path: root/mcs/docs/compiler

		       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 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. 

	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.