EECE 6083/5183 Compiler Project
1 Compiler Project
The class project is to build a simple recursive decent (LL(1)) compiler by hand (not using compiler
construction tools such as flex or antlr). You can use any imperative block structured programming language that supports recursion and for which I can install a standard debian package to
test your solution on my computer. Examples of languages that students have used for this class
include: c, c++, go, rust, java, and python. If you are not certain that your desired programming language is ok, please check with me. While you can use a wide selection of languages, you
cannot use any language features for constructing compiler subsystems (regular expression parsers,
etc). That said, I encourage you to use some of the more complex builtin data structures of these
languages such as hash tables. Again, if you have questions about what you can and cannot do,
please ask.
In addition to sending me your compiler source and build environment that I can run on my
Linux workstation (you are responsible for ensuring that it will build on a standard Linux box; if
you build it on some exotic system like Haiku, we can discuss a demo on that platform), you must
also turn in a one page report documenting your compiler. It should document your software, its
structure, the build process, and the language features that are correctly implemented as well as
those elements of the compiler that are not completed. Finally, the report should also highlight
any unique features you have implemented in the system.
I have organized the compiler project into 5 development phases with deadlines scattered
throughout the course semester period. While these deadline are soft, I will use your history
of early/late to assign plus/minus graduations to your final grade. I encourage you to attempt to
complete these phases early.
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2 Lexical Analysis: The Scanner
2.1 Tokens
Lexical analysis involves converting strings in the input language to tokens. Here is a representative
example of the object type for tokens.
// a globally visible enumeration (needed by both the lexer and by the parser
enum tokenType = {PLUS, MINUS, IF_RW, LOOP_RW, END_RW, L_PAREN, R_PAREN,
L_BRACKET, R_BRACKET, ... , NUMBER, IDENTIFIER}
class token
tokenType: tt
tokenMark: tm
end class
The token mark can be a complex data type that records secondary information about the token.
For many token types (e.g., PLUS) there is no token mark data required, the token type value fully
characterizes the token. For other token types, (e.g., IDENTIFIER) the token mark will contain
additional information to characterize the token; initially this will mostly be to hold an identifier
string, but later it may well contain information about complex types such as functions/procedures
and their argument list signature/return type, etc. In some systems, a compiler might combine
arithmetic operators, relational operators, multiplier operators, etc into a common token type and
use the token mark to record the specific member of that token class that is being represented.
2.2 Supporting functions/objects
There are some key support functions that can make building the compiler much easier; especially if
you encapsulate them with a strong API that permits the restructuring/extension of the underlying
implementation. I will organize these support functions into 3 parts: (i) input processing, (ii)
warning and error reporting, and (iii) symbol table management and reserved word setup. Below I
will present suggestions for each of these components. You are not required to setup your solution
this way, these are simply my recommendations to you. I will document each of these in an
object-oriented basis, you are not required to setup/use an object-oriented language, this is just a
convenient way for me to present the ideas.
2.2.1 Input Processing
I recommend that you create an object to manage your input file setup and location recording
(what line in the file is currently being processed). This might be a bit much for this project, but
it helps encapsulate stuff related to the input file being processed. It is a good plan to build it this
way as if you ever move to a more complicated language where multiple files are processed while
processing the designated input file, it is easy to have a stack of file points/line count variables to
record where the system is in processing the various files required.
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In general the lineCnt variable will be used to record which line in the input file the scanner
is currently working on. In this project, this value is used primarily to help generate meaningful
error messages.
class inFile
private:
file: filePtr = null // the input file
string: fileName
int: lineCnt = 0 // the line count; initialized to zero
public:
bool: attachFile(string) // open the named file
char: getChar() // get the next character
void: ungetChar(char) // push character back to the input file string
void: incLineCnt()
void: getLineCnt()
end class
2.2.2 Warning and Error Reporting
The main program should have an object for reporting errors and warnings. Ideally these functions
will output error messages using a standard format (e.g., https://gcc.gnu.org/onlinedocs/
gcc-3.3.6/gnat_ug_unx/Output-and-Error-Message-Control.html that supporting tools such
as emacs can use to, for example, automatically position your text editor/IDE to the correct file
and line number corresponding to the warning/error. The API for this is fairly simple.
class reporting
private:
bool: errorStatus = False // true if the compiler has discovered an error
public
void reportError(char *message)
void reportWarning(char *message)
bool getErrorStatus()
end class
While a compiler attempts to continue in the presence of both errors and warnings, an error
condition will generally cause the compiler to proceed only with the parse and type checking phases;
code optimization and generation should not occur when errors are encountered in the parsing of
the input program. In general the reportError function will set the private variable errorStatus
to True.
2.2.3 Symbol Table Management/Reserved Words
Most compilers will use a hash table(s) of the symbols seen in the input file; this is called the
symbol table. These symbols would be identifiers and functions. This is also a convenient place to
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drop entries for the reserved words in the language (such as if, loop, end, and so on). The symbol
table will be revised and extended as you build latter parts of the compiler, so it is imperative that
you keep access to it setup through and API so that its actual implementation is easily modified
later. The hashLook function will look into the symbol table and return the token for the string
argument. If this the first time an entry to the symbol table occurs for this string, then a new
entry is created with a default token definition (generally IDENTIFIER).
class symbolTable
private:
hashTable<token>: symTab
public:
token: hashLook(string) // lookup the string in the hash table
void: setToken(string) // change the token values for this symbol
end class
In general the lexer will lookup every candidate identifier string in the symbol table and return
the token stored in the symbol table for that string. Thus, to make life easy, a good idea is to preseed the symbol table with the reserved word strings and setup the tokens for each so that instead
of returning the token IDENTIFIER, the correct token for that reserved word is returned. Thus,
before you start processing the file, you will build a simple look to iterate through the reserved
word strings in order to initialize the symbol table with the reserved word tokens.
Constants
So the question sometimes comes up, how do we treat strings and numerical values in the lexical
analysis phase. There is no single uniform answer to this. You can treat them: (i) directly as
tokens, (ii), register them in the symbol table, or, (iii) build a separate string/numeric table(s)1
to
store the token representation of the item. If we assume that the lexer match string for the token is
stored as an ASCII string in the variable tokenString, then examples of the lexer return code for
each of these options can be outlined as (showing both for a STRING token and for an INTEGER
token):
(i) return new token(STRING, tokenString)
return new token(INTEGER, atoi(tokenString))
(ii) tok = symbolTable.hashLook(tokenString)
if (tok.tt != STRING) { tok.tt = STRING }
return tok
tok = symbolTable.hashLook(tokenString)
1Either a single common constantTable for both, or having two separate tables, one for strings and one for numerics
is possible.
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if (tok.tt != INTEGER) { tok.tt = INTEGER }
// optionally we might also build/add the integer value to the token
return tok
(iii) return new token(STRING, constantTable.hashLook(tokenString))
return new token(INTEGER, constantTable.hashLook(tokenString))
Of course there are many variations on this coding example. The option of using the symbol table
or a separate constant table is a good way to compress the final storage map. That is, if you store
them in the symbol table, then (at each scope) common constants will be represented as one item
that has to be mapped into memory during the code generation phase. If instead, you store all
constants in constant table(s) that transcend all scopes, you can potentially reduce the storage map
size even further.
2.3 The Scanner
I recommend that you build a scanner object with the principle API call scan() that returns the
next token in the input file. Normally your parser will call the scan() function to get the next
token to determine the next course of action for the parser. When the file has advanced to the end
of the file, you should have an end-of-file (EOF) token that can be returned to the parser.
For purposes of this step of your compiler, I recommend that you build a main program that
initializes the symbol table and iteratively calls scan() until the end-of-file is reached.
The scanner must skip white-space, newlines, tabs, and comments; comments are start with the
string ”//” and continue to the next newline character. It should count newlines to aid the error
reporting functions.
Illegal characters should be treated as white-space separators and reported as errors. These errors should not stop the parser or semantic analysis phases, but they should prevent code generation
from occurring.
The tokens your scanner should recognize are the tokens found in the project language specification.
I would also recommend defining character classes to streamline your scanner definition. In
short, what this means is you should define an array indexed by the input character that maps an
ASCII character into a character class. For example mapping all the digits [0-9] into the digit
character class, letters [a-zA-Z] into the letter character class, and so on (of course you have to
define the character classes in some enumeration type. I will go over this more in class for you.
I am leaving the remainder of this section in place; it is from an earlier version of
this document that may or may not be helpful to you.
While the scanner can be constructed to recognize reserved words and identifiers separately,
I strongly recommend that you fold them together as a common case in your scanner and seed
the symbol table with the reserved words and their corresponding token type. More precisely, I
recommend that you incorporate a rudimentary symbol table into your initial scanner implementation. While the data types of the symbol table entries are likely to expand as you build additional
capabilities into your compiler, initially you can have the symbol table entries record the token
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type and have a pointer to the string for the identifier/reserved word. For example, each element
in your symbol table could have the following structure:
sym_table_entry : record
token_type : TOKEN_TYPES;
token_string : *char;
end record
where TOKEN TYPES is the enumeration type of all your token types.
Operationally, I would build the symbol table so that new entries are created with the token type
field initialized to IDENTIFIER. You can then seed the symbol table (during the scanner initialization step described above) with reserved words in the scanner’s initialize method. The easiest
way to do this is to setup an array of reserved word and their token type. Then walk through the
array to do a hash look up with each reserved word string and change the token type field to the
specified token type. We will go over this in class.
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3 The Parser
Build a recursive decent parser that looks only at the immediate next token to control the parse.
That is, build an LL(1) parser from the project programming language specification given elsewhere
in these webpages. If you really would prefer to build a LALR parser that is possible, but please
discuss it with me first.
The parser should have at least one resync point to try to recover from a parsing error.
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4 Type Checking
Incorporate type checking into the parser and perform type checking while the statements are
parsed. Your principle concern is with scoping and type matching. At least for expressions and
statements, your parsing rules will now have to be expanded to return the type result for the
construct just parsed. The upper rules will use that type information to assert type checks at its
level.
A full symbol table complete with scoping data must be constructed. You must be able to
define a scope and remove a scope as the parse is made. You can achieve scoping by having nested
symbol tables or by chaining together the entries in the symbol table and placing scope entry points
that can be used to control how symbols are removed when your parser leaves a scope.
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5 Code Generation
You have two options for code generation. The first (and recommended) option is to use the LLVM
back-end optimizer and code generator. In this case your code generation phase would really be a
translator to the LLVM intermediate form (either the memory resident IR or the llvm assembly).
The second option is to generate a file containing a restricted C program space as documented
below.
5.1 Generating C
Basically the generated file should have declarations for your memory space, register space and a
flat C (no subroutines) with goto’s used to branch around the generated C file.
Your generated C must follow the style of a load/store architecture. You may assume a register
file sized to your largest need and a generic 2-address instruction format. You do not have to
worry about register allocation and you should not carryover register/variable use from expression
to expression. Thus a program with two expressions:
c := a + b;
d := a + c + b;
would generate something like:
// c := a + b;
R[1] = MM[44]; // assumes variable a is at MM location 44
R[2] = MM[56]; // assumes variable b is at MM location 56
R[3] = R[1] + R[2];
MM[32] = R[3]; // assumes variable c is at MM location 32
// d := a + c + b;
R[1] = MM[44];
R[2] = MM[32];
R[3] = R[1] + R[2];
R[4] = MM[56];
R[5] = R[3] + R[4];
MM[144] = R[5]; // assumes variable d is at MM location 144
You can also use indirection off the registers to define memory locations to load into registers.
For example your code generator can generate something like this:
R[1] = MM[R[0]+4];
You can statically allocate/assign some of the registers for specific stack operation (pointers).
The stack must be built in your memory space.
For conditional branching (goto) you can use an if statement with a then clause but not with
an else clause. Furthermore the condition must be evaluated to true/false (0/1) prior to the if
statement so that the condition in the if statement is limited to a simple comparison to true/false.
Thus for conditional branching only this form of an if statement is permitted:
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EECE 6083/5183 Compiler Project
if (R[2] = true) then goto label;
The code generator is to output a restricted form of C that looks much like a 3-address load/store
architecture. You can assume an unbounded set of registers, a 64M bytes of memory space containing space for static memory and stack memory. Your machine code should look something like
(I forget C syntax, so you may have to translate this to real C):
Reg[3] = MM[Reg[SP]];
Reg[SP] = Reg[SP] { 2;
Reg[4] = MM[12]; // assume a static
// variable at
// location 12
Reg[5] = Reg[3] + Reg[4]
MM[12] = Reg[5];
You must use simple C: assignment statements, goto statements, and if statements. No procedures, switch statements, etc.
You must evaluate the conditional expressions in “if statements” and simply reference the result
(stored in a register) in the if statement of your generated C code.
Basically you should generate C code that looks like a simple 3-address assembly language.
5.2 Generating LLVM Assembly
See other lecture notes on LLVM.
5.3 Activation Records
See other lecture notes on Code Generation and Figure 1.
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FP
SP
return address
arg 2
local var 1
local var 2
arg 1
return value ptr
old SP
old FP
old FP
old SP
FP: frame pointer
SP: stack pointer Activation record k+1 Activation record k
prev
next
prev
next
Activation record k Activation record k+1
Curr activation record
Figure 1: The call chain of activation records; stack model on left and heap model on right
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6 Runtime
If you are using the LLVM infrastructure, you can use the libc supported gets, puts, atoi, etc
functions as your runtime system. This means that you will not end up writing the runtime library
other than adapting the code generator to interface to the libc standard.
For the runtime environment, you should enter the runtime function names and type signatures
into your symbol table prior to starting the parse of the input files. To code generate for these
functions, you can either special case them and use C function calls or you can have a static
(handwritten) C program with predefined labels (on the hand written code C code that calls your
library functions) that you generated code can goto. This second option sounds more difficult but
is probably much easier to implement as it’s not a special case in your code generator.
There are several (globally visible) predefined procedures provided by the runtime support
environment, namely the functions described in the project language description.
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