algorithm-in-python/parser/PL0-compiler

PL0-compiler

A compiler for c-like programming language based on PL0, which is a dynamic, strong typing language.

See grammar here, wikipedia-PL0, and download this pdf(zh) for more details.

QuickStart

usage: parser.py [-h] [-i] [-s] [-t] [-v] [-f FILE]

optional arguments:
  -h, --help            show this help message and exit
  -i, --instruction     output instructions
  -s, --stack           output data stack when executing each instruction
  -t, --token           output tokens when parsing
  -v, --varible         output varibles for every static environment
  -f FILE, --file FILE  compile and run codes. Without this arg, enter
                        interactive REPL

Run python parse.py and enter a REPL state, you can type and run sentences and expressions interactively

Examples

Note that when in REPL, every sentence or expresion or block ends with '.'. But in program codes, only the whole program ends with a dot.

interactive-expression

Therer are some expressions and sentence in file expr.txt, now test it. python parser.py -f test/expr.txt

>> codes:
1     // expression
2     var a=3,b=2,c;.

>>  c:=a+1.
>> begin c; c+1!=1 ; c+1=5 end.
result: 4.0; True; True;
>> for(;b>=0;b:=b-1) print('random(100): %d',random(100)) .
random(100): 14
random(100): 60
random(100): 58
>> begin ++1--1; 1<<2+3%2; 2&1 end.
result: 2.0; 8; 0;
>>   -1+2*3/%2.
result: 2.0;
>>    (1+2.
line 1: ( 1 + 2 .
                ^
[Error]: Expected ")", got "."
>> 4!!.
result: 620448401733239439360000;
>> codes:
1     if   0 then 1
2     elif 1>2 then 2
3     elif false then 3
4     else 4.

result: 4.0;

fibonacci

Run python parser.py -f test/fibonacci.txt

>> codes:
1     func fib(n)
2     begin
3         if n=1 || n=2 then return 1;
4         return fib(n-1)+fib(n-2);
5     end ;
6     var n=1;
7     begin
8         while n<15 do
9         begin
10            print('fib[%d]=%d',n,fib(n));
11            n :=n+1;
12        end;
13    end
14    .

fib[1]=1
fib[2]=1
fib[3]=2
fib[4]=3
fib[5]=5
fib[6]=8
fib[7]=13
fib[8]=21
fib[9]=34
fib[10]=55
fib[11]=89
fib[12]=144
fib[13]=233
fib[14]=377

Try the following commands to explore more examples.

python parser.py -f test/factorial.txt
python parser.py -f test/closure.txt
python parser.py -f test/closure.txt -i
python parser.py -f test/closure.txt -t
python parser.py -f test/closure.txt -s
python parser.py -f test/closure.txt -istv
python parser.py  # enter interactive repl

Description

ident type

• constant
• varible
• function

operator

relation opr

• <
• >
• <=
• >=
• = equal
• !=
• odd

bit opr

• & bitand
• | bitor
• ~ bitnot
• << left shift
• >> right shift

arithmetic opr

• + add/plus
• - sub/minus
• * multiply
• / divide
• /% integer div
• % mod
• ^ power
• ! factorial

conditon opr

• ?: eg a>b ? c:d

control structure

• if elif else
• for
• while
• break
• continue
• return

builtin function

• print(formatStr,arg1,...)
• random(), random(n)

Grammar

program =  body "."
body = {varDeclaration ";" |  constDeclaration ";" |  "func" ident "(" arg_list  ")" body ";"}  sentence

varDeclaration = "var"  varIdent { "," varIdent}
varIdent  = ident ["=" number] | ident  { "[" number "]" } 
constDeclaration = "const" ident "=" number {"," ident "=" number}

sentence = [ ident ":=" { ident ":=" } sentenceValue 
                |  "begin" sentence { ";" sentence}  "end"
                |  "if" sentenceValue "then" sentence {"elif" sentence} ["else" sentence]
                |  "while" sentenceValue "do" sentence
                |  "do" sentence "while" sentenceValue 
                |  "switch" sentenceValue {"case" sentenceValue {"," sentenceValue} ":" [setenceValue]}  (* ["default" ":" sentenceValue]   to do *)
                |  "break"
                |  "continue"
                |  ["return"] sentenceValue
                |  "print" "(" str,real_arg_list ")" ]

sentenceValue =   condition

arg_list =  ident { "," ident}

real_arg_list = sentenceValue {"," sentenceValue }


condition = condition_or [ "?" sentenceValue ":" sentenceValue ]
condition_or  = condition_and { "||" condition_or }
condition_and = condition_not { condition_not "&&" condition_and}
condition_not = {"!"} condition_unit
condiiton_unit = ["odd"] expression
                        | expression ("<" | ">" | "<=" | ">=" | "=" | "!=") expression

expression =  level1 { ("<<"| ">>" | "&" | "|") level1 }
level1  = level2 { ( "+" | "-" ) level2 }
level2 = level3 { "*" | "/" | "/%" | "%" ) level3 }
level3 = level4 {"^" level4}
level4 = item {"!"}          (*  factorial *)
item =  number|"true"|"false" | ident { "(" real_arg_list ")" }| "(" sentenceValue" )" | ("+" | "-" | "~" ) item

syntax

Writet down syntax, then convert left recursion to right recursion. Namely we should change the following productions: expr, level0, level, level3

We notice that

A -> Aa|b

equls to

A -> bR
R -> nil | aR

so here are the right-recursion productions

expr   -> level1 interval1
interval1 -> nil | {&|'|'|>>|<<|} interval1

level1 -> level2 interval2
interval2 -> nli | {+|-} interval2

level2 -> level3 interval3
interval3 -> nil | {*|/|//|%} interval3

level3 -> level4 | level4 ^ level3

level4 -> item interval4 
interval4 -> nil |! interval4

item   -> NUM|E|PI|ln(expr)|(expr)| + item| - item| ~ item

When implementing the parser, we can use a loop structure to implement the right recursion because it's tail-recursive.

For instance, we can simply find that the production for level4 is

level4 -> item | item ! | item!! |item !!! | ...

Though we can't write a production with infinite loops, we can write it in code like this:

match_level4():
    result = match(item)
    while lookAhead  matches item:
        match("!")
        result = factorial(item)
    return result

Instruction generation

We designed several instructions that can be generated for the target machine. To simplify this problem, we will emulate this virtual machine and execute instructions in python.

register

This machine has three registers:

• b is the base register that contains the base pointer to locate a varible in the data stack
• regs are a series of registers. Currently the first one is used for returning value of latest function call, and the second one is used to store the switch value
• pc is the pc register that points to the instruction

stack

There are two stack in this virtual machine. One contains the instructions, visited by register pc. It won't change when executing instructions, so we can assume it's readonly The other is data stack. It dynamiclly changes when running the program.

For each level, the first is the base address of this level. The second place is the static chain to visit the upper level's varibles. The third place contains the return address of the upper level. And the other places in one level contains local varibles and real time data for calculation.

Each time we call a function, the level increases 1. Also, the level decreases 1 when we return from a function.

instruction

Every instruction consists of three parts. The first is the name of the instruction. Generally, the second is the level diifference of a identifier(if it has). And the third part is the address.

name	levelDiff	address	explanation
INT	0	n	allocate n space for one level
INT	1	n	rewind stk.top backward n steps
INT	2	n	print the top n elements of stack
LIT	-	constant value	push a constant value to the top of the data stack
LOD	levelDiff	addr	load a varible value to the top of the data stack. The var can be found use levelDiff and addr
STO	levelDiff	addr	store the stack top value to a varible, top decreases.
CAL	levelDiff	addr	call a function
JMP	-	addr	jmp to addr, namely set addr to pc
JPC	-	addr	pop stack, if the value is not True, jmp addr
MOV	n1	n2	stk[top-n2] = stk[top-n1]
RET	-	-	return to the upper level, use current level's first three value to change pc, data stack, base register.
POP	-	-	pop the data stack, store the value in reg register
PUSH	-	-	push reg to stack top
OPR	-	operator type	variout operation on value

Design

We can generate instruction when analysing grammar. Some keypoints is the control structures' instruction traslation.

if elif else

while/break

continue, for can be translated in the same way.

switch

eg

switch n
    case 1,2:print('1 or 2')
    case 1+5:print('6')
    case func_add(1,6):print('7')
;

function arguments pass

When analysing the function's defination, we can store the formal arguments as function's local varibles. As soon as we call this function, we should calculate the real arguments in the level upper the function, and then pass value to the function's formal varibles one by one.

I use an instruction MOV to achive this goal. MOV addr1, addr2 will store value stk[top-n2] in stk[top-n1]. Let's have a look at how to call a function and pass args value.

Before we call a function, its real args will be calculated in the level upper this function. Note function level is n+1, and we call this function in level n. In level n, we calculated function's args, all values are stored in the data stack of level n. Now call function and enter it. Data stack reaches level n+1 and grows three spaces for DL,SL,RA. The following space are for function's local varibles. So we can mov level n's real args value to these places according to function's argument num and varible num.

For example, function has n1 args, n2 local varibles(excluding args), then

for i in [0,1..,n1-1]:
    mov , n2+n1+3+i, n2 + i

The moment we returned level n, we should rewind top for n1 spaces, OPR,n1,'BACK' can make it.

function return

Also, mark function level as n+1, and outer(upper) is level n. To implement return sentence, we just need to do two things:

• calculate return sentence value in level n+1
• pass this value to level n It seems that it's hard to pass level n+1 's value to level n. Once we returned to level n, level n+1 's data in data stack will be cleared.

I use a extra register reg to achive this. Before we return,

• calculate return value
• OPR ,0,'POP' will pop the value and store it in reg
• return level n
• OPR,0,'PUSH' will push reg value to stack top

Now the return value has be passed from level n+1 to level n

instruction backpatching

Taking while block as an example, Note that we don't know the JPC instruction's target addr until we finish analysing the whole block.The Solution is that after we analyse while condition, we generate an instruction with no target address, just take a place. We note down this instruction's address. As soon as we finish analysing the whole while block, the instruction pointer, namely ip, pointing to the target address of JPC. Then we backpatch the JPC instruction with the target address along to ip.

symbol table

When analysing and translating, we want to get the symbol which including level, address,(value for constant) according to its name. The following shows how to achive it elegantly

There are three types of symbols:

• constant
• varible
• function name Every function has an environment that contains this level's symbols, and an outer environment(except main function). Every environment has the three symbols mentioned above.

Defaultly, we are in the main function in the beginning of this program.

In an enviroment, when we meet a symbol, we should seek it in current environment. If not found, go for the outer environment recursively until we found it.

It gurantees that every environment has no same names for different symbols but may have same names in different environment.

So there won't be conflits when different functions have same local varibles or arguments.

I create class closure to describe this kind of environment and varible curClosure to mark down current environment. Every time when calling a function, we enter a more inner environment. We do the following things to make sure that environment changes creately.

saved = curClosure
curClosure = function.closure
call function
curClosure = saved

builtin function--print

This function is just like function printf in clang. Call it in the following format: print(FORMAT[,arg1,arg2...]) The format string supports two kinds of format currently:

• %d: integer
• %f: float

If you want to print raw %d, not formatting. You can add a back slash in front of %. (So it's with %f...)

For example:

>> print('a=%d, % \%d',1)
a=1, % %d

To implement this builtin function, we should firstly parse the formatting str. I parse the format-str and generate segs seperated by %d or %f. For instance, 'fib[%d]=%d' generates segs ['fib[','%d',']=','%d']. For every seg, if it's string, generate instruction ('LIT',0,c), c is one chracter that consist of seg. If it's %d or %f, we should first match comma, and then parse the followwing value and generate instructions. When in runtime, after executing there instructions, we will get a value(only take place one data-stack unit).

After handling all segs, we generate an instruction ('INT',2,n), which represents printing the top n units of data stack, and stk.top = stk.top-n. N can be calculated by suming all lengths of str-seg, and num of format-seg.

To do

• array
• different value pass
• function pass
• type
• struct