1 * MCS: The Ximian C# compiler
3 The Mono C# compiler is considered feature complete at this
4 point and relatively mature. MCS is able to compile itself
5 and many more C# programs (there is a test suite included that
6 you can use). It is routinely used to compile Mono, roughly
7 half a million lines of C# code.
11 The Mono C# compiler is part of the `mcs' module in the Mono CVS
12 you can get it from our <a href="anoncvs.html">Anonymous CVS</a> server,
13 or you can get nightly <a href="download.html">download page</a>.
17 MCS is written in C# and uses heavily the .NET APIs. MCS runs
18 on Linux with the Mono runtime and on Windows with both the
19 .NET runtime and the Mono runtime.
21 ** Reporting Bugs in MCS
23 When you report a bug, try to provide a small test case that would
24 show the error so we can include this as part of the Mono C# regression
27 If the bug is an error or a warning that we do not flag, write
28 a sample program called `csXXXX.cs' where XXXX is the code number
29 that is used by the Microsoft C# compiler that illustrates the
30 problem. That way we can also do regression tests on the invalid
33 ** Phases of the compiler
35 The compiler has a number of phases:
38 * Lexical analyzer: hand-coded lexical analyzer that
39 provides tokens to the parser.
41 * The Parser: the parser is implemented using Jay (A
42 Berkeley Yacc port to Java, that I ported to C#).
43 The parser does minimal work and syntax checking,
44 and only constructs a parsed tree.
46 Each language element gets its own class. The code
47 convention is to use an uppercase name for the
48 language element. So a C# class and its associated
49 information is kept in a "Class" class, a "struct"
50 in a "Struct" class and so on. Statements derive
51 from the "Statement" class, and Expressions from the
54 * Parent class resolution: before the actual code
55 generation, we need to resolve the parents and
56 interfaces for interface, classe and struct
59 * Semantic analysis: since C# can not resolve in a
60 top-down pass what identifiers actually mean, we
61 have to postpone this decision until the above steps
64 * Code generation: The code generation is done through
65 the System.Reflection.Emit API.
70 The compiler performs a number of simple optimizations on its input:
71 constant folding (this is required by the C# language spec) and
72 can perform dead code elimination.
74 Other more interesting optimizations like hoisting are not possible
75 at this point since the compiler output at this point does not
76 generate an intermediate representation that is suitable to
77 perform basic block computation.
79 Adding an intermediate layer to enable the basic block
80 computation to the compiler should be a simple task, but we
81 are considering having a generic CIL optimizer. Since all the
82 information that is required to perform basic block-based
83 optimizations is available at the CIL level, we might just skip
84 this step altogether and have just a generic IL optimizer that
85 would perform hoisting on arbitrary CIL programs, not only
86 those produced by MCS.
88 If this tool is further expanded to perform constant folding
89 (not needed for our C# compiler, as it is already in there)
90 and dead code elimination, other compiler authors might be
91 able to use this generic CIL optimizer in their projects
92 reducing their time to develop a production compiler.
96 See the <a href="bugs.html">bugs page</a> for more information.
98 A test suite is maintained to track the progress of
99 the compiler and various programs are routinely compiled and
104 Slides for the Mono C# Compiler presentation at .NET ONE are
106 href="http://primates.ximian.com/~miguel/slides-europe-nov-2002/Mono_C_Sharp_Overview_1007.sxi">here</a>
107 in StarOffice format.
111 MCS was able to parse itself on April 2001, MCS compiled itself
112 for the first time on December 28 2001. MCS became self hosting
113 on January 3rd, 2002.
115 The Mono Runtime and the Mono execution engine were able to make
116 our compiler self hosting on March 12, 2002.
118 ** Questions and Answers
120 Q: Does the Mono C# compiler support C# 2.0?
122 A: At this point the Mono C# compiler supports some of the features of
123 C# 2.0, but the support has not been completed. To enable 2.0 features
124 you must use the -2 flag to the compiler.
126 Q: What features are available as of Feb 2004?
128 A: Iterators have been implemented as well as method group implicit
129 conversion to delegates on the main compiler branch.
131 We have a branch of the compiler in the module `mcs/gmcs' which is
132 where we are developing the Generics support for the compiler. Plenty
133 of tests work (see mcs/tests/gen-*.cs for a list of tests), but work
136 Q: Will the C# 2.0 features be part of the Mono 1.0 release?
138 A: Only a few, the generic compiler will not be part of the 1.0
139 stable release, but a beta preview will be distributed.
141 Q: Why not write a C# front-end for GCC?
143 A: I wanted to learn about C#, and this was an exercise in this
144 task. The resulting compiler is highly object-oriented, which has
145 lead to a very nice, easy to follow and simple implementation of
148 I found that the design of this compiler is very similar to
149 Guavac's implementation.
151 Targeting the CIL/MSIL byte codes would require to re-architecting
152 GCC, as GCC is mostly designed to be used for register machines.
154 The GCC Java engine that generates Java byte codes cheats: it does
155 not use the GCC backend; it has a special backend just for Java, so
156 you can not really generate Java bytecodes from the other languages
159 Q: If your C# compiler is written in C#, how do you plan on getting
160 this working on a non-Microsoft environment.
162 We will do this through an implementation of the CLI Virtual
163 Execution System for Unix (our JIT engine).
165 Our JIT engine is working for the purposes of using the compiler.
166 The supporting class libraries are being worked on to fully support
171 A: No, currently I am using Jay which is a port of Berkeley Yacc to
172 Java that I later ported to C#. This means that error recovery is
173 not as nice as I would like to, and for some reason error
174 productions are not being caught.
176 In the future I want to port one of the Bison/Java ports to C# for
179 Q: Should someone work on a GCC front-end to C#?
181 A: I would love if someone does, and we would love to help anyone that
182 takes on that task, but we do not have the time or expertise to
183 build a C# compiler with the GCC engine. I find it a lot more fun
184 personally to work on C# on a C# compiler, which has an intrinsic
187 We can provide help and assistance to anyone who would like to work
190 Q: Should someone make a GCC backend that will generate CIL images?
192 A: I would love to see a backend to GCC that generates CIL images. It
193 would provide a ton of free compilers that would generate CIL
194 code. This is something that people would want to look into
195 anyways for Windows interoperation in the future.
197 Again, we would love to provide help and assistance to anyone
198 interested in working in such a project.
200 Q: What about making a front-end to GCC that takes CIL images and
201 generates native code?
203 A: I would love to see this, specially since GCC supports this same
204 feature for Java Byte Codes. You could use the metadata library
205 from Mono to read the byte codes (ie, this would be your
206 "front-end") and generate the trees that get passed to the
209 Ideally our implementation of the CLI will be available as a shared
210 library that could be linked with your application as its runtime
213 Again, we would love to provide help and assistance to anyone
214 interested in working in such a project.
216 Q: But would this work around the GPL in the GCC compiler and allow
217 people to work on non-free front-ends?
219 A: People can already do this by targeting the JVM byte codes (there
220 are about 130 compilers for various languages that target the JVM).
222 Q: Why are you writing a JIT engine instead of a front-end to GCC?
224 A: The JIT engine and runtime engine will be able to execute CIL
225 executables generated on Windows.
227 You might also want to look at the <a href="faq.html#gcc">GCC</a>
228 section on the main FAQ