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                       The Internals of the Mono C# Compiler
        
                                Miguel de Icaza
                              (miguel@ximian.com)
                                      2002

* Abstract

        The Mono C# compiler is a C# compiler written in C# itself.
        Its goals are to provide a free and alternate implementation
        of the C# language.  The Mono C# compiler generates ECMA CIL
        images through the use of the System.Reflection.Emit API which
        enable the compiler to be platform independent.
        
* Overview: How the compiler fits together

        The compilation process is managed by the compiler driver (it
        lives in driver.cs).

        The compiler reads a set of C# source code files, and parses
        them.  Any assemblies or modules that the user might want to
        use with his project are loaded after parsing is done.

        Once all the files have been parsed, the type hierarchy is
        resolved.  First interfaces are resolved, then types and
        enumerations.

        Once the type hierarchy is resolved, every type is populated:
        fields, methods, indexers, properties, events and delegates
        are entered into the type system.  

        At this point the program skeleton has been completed.  The
        next process is to actually emit the code for each of the
        executable methods.  The compiler drives this from
        RootContext.EmitCode.

        Each type then has to populate its methods: populating a
        method requires creating a structure that is used as the state
        of the block being emitted (this is the EmitContext class) and
        then generating code for the topmost statement (the Block).

        Code generation has two steps: the first step is the semantic
        analysis (Resolve method) that resolves any pending tasks, and
        guarantees that the code is correct.  The second phase is the
        actual code emission.  All errors are flagged during in the
        "Resolution" process. 

        After all code has been emitted, then the compiler closes all
        the types (this basically tells the Reflection.Emit library to
        finish up the types), resources, and definition of the entry
        point are done at this point, and the output is saved to
        disk. 

        The following list will give you an idea of where the
        different pieces of the compiler live:

        Infrastructure:

            driver.cs:
                This drives the compilation process: loading of
                command line options; parsing the inputs files;
                loading the referenced assemblies; resolving the type
                hierarchy and emitting the code. 

            codegen.cs:
                
                The state tracking for code generation. 

            attribute.cs:

                Code to do semantic analysis and emit the attributes
                is here.

            rootcontext.cs:

                Keeps track of the types defined in the source code,
                as well as the assemblies loaded.  

            typemanager.cs:

                This contains the MCS type system.

            report.cs:

                Error and warning reporting methods.

            support.cs:

                Assorted utility functions used by the compiler.
                
        Parsing

            cs-tokenizer.cs:

                The tokenizer for the C# language, it includes also
                the C# pre-processor.

            cs-parser.jay, cs-parser.cs:

                The parser is implemented using a C# port of the Yacc
                parser.  The parser lives in the cs-parser.jay file,
                and cs-parser.cs is the generated parser.

            location.cs:

                The `location' structure is a compact representation
                of a file, line, column where a token, or a high-level
                construct appears.  This is used to report errors.

        Expressions:
          
            ecore.cs
        
                Basic expression classes, and interfaces most shared
                code and static methods are here.

            expression.cs:

                Most of the different kinds of expressions classes
                live in this file.

            assign.cs:

                The assignment expression got its own file.

            constant.cs:

                The classes that represent the constant expressions.

            literal.cs
                
                Literals are constants that have been entered manually
                in the source code, like `1' or `true'.  The compiler
                needs to tell constants from literals apart during the
                compilation process, as literals sometimes have some
                implicit extra conversions defined for them. 

            cfold.cs:

                The constant folder for binary expressions.

        Statements

            statement.cs:

                All of the abstract syntax tree elements for
                statements live in this file.  This also drives the
                semantic analysis process.

        Declarations, Classes, Structs, Enumerations

            decl.cs

                This contains the base class for Members and
                Declaration Spaces.   A declaration space introduces
                new names in types, so classes, structs, delegates and
                enumerations derive from it.

            class.cs:
                
                Methods for holding and defining class and struct
                information, and every member that can be in these
                (methods, fields, delegates, events, etc).

                The most interesting type here is the `TypeContainer'
                which is a derivative of the `DeclSpace' 

            delegate.cs:

                Handles delegate definition and use. 

            enum.cs:

                Handles enumerations.

            interface.cs:

                Holds and defines interfaces.  All the code related to
                interface declaration lives here.

            parameter.cs:

                During the parsing process, the compiler encapsulates
                parameters in the Parameter and Parameters classes.
                These classes provide definition and resolution tools
                for them.

            pending.cs:

                Routines to track pending implementations of abstract
                methods and interfaces.  These are used by the
                TypeContainer-derived classes to track whether every
                method required is implemented.

        
* The parsing process

        All the input files that make up a program need to be read in
        advance, because C# allows declarations to happen after an
        entity is used, for example, the following is a valid program:

        class X : Y {
                static void Main ()
                {
                        a = "hello"; b = "world";
                }
                string a;
        }
        
        class Y {
                public string b;
        }

        At the time the assignment expression `a = "hello"' is parsed,
        it is not know whether a is a class field from this class, or
        its parents, or whether it is a property access or a variable
        reference.  The actual meaning of `a' will not be discvored
        until the semantic analysis phase.

** The Tokenizer and the pre-processor

        The tokenizer is contained in the file `cs-tokenizer.cs', and
        the main entry point is the `token ()' method.  The tokenizer
        implements the `yyParser.yyInput' interface, which is what the
        Yacc/Jay parser will use when fetching tokens.  

        Token definitions are generated by jay during the compilation
        process, and those can be references from the tokenizer class
        with the `Token.' prefix. 

        Each time a token is returned, the location for the token is
        recorded into the `Location' property, that can be accessed by
        the parser.  The parser retrieves the Location properties as
        it builds its internal representation to allow the semantic
        analysis phase to produce error messages that can pin point
        the location of the problem. 

        Some tokens have values associated with it, for example when
        the tokenizer encounters a string, it will return a
        LITERAL_STRING token, and the actual string parsed will be
        available in the `Value' property of the tokenizer.   The same
        mechanism is used to return integers and floating point
        numbers. 

        C# has a limited pre-processor that allows conditional
        compilation, but it is not as fully featured as the C
        pre-processor, and most notably, macros are missing.  This
        makes it simple to implement in very few lines and mesh it
        with the tokenizer.

        The `handle_preprocessing_directive' method in the tokenizer
        handles all the pre-processing, and it is invoked when the '#'
        symbol is found as the first token in a line.  

        The state of the pre-processor is contained in a Stack called
        `ifstack', this state is used to track the if/elif/else/endif
        nesting and the current state.  The state is encoded in the
        top of the stack as a number of values `TAKING',
        `TAKEN_BEFORE', `ELSE_SEEN', `PARENT_TAKING'.

** Locations

        Locations are encoded as a 32-bit number (the Location
        struct) that map each input source line to a linear number.
        As new files are parsed, the Location manager is informed of
        the new file, to allow it to map back from an int constant to
        a file + line number.

        Prior to parsing/tokenizing any source files, the compiler
        generates a list of all the source files and then reserves the
        low N bits of the location to hold the source file, where N is
        large enough to hold at least twice as many source files as were
        specified on the command line (to allow for a #line in each file).
        The upper 32-N bits are the line number in that file.

        The token 0 is reserved for ``anonymous'' locations, ie. if we
        don't know the location (Location.Null).

        The tokenizer also tracks the column number for a token, but
        this is currently not being used or encoded.  It could
        probably be encoded in the low 9 bits, allowing for columns
        from 1 to 512 to be encoded.

* The Parser

        The parser is written using Jay, which is a port of Berkeley
        Yacc to Java, that I later ported to C#. 

        Many people ask why the grammar of the parser does not match
        exactly the definition in the C# specification.  The reason is
        simple: the grammar in the C# specification is designed to be
        consumed by humans, and not by a computer program.  Before
        you can feed this grammar to a tool, it needs to be simplified
        to allow the tool to generate a correct parser for it. 

        In the Mono C# compiler, we use a class for each of the
        statements and expressions in the C# language.  For example,
        there is a `While' class for the the `while' statement, a
        `Cast' class to represent a cast expression and so on.

        There is a Statement class, and an Expression class which are
        the base classes for statements and expressions. 

** Namespaces
        
        Using list.

* Internal Representation

** Expressions

        Expressions in the Mono C# compiler are represented by the
        `Expression' class.  This is an abstract class that particular
        kinds of expressions have to inherit from and override a few
        methods.

        The base Expression class contains two fields: `eclass' which
        represents the "expression classification" (from the C#
        specs) and the type of the expression.

        Expressions have to be resolved before they are can be used.
        The resolution process is implemented by overriding the
        `DoResolve' method.  The DoResolve method has to set the
        `eclass' field and the `type', perform all error checking and
        computations that will be required for code generation at this
        stage. 

        The return value from DoResolve is an expression.  Most of the
        time an Expression derived class will return itself (return
        this) when it will handle the emission of the code itself, or
        it can return a new Expression.

        For example, the parser will create an "ElementAccess" class
        for:

                a [0] = 1;

        During the resolution process, the compiler will know whether
        this is an array access, or an indexer access.  And will
        return either an ArrayAccess expression or an IndexerAccess
        expression from DoResolve.



*** The Expression Class

        The utility functions that can be called by all children of
        Expression. 

** Constants

        Constants in the Mono C# compiler are reprensented by the
        abstract class `Constant'.  Constant is in turn derived from
        Expression.  The base constructor for `Constant' just sets the
        expression class to be an `ExprClass.Value', Constants are
        born in a fully resolved state, so the `DoResolve' method
        only returns a reference to itself.

        Each Constant should implement the `GetValue' method which
        returns an object with the actual contents of this constant, a
        utility virtual method called `AsString' is used to render a
        diagnostic message.  The output of AsString is shown to the
        developer when an error or a warning is triggered.

        Constant classes also participate in the constant folding
        process.  Constant folding is invoked by those expressions
        that can be constant folded invoking the functionality
        provided by the ConstantFold class (cfold.cs).   

        Each Constant has to implement a number of methods to convert
        itself into a Constant of a different type.  These methods are
        called `ConvertToXXXX' and they are invoked by the wrapper
        functions `ToXXXX'.  These methods only perform implicit
        numeric conversions.  Explicit conversions are handled by the
        `Cast' expression class.

        The `ToXXXX' methods are the entry point, and provide error
        reporting in case a conversion can not be performed.

** Constant Folding

        The C# language requires constant folding to be implemented.
        Constant folding is hooked up in the Binary.Resolve method.
        If both sides of a binary expression are constants, then the
        ConstantFold.BinaryFold routine is invoked.  

        This routine implements all the binary operator rules, it
        is a mirror of the code that generates code for binary
        operators, but that has to be evaluated at runtime.

        If the constants can be folded, then a new constant expression
        is returned, if not, then the null value is returned (for
        example, the concatenation of a string constant and a numeric
        constant is deferred to the runtime). 

** Side effects

        a [i++]++ 
        a [i++] += 5;

** Statements

* The semantic analysis 

        Hence, the compiler driver has to parse all the input files.
        Once all the input files have been parsed, and an internal
        representation of the input program exists, the following
        steps are taken:

                * The interface hierarchy is resolved first.
                  As the interface hierarchy is constructed,
                  TypeBuilder objects are created for each one of
                  them. 

                * Classes and structure hierarchy is resolved next,
                  TypeBuilder objects are created for them.

                * Constants and enumerations are resolved.

                * Method, indexer, properties, delegates and event
                  definitions are now entered into the TypeBuilders. 

                * Elements that contain code are now invoked to
                  perform semantic analysis and code generation.

* Output Generation

** Code Generation

        The EmitContext class is created any time that IL code is to
        be generated (methods, properties, indexers and attributes all
        create EmitContexts).  

        The EmitContext keeps track of the current namespace and type
        container.  This is used during name resolution.

        An EmitContext is used by the underlying code generation
        facilities to track the state of code generation:

                * The ILGenerator used to generate code for this
                  method.

                * The TypeContainer where the code lives, this is used
                  to access the TypeBuilder.

                * The DeclSpace, this is used to resolve names through
                  RootContext.LookupType in the various statements and
                  expressions. 
        
        Code generation state is also tracked here:

                * CheckState:

                  This variable tracks the `checked' state of the
                  compilation, it controls whether we should generate
                  code that does overflow checking, or if we generate
                  code that ignores overflows.
                  
                  The default setting comes from the command line
                  option to generate checked or unchecked code plus
                  any source code changes using the checked/unchecked
                  statements or expressions.  Contrast this with the
                  ConstantCheckState flag.

                * ConstantCheckState
                  
                  The constant check state is always set to `true' and
                  cant be changed from the command line.  The source
                  code can change this setting with the `checked' and
                  `unchecked' statements and expressions.
                  
                * IsStatic
                  
                  Whether we are emitting code inside a static or
                  instance method
                  
                * ReturnType
                  
                  The value that is allowed to be returned or NULL if
                  there is no return type.
                  
                  
                * ContainerType
                  
                  Points to the Type (extracted from the
                  TypeContainer) that declares this body of code
                  summary>
                  
                  
                * IsConstructor
                  
                  Whether this is generating code for a constructor

                * CurrentBlock

                  Tracks the current block being generated.

                * ReturnLabel;
                
                  The location where return has to jump to return the
                  value

        A few variables are used to track the state for checking in
        for loops, or in try/catch statements:

                * InFinally
                
                  Whether we are in a Finally block

                * InTry

                  Whether we are in a Try block

                * InCatch
                  
                  Whether we are in a Catch block

                * InUnsafe
                  Whether we are inside an unsafe block
                
* Miscelaneous

** Error Processing.

        Errors are reported during the various stages of the
        compilation process.  The compiler stops its processing if
        there are errors between the various phases.  This simplifies
        the code, because it is safe to assume always that the data
        structures that the compiler is operating on are always
        consistent.

        The error codes in the Mono C# compiler are the same as those
        found in the Microsoft C# compiler, with a few exceptions
        (where we report a few more errors, those are documented in
        mcs/errors/errors.txt).  The goal is to reduce confussion to
        the users, and also to help us track the progress of the
        compiler in terms of the errors we report. 

        The Report class provides error and warning display functions,
        and also keeps an error count which is used to stop the
        compiler between the phases.  

        A couple of debugging tools are available here, and are useful
        when extending or fixing bugs in the compiler.  If the
        `--fatal' flag is passed to the compiler, the Report.Error
        routine will throw an exception.  This can be used to pinpoint
        the location of the bug and examine the variables around the
        error location.

        Warnings can be turned into errors by using the `--werror'
        flag to the compiler. 

        The report class also ignores warnings that have been
        specified on the command line with the `--nowarn' flag.

        Finally, code in the compiler uses the global variable
        RootContext.WarningLevel in a few places to decide whether a
        warning is worth reporting to the user or not.