curOpt) {
+          assert(curOpt.HasArgument());
+          generatedAction.AddOption(Option("-D", curOpt.GetArgument()));
+       }
+
+       curOpt = providedOptions.Get("-Wall");
+       if (curOpt)
+         generatedAction.AddOption(Option("-Wall"));
+
+       curOpt = providedOptions.Get("-Wextra");
+       if (curOpt)
+         generatedAction.AddOption(Option("-Wall"));
+
+       curOpt = providedOptions.Get("-W");
+       if (curOpt)
+         generatedAction.AddOption(Option("-Wall")); }
+
+       return generatedAction;
+     }
+
+    };
+
+    // defined somewhere...
+
+    class Action { public: void AddOption(const Option& opt) {...}
+    int Run(const Filenames& fnms) {...}
+
+    }
+
+Option handling
+===============
+
+Since one of the main tasks of the compiler driver is to correctly
+handle user-provided options, it is important to define this process
+in exact way. The intent of the proposed scheme is to function as a
+drop-in replacement for GCC.
+
+Option syntax
+-------------
+
+Option syntax is specified by the following formal grammar::
+
+        <command-line>      ::=  <option>*
+        <option>            ::=  <positional-option> | <named-option>
+        <named-option>      ::=  -[-]<option-name>[<delimeter><option-argument>]
+        <delimeter>         ::=  ',' | '=' | ' '
+        <positional-option> ::=  <string>
+        <option-name>       ::=  <string>
+        <option-argument>   ::=  <string>
+
+This roughly corresponds to the GCC option syntax. Note that grouping
+of short options(as in ``ls -la``) is forbidden.
+
+Example::
+
+        llvmc -O3 -Wa,-foo,-bar -pedantic -std=c++0x a.c b.c c.c
+
+Option arguments can also have special forms: for example, an argument
+can be a comma-separated list (like in -Wa,-foo,-bar). In such cases,
+it's up to the option handler to parse the argument.
+
+Option semantics
+----------------
+
+According to their meaning, options are classified into following
+categories:
+
+- Global options - Options that influence compilation graph
+  construction/traversal. Example: -E (stop after preprocessing).
+
+- Local options - Options that influence one or several Actions in
+  the generated action sequence. Example: -O3 (turn on optimization).
+
+- Prefix options - Options that influence meaning of the following
+  command-line arguments. Example: -x language (specify language for
+  the input files explicitly). Prefix options can be local or global.
+
+- Built-in options - Options that are hard-coded into
+  driver. Examples: --help, -o file/-pipe (redirect output). Can be
+  local or global.
+
+Naming
+======
+
+Since the compiler driver, as a single point of access to the LLVM
+tool set, is a very often used tool, it would be desirable to make its name
+as short and easy to type as possible. Some possible names are 'llcc' or
+'lcc', by analogy with gcc.
+
+
+Issues
+======
+
+1. Should global-options-influencing hooks be written by hand or
+   auto-generated from TableGen specifications?
+
+2. More?
+
+References
+==========
+
+.. [1] LLVM Bug#686
+
+       http://llvm.org/bugs/show_bug.cgi?id=686
+
+.. [2] TableGen Fundamentals
+
+       http://llvm.org/docs/TableGenFundamentals.html