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CSE 302: Compilers | Lab 1
Generating Three Address Code (TAC)
1 INTRODUCTION
This lab is primarily intended to get you set up with your development libraries and associated tools. The
goal of this lab is to write a pair of maximal munch stages to transform the AST for straightline BX to TAC..
This lab will be assessed. It is worth 5% of your final grade. Your compiler pass is due in 1 week, i.e., on or
before 2021­09­16 23:59:59 (Paris time).
Every student must work individually for lab 1.
2 STRAIGHTLINE BX
Source Language The source language for this lab is a drastically simplified fragment of BX. There
are no control structures such as loops, conditionals, or functions, nor is there any way to produce any
output except by means of the print statement. This language works with data of a single type, signed
64 bit integers in 2’s complement representation. This can represent all integers in the range [−2
63
, 2
63).
The relevant grammar for this fragment of BX is shown in figure 1. Every BX program is produced
by the ⟨program⟩ non-terminal, which in this case will be a single procedure called main that takes no
arguments. Within the body of this function is a sequence of local variable declarations, given by the
⟨vardecl⟩ non-terminal, followed by a sequence of statements given by the ⟨stmt⟩ non-terminal. Every
variable declaration declares an integer variable initialized to 0. Statements are of two kinds: assignment
statements (⟨assign⟩) and print statements (⟨print⟩). Both of these make use of expressions (⟨expr⟩).
JSON Representation For this lab, you will be not need to worry about reading BX source directly.
Instead, you will start from a representation of the BX source file in the form of a JSON object, which
makes it trivial to load in almost any programming language. The JSON representation of a BX file always
has the following external structure:
{
“provenance”: “/path/to/sourcefile.bx”, // the source file that was parsed
“source”: “…”, // the original text of the source file
“ast”: … // the AST (described here)
}
The provenance and source fields may safely be ignored. They can be used to create fancy error messages
(not required!) as described below. We will primarily concern ourselves with the ast field, which contains
a compact representation of the BX abstract syntax tree (AST).
Most AST subtrees are encoded as JSON lists of length 2 with the following shape:
[ payload, location ]
[2021­09­11 15:16] 1
⟨program⟩ ::= “def main() {” ⟨vardecl⟩

⟨stmt⟩
∗ “}”
⟨vardecl⟩ ::= “var” IDENT “= 0 : int;”
⟨stmt⟩ ::= ⟨assign⟩ | ⟨print⟩
⟨assign⟩ ::= IDENT “=” ⟨expr⟩ “;”
⟨print⟩ ::= “print(” ⟨expr⟩ “);”
⟨expr⟩ ::= IDENT | NUMBER
| ⟨expr⟩ “+” ⟨expr⟩ | ⟨expr⟩ “­” ⟨expr⟩
| ⟨expr⟩ “*” ⟨expr⟩ | ⟨expr⟩ “/” ⟨expr⟩ | ⟨expr⟩ “%” ⟨expr⟩
| ⟨expr⟩ “&” ⟨expr⟩ | ⟨expr⟩ “|” ⟨expr⟩ | ⟨expr⟩ “^” ⟨expr⟩
| ⟨expr⟩ “<<” ⟨expr⟩ | ⟨expr⟩ “>>” ⟨expr⟩
| “­” ⟨expr⟩ | “~” ⟨expr⟩
| “(” ⟨expr⟩ “)”
IDENT ::≈ /[A­Za­z][A­Za­a0­9_]*/ (except reserved words)
NUMBER ::≈ /0|[1­9][0­9]*/ (value must fit in 63 bits)
Figure 1: Grammar of the straightline fragment of BX
where payload contains the actual AST structure, and location is a 6 element list of natural numbers of
the form:
[ start_lnum, start_bol, start_cnum, end_lnum, end_bol, end_cnum ]
In this lab you can completely ignore the location tuple if you want. However, here is is what it
means: the AST payload is located in the original source between character offsets start_cnum and
end_cnum, counted from the start of the BX source file. To convert these character offsets into line and
column numbers, we use the corresponding lnum and bol values which give the line number and the
character offset of the start of the line respectively. In other words, the column number is (cnum ­ bol).
For example, consider the following location:
[ 10, 185, 190, 11, 230, 231 ]
This indicates the fragment of the source that begins on line 10, column 5 (i.e., 190 − 185) and ends on
line 11, column 1.
The ast field of the overall JSON object will be a list of toplevel declarations of BX. For this lab, there is
only one toplevel declaration, which is the declaration of the main function. This is encoded in the JSON
object as follows, where we use ⊠ to indicate the location components (which, recall, you can ignore for
this lab).
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[ [ “<procdef>”,
[ “main”, ⊠ ], // name of procedure
[], // arguments and their types ­­ in this case, none
[ [ “VOID” ], ⊠ ], // return type ­­ in this case, the special type “VOID”
[ [ “<block>”, [ · · · ] ], ⊠ ]
], ⊠ ]
Most of the complexity of this definition comes from supporting the full BX language. However, in this
lab you can take the above as a fixed structure and focus on the contents of the body of the procedure,
indicated by · · · above. (Pay close attention to the brackets and nesting!)
The body of the procedure, shown by · · · above, consists of a sequence of local variable declarations
followed by a sequence of statements.
• Local variables: all such declarations have the following shape:
[ [ “<vardecl>”,
[ “x”, ⊠ ], // name of the variable
[ [ “<number>”, 0 ], ⊠ ], // initial value
[ [ “INT” ], ⊠ ] // type
], ⊠ ]
The only compoenent that will vary in this lab is the name of the variable, which is x in the above
shape. The initial value and type are fixed at 0 and int respectively.
• Assignment statement (⟨assign⟩): these statements have the following shape:
[ [ “<assign>”,
[ [ “<var>”, “x” ], ⊠ ],
⟨expr⟩
], ⊠ ]
The left hand side of the assignment, here shown with the variable x, will change from one assignment statement to the next. The right hand side of the assignment is shown with ⟨expr⟩ above, whose
JSON shapes are specified below.
• Print statements (⟨print⟩) have the following shape:
[ [ “<eval>”,
[ [ “<call>”,
[ “print”, ⊠ ],
[ ⟨expr⟩ ]
], ⊠ ]
], ⊠ ]
The argument to the print statement is shown with ⟨expr⟩ above. The JSON for print statements
is more complex than strictly necessary, but it anticipates the extension of to the full BX to come,
when print() will become a part of the BX standard library of functions.
Expressions (⟨expr⟩) are represented by any of the following JSON object shapes.
• Variables are represented by the following shape:
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[ [ “<var>”, “x” ], ⊠ ]
(shown here with the example of the variable x).
• Numbers are represented by the following shape:
[ [ “<number>”, 42 ], ⊠ ]
(shown here with the example of 42).
• Unary operator applications are represented by the following shape:
[ [ “<unop>”,
[ [ ⟨op⟩ ], ⊠ ],
⟨expr⟩
], ⊠ ]
where ⟨op⟩ ∈ {“UMINUS”, “BITCOMPL”}. (“Uminus” stands for “unary minus”.)
• Binary operator applications are represented by the following shape:
[ [ “<binop>”,
⟨expr⟩,
[ [ ⟨op⟩ ], ⊠ ],
⟨expr⟩
], ⊠ ]
where ⟨op⟩ ∈ {
“PLUS”, “MINUS”, “TIMES”, “DIV”, “MODULUS”, “BITAND”, “BITOR”, “BITXOR”,
“BITSHL”, “BITSHR”
}
.
Loading an AST in JSON Format To load JSON from a file named file.json in Python, you can
proceed as follows:
with open(‘file.json’, ‘r’) as fp:
js_obj = json.load(fp)
# js_obj is a representation of the object using Python data structures
It is strongly recommended that you don’t use the js_obj object directly, since their specific form
is very closely tied to the JSON input that will evolve over the course of the labs. Instead, build a class
hierarchy of expressions and statements such as:
[2021­09­11 15:16] 4
class Expr: pass
class Variable(Expr):
def __init__(self, name):
self.name = name
class Number(Expr):
# … (similar)
class UnopApp(Expr):
def __init__(self, op, arg):
self.op = op
self.arg = arg
class BinopApp(Expr):
# … (similar)
Then, write a recursive JSON loader function that transforms the JSON object into the corresponding
element of the class hierarchy. For example, for loading expressions from JSON objects into the hierarchy
depicted earlier, you can write a function that has the following structure.
def json_to_expr(js_obj):
js_obj = js_obj[0] # ignore the location component
if js_obj[0] == ‘<var>’:
return Variable(js_obj[1])
elif js_obj[0] == ‘<number>’:
return Number(js_obj[1])
elif js_obj[0] == ‘<unop>’:
op = js_obj[1][0][0] # careful of all the nesting!
# OP will be ‘UMINUS’ or ‘BITCOMPL’
arg = json_to_expr(js_obj[2]) # recursive call
return UnopApp(op, arg)
elif js_obj[0] == ‘<binop>’:
# …
# …
else:
print(f’Unrecognized <expr> form: {js_obj[0]}’)
raise ValueError # or whatever
3 THREE ADDRESS CODE (TAC)
The purpose of this lab is to compile a BX program to a TAC program. The lexical tokens and grammar
of TAC are shown in fig. 2. A TAC ⟨program⟩ is a sequence of zero or more TAC ⟨instr⟩uctions that are
placed in the procedure named @main. (Note the change from main in BX to @main in TAC– this will be
explained later when we discuss global vs. local symbols.)
While TAC has a grammar, for this lab you will work instead with a JSON representation of TAC
programs. The TAC programs you generate in this lab will have the following overall structure.
[ { “proc”: “@main”, “body”: [ ⟨instr⟩, ⟨instr⟩, … ] } ]
Each instruction is represented by a JSON object with three fields, opcode, args, and result. The valid
[2021­09­11 15:16] 5
TEMP ::≈ /%(0|[1­9][0­9]*|[A­Za­z][A­Za­z0­9_]*)/
NUM64 ::≈ /0|­?[1­9][0­9]*/ (numerical value ∈ [−2
63
, 2
63))
BINOP ::≈ /add|sub|mul|div|mod|and|or|xor|shl|shr/
UNOP ::≈ /neg|not/
⟨program⟩ ::= “proc @main:” (⟨instr⟩ “;”)

⟨instr⟩ ::= TEMP “=” “const” NUM64
| TEMP “=” “copy” TEMP
| TEMP “=” UNOP TEMP
| TEMP “=” BINOP TEMP “,” TEMP
| “print” TEMP
| “nop” (does nothing)
Figure 2: Tokens and grammar of the TAC language (not directly relevant for this lab)
opcodes are all shown in figure 2. The args field is a tuple of temporaries or numbers, with temporaries
represented as JSON strings (i.e., “%42” etc.), while numbers are represented as JSON ints. The result
field is either a temporary or null. Here are some examples of instructions in JSON form:
{“opcode”: “const”, “args”: [42], “result”: “%0”}
{“opcode”: “copy”, “args”: [“%0”], “result”: “%1”}
{“opcode”: “mul”, “args”: [“%2”, “%1”], “result”: “%3”}
{“opcode”: “neg”, “args”: [“%8”], “result”: “%9”}
{“opcode”: “print”, “args”: [“%15”], “result”: null}
{“opcode”: “nop”, “args”: [], “result”: null}
An example of a complete TAC file in JSON form is shown in figure 3.
You are provided a compiler pass from TAC in JSON form to AMD64 assembly and executable using
GCC as the assembler and linker. This compiler pass is called tac2asm.py and can be used as follows:
$ python3 tac2asm.py tacfile.tac.json
tacfile.tac.json ­> tacfile.s
tacfile.s ­> tacfile.exe
$ ./tacfile.exe

4 TASK: MAXIMAL MUNCH
Your task in this lab is to write a compiler pass that goes from an AST represented in JSON format to TAC.
Remember that your compiler needs to be correct, meaning that any TAC program you produce must have
exactly the same behavior as the source BX program.
Setup Create a private Github repository with a name such as cse302labs and assign me (@chaudhuri)
as a collaborator. Then, start working in a subdirectory such as cse302labs/1. Please do not create new
repositories for each lab as that will drastically increase my grading workload.
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[ { “proc”: “@main”,
“body”: [
{“opcode”: “const”, “args”: [10], “result”: “%0”},
{“opcode”: “copy”, “args”: [“%0”], “result”: “%1”},
{“opcode”: “const”, “args”: [2], “result”: “%2”},
{“opcode”: “mul”, “args”: [“%2”, “%1”], “result”: “%3”},
{“opcode”: “copy”, “args”: [“%3”], “result”: “%4”},
{“opcode”: “mul”, “args”: [“%4”, “%4”], “result”: “%5”},
{“opcode”: “const”, “args”: [2], “result”: “%6”},
{“opcode”: “div”, “args”: [“%5”, “%6”], “result”: “%7”},
{“opcode”: “copy”, “args”: [“%7”], “result”: “%1”},
{“opcode”: “const”, “args”: [9], “result”: “%8”},
{“opcode”: “mul”, “args”: [“%8”, “%1”], “result”: “%9”},
{“opcode”: “mul”, “args”: [“%9”, “%1”], “result”: “%10”},
{“opcode”: “const”, “args”: [3], “result”: “%11”},
{“opcode”: “mul”, “args”: [“%11”, “%1”], “result”: “%12”},
{“opcode”: “add”, “args”: [“%10”, “%12”], “result”: “%13”},
{“opcode”: “const”, “args”: [8], “result”: “%14”},
{“opcode”: “sub”, “args”: [“%13”, “%14” ], “result”: “%15”},
{“opcode”: “print”, “args”: [“%15”], “result”: null}
]
}
]
Figure 3: A complete TAC program in JSON form
Given On the Moodle you can find cse302_lab1_starter.zip that contains the TAC compiler phase
(tac2asm.py) and a collection of example ASTs in JSON format in the examples/ subdirectory. Start by
unpacking this in your cse302labs/1 directory.
Deliverables You must build a program called ast2tac.py.
1 This program will be given a single
AST in JSON format in the command line, and it should produce the corresponding TAC file (also in
JSON format).
$ python3 ast2tac.py astfile.json # produces astfile.tac.json
To get an A in this lab, you must implement at least one of top-down or bottom-up maximal munch.
For an A+ in the lab, you should implement both maximal munches and allow the user to pick between
them using the ­­tmm or ­­bmm flags.
$ python3 ast2tac.py ­­tmm astfile.json # top down
$ python3 ast2tac.py ­­bmm astfile.json # bottom up
Creating Fresh AST Files In order to test your compiler pass, you can directly write your test cases
using the JSON format for ASTs. Altenatively, you can use the following web-site to convert BX source
files to ASTs:
1
If you’re not using Python, make sure that your source compiles to ast2tac.exe or ast2tac.sh. Document this clearly in
a README.txt.
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https://cse302.chaudhuri.info/bx2ast/
Note that this site is designed to work with the full BX language, not just the restricted fragment of this
lab. You should make sure to give it it reasonable input.
Some Hints
• Create a mechanism to get a fresh temporary that has definitely not been used anywhere before.
There are many ways to do this, such as with a global counter.
• In your maximal munch implementations, you will need to maintain a mapping from the local variables in your main() function to anonymous temporaries. You can use the collection of ⟨vardecl⟩s
at the top of the body of main() to seed this mapping.
• Give some thought to your class hierarchies for expressions and statements. Both categories will
expand significantly in the coming weeks.
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