path: root/cibic.tex

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\begin{document}
\title{\Large{CIBIC: C Implemented Bare and Ingenuous Compiler}}

\author{\textsc{尹茂帆 Ted Yin}\\ \\\multicolumn{1}{p{.7\textwidth}}
    {\centering\emph{F1224004 5120309051\\
Shanghai Jiao Tong University ACM Class}}}
\maketitle
\begin{abstract}
    This report presents the design and features of a simple C compiler which
    generates MIPS assembly code. Although not all the language requirements
    and features are implemented according to the standard, it still supports
    major C features, such as basic types (void, int, char), basic flow control
    syntax (if, while-loop, for-loop), user-defined types (aka. typedef),
    functions, pointers (including function pointers), struct/union (may be
    nested), etc. Besides, it makes use of SSA (Single Static Assignment) form
    for the IR (Intermediate Representation) and optimization. The whole compiler is
    written in pure C, obeying C89/C99 standard. The report first introduces
    the lexer and parser generation, then focuses on the data structures being
    used in the AST (Abstract Syntax Tree) and symbol tables, and makes a
    conclusion on the supported syntactical features. Next, the report shows
    the intermediate representation design and claims the techniques that have
    been used in the project. Finally, various optimization techniques are
    presented and discussed, some are accompanied with code snippets.
\end{abstract}
\section{Lexer and Parser}
CIBIC makes good use of existing lexer and parser generators. It uses Flex to
generate lexer while using Bison as parser generator. Both generators read
boilerplate text which contains C code fragments to be filled in the generated
lexer/parser. The best practice, I suggest, is to avoid embedding too much
concrete code in the boilerplate. Instead, we shall write well-designed
functions in a separate file (``\texttt{ast.c}'' in CIBIC) and invoke functions in the
boilerplate. The reason is that it is almost not practical nor convenient to
debug the generated lexer or parser. Using the seperation method, we can set up
breakpoints in the seperate file easily and debug on the fly. The declarations
of all functions that may be invoked during parsing can be found in ``\texttt{ast.h}''.

It requires a little more effort to track the location of each token using
Flex. Luckily, Flex provides with a macro called ``\texttt{YY\_USER\_ACTION}''
to let the user do some extra actions after each token is read. So I maintained
a global variable ``\texttt{yycolumn}'' to keep the current column, a global
char array ``\texttt{linebuff}'' to buffer the text being read for error
report.
\begin{listing}[H]
    \centering
    \RecustomVerbatimEnvironment{Verbatim}{BVerbatim}{}
    \begin{minted}{c}
int yycolumn = 1;
char linebuff[MAX_LINEBUFF], *lptr = linebuff;

#define YY_USER_ACTION \
    do { \
        yylloc.first_line = yylloc.last_line = yylineno; \
        yylloc.first_column = yycolumn; \
        yylloc.last_column = yycolumn + yyleng - 1; \
        yycolumn += yyleng; \
        memmove(lptr, yytext, yyleng);  \
        lptr += yyleng; \
    } while (0);

#define NEW_LINE_USER_ACTION \
    do { \
        yycolumn = 1; \
        lptr = linebuff; \
    } while (0)
\end{minted}
\caption {Code Snippet to Track Down Location}
\end{listing}

\section{Semantic Analysis and Implementation}
The parsing process is also an AST (Abstrct Syntax Tree) construction process.
By calling the corresponding functions, the generated parser create tree nodes
and merge them into subtrees. The major issue here is how to design a proper
data structure to represent and maintain the AST. In CIBIC, all nodes in an AST
are instances of struct ``CNode'' in figure.
\begin{figure}[H]
    \centering
    \texttt {
    \begin{tabular}{|c|}
        \hline
        type \\
        \footnotesize{describe syntax information e.g. expression or declarator} \\ \hline
        rec: \\
        \footnotesize{\textbf{union of intval / subtype / strval}} \\
        \footnotesize{stores detailed information e.g. which kind of expression it is} \\ \hline
        ext: \\
        \footnotesize{\textbf{struct of type, var, autos, const\_val, is\_const, offset}} \\
        \footnotesize{extended info, where annotation is made} \\ \hline
        chd \\
        \footnotesize{the left most child of the node} \\ \hline
        next \\
        \footnotesize{the next sibling} \\ \hline
        loc \\
        \footnotesize{\textbf{struct of row, col}} \\
        \footnotesize{the location in the source code, for error report} \\ \hline
        \end{tabular}
    }
    \caption{The Structure of a CNode instance}
\end{figure}
Since a node may have variable number of children, the most common and
efficient approach is to implement a left-child right-sibling binary tree. The
field ``\texttt{chd}'' points to the child and ``\texttt{next}'' points to the
next sibling. This implementation is extremely flexible because we do not need
to know the number of children in advance, which brings us the convenience of
appending argument nodes to a function invocation node in the boilerplate.

After construction of the AST, the tree will be traversed by calling mutually
recursive functions declared in ``\texttt{semantics.h}''. The entry call is ``\texttt{semantics\_check}'' which will set up the symbol tables and call ``\texttt{semantics\_func}'' and ``\texttt{semantics\_decl}'' to analyze function definitions and global declarations. These functions will further invoke more basic functions such as ``\texttt{semantics\_comp}'' (handles compound statements) to fully check the semantics and gather variable typing information.

The key data structures in the semantic checking are \textbf{symbol tables}. Symbol tables maintain variables and types in different scopes across different name spaces. When a variable is defined, the corresponding type specifer will be checked and binds to the variable. Also, when the scope ends, all variable bindings living within the scope will be removed from the table. Although they are removed from the table, the bindings are still referenced by some nodes on the AST, so that the AST becomes ``\textbf{annotated AST}''. In CIBIC, the variable reference is stored in ``\texttt{ext.var}'' field of ``\texttt{CNode}'' and the type information of an subexpression is annotated at ``\texttt{ext.type}''. Thus, in a word, symbol tables stores all symbols that are currently \textbf{visible}.

In C language, there are four name spaces: for label names, tags, members of structures or unions and all other identifiers. Goto statments are not implemented in CIBIC, so there're actually three name spaces. Since each kind of structures or unios have its own name space for its fields, we treat them specially and create a new table for each of them. For tags and all other identifiers, we set up two global tables. Besides name spaces, scoping is also a very important concept in C. It seems to be an orthogonal concept to name spaces.

Considering these two concepts, CIBIC implements a struct named ``\texttt{CScope}'' to maintain all the information as shown in figure.
\begin{figure}[H]
    \centering
    \texttt {
    \begin{tabular}{|c|}
        \hline
        lvl \\
        \footnotesize{current nesting level of scopes, e.g. 0 for global scope} \\ \hline
        func \\
        \footnotesize{current function, helpful when analyzing a return statement} \\ \hline
        inside\_loop \\
        \footnotesize{if the scope is inside a loop, help full when analyzing a break statement} \\ \hline
        top \\
        \footnotesize{points to the top of the scope stack} \\ \hline
        ids \\
        \footnotesize{name space for all other variables} \\ \hline
        tags \\
        \footnotesize{name space for all tags (name of structs or unions)} \\ \hline
        \end{tabular}
    }
    \caption{The Structure of a CScope instance}
\end{figure}

Note that ``\texttt{top}'' points to an instance of ``\texttt{CSNode}'' which
has two fields ``\texttt{symlist}'' and ``\texttt{next}''. ``\texttt{symlist}''
points to a list of symbols in the same scope while ``\texttt{next}'' links to
the outer scope which is another instance of ``\texttt{CSNode}''. As for
``\texttt{ids}'' and ``\texttt{tags}'', they are pointers to
``\texttt{CTable}'',stores all the current symbols. As mentioned above, for
each struct or union, there is also a pointer to ``\texttt{CTable}'' stores all
field names. ``\texttt{CTable}'' is an open addressing hash table
containing nodes of the type ``\texttt{CTNode}''. The structure of each node is
depicted in figure.

\begin{figure}[H]
    \centering
    \texttt {
    \begin{tabular}{|c|}
        \hline
        key \\
        \footnotesize{char *} \\ \hline
        val \\
        \footnotesize{void *, in order to also be used in checking duplicate parameters, etc.} \\ \hline
        next \\
        \footnotesize{the next element which has the same hash value} \\ \hline
        lvl \\
        \footnotesize{scope level} \\ \hline
        \end{tabular}
    }
    \caption{The Structure of a CTNode instance}
\end{figure}

Thanks to the level information kept in each ``\texttt{CTNode}'', we do not
have to establish a hash table for every scopes, which may be memory consuming.
Instead, whenever a new scope begins, CIBIC simply pushes a new frame to scope
stack. This is achieved by creating an instance of ``\texttt{CSNode}'', setting
its ``\texttt{next}'' field to the ``\texttt{top}'' field of the
``\texttt{CScope}'' then letting the ``\texttt{top}'' points to the new frame,
finally increasing ``\texttt{lvl}'' by one. Whenever a new symbol is being
added to ``\texttt{CScope}'', CIBIC adds the symbol to one of the tables
``\texttt{ids}'' and ``\texttt{tags}'', then also appends the symbol to the
``\texttt{symlist}'' of the top of the scope stack. The most elegant
characteristics of open addressing hash tables is, for the same name appears in
different scopes, the symbol defined at the inner most is always ahead of
others in the chain of the table because it is the most recently added one. So,
for lookups, CIBIC just need to return the first node having the same name in
the chain, which is very time-efficient. At last, when a scope ends, CIBIC
scans the whole ``\texttt{symlist}'' of the top scope frame, and tries to
remove these symbols from the table. Figure presents the content of the scope
stack when the analysis proceeds into the inner most declaration of a. See
chains with hash code 0097 and 0098.
\begin{figure}[H]
%    \centering
    \begin{minipage}{0.35\textwidth}
%    \RecustomVerbatimEnvironment{Verbatim}{BVerbatim}{}
    \begin{minted}{c}
int main() {
    int a, b;
    if (a > 0)
    {
        int a, b;
        if (a > 0)
        {
            int a, b;
        }
    }
}
    \end{minted}
 \caption {Source Code Being Proceeded}
\end{minipage}
%    \centering
    \begin{minipage}{0.5\textwidth}
    \begin{BVerbatim}
    [0072]->[int@747e780:0]
    [0097]->[a@7484ae0:3]->[a@7484890:2]->[a@7484580:1]
    [0098]->[b@7484bb0:3]->[b@7484960:2]->[b@7484710:1]
    [0108]->[printf@747f150:0]
    [0188]->[scanf@747f640:0]
    [0263]->[__print_string@7484010:0]
    [0278]->[__print_char@7483fc0:0]
    [0623]->[__print_int@747f8b0:0]
    [0778]->[malloc@7484100:0]
    [0827]->[void@747eee0:0]
    [0856]->[main@7484530:0]
    [0908]->[memcpy@7484060:0]
    [0971]->[char@747e9f0:0]
    \end{BVerbatim}
    \caption {Dump Info from CIBIC: Scope Stack}
\end{minipage}
\end{figure}

\subsection{Type System}
C has a small set of basic types. In CIBIC, basic types include \texttt{char},
\texttt{int} and \texttt{void} (literally, \texttt{void} is not an actual
type). However, C supports two powerful type aggregation: arrays and structures
(unions), and also supports an indirect access tool: pointer. This makes the
type system much more complex and usable in practice. CIBIC uses the concept
``type tree'' to organize types. All basic types are the leaves of a type tree,
while aggregate types and pointers are the intermediate nodes. Figure shows a
typical type tree of a C struct. 

A type tree is good at preserving type hierarchy info which may be extremely
useful in type checking. For example, when checking a expression \texttt{*a},
compiler first check if the root of the type tree of a is a pointer. If not,
error message will be printed and semantic checking fails. Otherwise, the type
of the expression result is the subtree of the only child of the root. Also, a
type tree enables us to implement complex type nesting, which will be discussed
later on.

CIBIC has support for user-defined types, which are defined via the keyword
``\texttt{typedef}''.  However, ``\texttt{typedef}'' is notoriously difficult
to deal with due to the ambiguity caused by the language design. For example,
in ``\texttt{int A}'', A is a \textbf{variable} of integer type, but in
``\texttt{A a}'', A is a user-defined \textbf{type}. The subtle semantic
difference is caused by context. In former case, A is identified as a
identifier token, while in latter case, identified as a type specifier. The
meaning of a token should be made clear during lexical analysis which does not
take in account of context. So the most direct and effective way to implement
\texttt{typedef} is to hack the lexer and parser. CIBIC maintains a
``\texttt{typedef\_stack}'' to denote current parsing status. When a parser has
just read a type specifier, before it moves on, it invokes
``\texttt{def\_enter(FORCE\_ID)}'' to notify ``\texttt{typedef\_stack}'' the
subsequent string token must be an identifier instead of a type specifier. As
for ``\texttt{typedef}'' statements, the parser will invoke
``\texttt{def\_enter(IN\_TYPEDEF)}'' to record the newly defined typename by
adding an entry to a hash table. In the lexer, when a string token is being
read, it invokes ``\texttt{is\_identifier}'' to make sure whether the string is
an \texttt{IDENTIFIER} or a \texttt{USER\_TYPE}. 

Figure demonstrates a very subtle use of \texttt{typedef}, which can be parsed perfectly by CIBIC.

\begin{listing}[H]
    \centering
    \RecustomVerbatimEnvironment{Verbatim}{BVerbatim}{}
    \begin{minted}{c}
/* It's quite common to define a shorthand `I' for `struct I'. */
typedef struct I I; 
/* Declaration of a function with parameter `a' of user-defined type `I'. */
int incomp(I a);
/* The definition of `I' is postponed. */
struct I { 
    int i, j;
};
/* Function definition of previous declared one. */
int incomp(I a) {}
/* Define `b' as an int type. */
typedef int b;
int main() {
    /* Variable `b' is of type `b', an integer actually. */
    I i;
    b b;
    incomp(i);
}
\end{minted}
\caption {Code Snippet to Track Down Location}
\end{listing}


\subsection{Complex Declaration and Function Pointer}
\section{Intermadiate Representation}
\subsection{Single Static Assignment Form}
\subsection{Phi-functions}
\subsection{Register Allocation}
\section{Performance and Optimization}
\end{document}