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Project step 1 – a compiler for a simple language

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Project step 1 – a compiler for a simple language. v1.01
Change log:
v1.01 Wording changes in the introductory text and high level actions section. Changes to the code
generated for ret, retr var, print<type>, printv, call, callr
v1.0, changes from 0.18. Use the popa instruction to clear the stack during a return. Coloring for
previous changes removed. Overstruck text from previous changes is removed. These changes are
colored blue.
v0.18, changes from 0.17. “,” (commas) between arguments have been removed from statements.
They serve no purpose and add extra steps to the parsing.
v0.17, changes from 0.16. Added peek, poke and swp to list of instructions in the High Level Actions
of the Compiler section. Changed base print opcode to 144, it had been conflicting with the cmpe
instruction. Removed crossed out text. Made ret, call and pushv opcodes no longer crossed out.
Changed instructions that use pushv (retr , printv) to use it correctly. Change code for call and callr as
a result of changes to pushv. Change the peek statement and peek bytecode to be typed. Cross out
bytecode documentation and refer the reader to the interpreter document. Changes shown in green
v0.16, changes from 0.15 Make all push types in compiler actions explicit. Simplified and better
documentation of call and callr instruction compiler actions. Let the print statement print characters
and numbers. Added a printv statement to print variables. Changed compiler actions for retr to push a
variable value, not a literal value.
v0.15, changes from 0.14 Change compiler actions for ret, retr, jmp. Change the description and
compiler actions for poke. Change the description for swp. Change the compiler actions for call and
callr.
v0.14, changes from 0.13 Add peek, poke and swp instructions. Change popm compiler actions.
Change callr compiler actions. Other small changes to wording.
v0.13, changes from 0.12 Add a count field to subr, call and callr to simplify code generation.
v0.12 Changes from 0.11. Added a callr statement that takes a return type. Fix the generated code for
this and for call to allow arguments to be pushed by the call. Add a retr that returns a value and update
the reg.
v0.11: changes from 0.10. Put typing into push operators. Put opcodes for compare operators. fix
actions for call. Make declarations reserve a stack location. Remove redundant store instruction (popv
does the same thing.)
v0.10: changes from 0.0. Comparison operators (cmpe, cmplt, cmpgt) added. jump conditional
(jmpc) added. bytecode values added. Font changed to Times New Roman.
This project builds a compiler for a small language. The input language is described below. The
output language is a bytecode that is interpreted. You will be supplied a binary for the interpreter, and
will write an interpreter as a second project.
High level organization of the language:
The first non-comment statement in the language is the start of the main routine. After the end of the
main routine, other functions are declared and defined.
Within each function the first non-comment statements are variable declarations. All variable
declarations must be at the top of the function.
The first non-declaration statement defines the start of the executable and label statements. The various
statements are defined below.
At the end of the function there will be a ret statement. The next statement, if it exists, should be a new
function.
Four kinds of values can be operated on by a program, in addition to labels.
integer: 32 bit integer values. Specified as a string of decimal digits, i.e., 56.
short: 16 bit integer values. specified as a string of decimal digits with an s appended, i.e., 56s.
float: 32 bit floating point values, specified as a string of decimal digits, a decimal point, and
a fractional part, i.e., 56.04.
char: these are only present in print statements and are always literals, e.g., ‘c’.
Native representations can be used for integers, shorts and floats. Operations, described below, can be
applied to mixed values, i.e., a float and a short can be added and stored in an integer.
Details of different kinds of statements in programs are given in the section Input language below.
High level actions of the compiler:
The compiler will read each statement in turn from the source program file, and determine
the operation the statement performs and the operands. Each statement has the structures
operation op op, …
i.e., and operation plus zero or more operands.
If the statement is a variable declaration, the compiler will create an entry in the symbol table . There is
one symbol table for the entire program. The symbol table is a map whose key is one of two kinds. The
first is a variable name, and the data for the variable name is the offset on the runtime stack (of the
function the variable is declared in) for the variable. Clearly, the compiler needs to have a counter that
keeps track of the number of variables declared in a function so that its runtime stack location is
known. The second is a label number, where the data is the offset in the byte code of the label. Clearly,
the compiler needs to have a counter that keeps track of the offset in the generated bytecode that the
label is found.
For symbol table entries, (x) indicates the contents of the symbol table for the entry for key x.
After processing dec statements that only create entries in the symbol table and do not cause any code
to be generated, the executable part of the function begins. The executable part of the function consists
of lab, subr, ret, print, jmp, jmpc, cmpe, cmplt, cmpgt, call, push, popm, popv, peek, poke, swp, st, add,
sub, mul and div.
Actions for many statements are straightforward, and are described in the section Input language,
below. Because variables have to be declared first before executable statements, every variable needed
will be present in the symbol table. When we encounter a jmp or a call statement, however, we may
not have visited the corresponding lab or subr statement, and the location jumped to, or the location of
the function being called, may not be in the symbol table, and th compiler will no know how to
generate code to jump to that location. In this case the compiler should keep track of the location of
all code generated that requires the value of a jmp target or the start of a function. When the entire
program has been traversed all of these values are known, and the compiler can go to the locations
that need to be updated and update them.
At this time the program can be written to a file with an extension of .smp as a stream of bytes.
Input language.
The input language processes arithmetic expressions with numeric operations of +, -, *, and /. The
bytecode representation of an operation is given by bc.op. For simplicity, the runtime stack contains
one value in each position, i.e., a short and an int take the same amount of space. The runtime stack
can be implemented as a vector whose elements point to objects containing the actual stack value.
Language statements:
The descriptions below give the semantics of the statement when the program is executed (which will be
done by another program) and the code generated, or other actions taken, by this program. For all
statements that generate byte codes an internal compiler counter giving the bytecode offset should be
incremented appropriately. (x) indicates the contents of the symbol table for the entry for (x).
Statements should all be parseable by a state machine/DFA.
All language statements fit on a single line.
/ string: A comment that can be ignored
compiler actions: throw away the line of the program that is the comment.
decl var type: declares a variable whose name is given by var. The name will be 8 alpha characters or
less. type is one of the three numeric types above, and are specified as int, short or float. All variables
are local. There are no global variables.
compiler actions: create an entry in the symbol table (see below). The entry will be of the form <key,
value>, where the key will be the concatenation of the function label of the function being compiled
and var, and the value will be an object containing the stack offset within the stack frame of the
function that will hold the variable and the type. The first variable declared in a function will have an
offset of 0, the next an offset of 1, and so forth. Reserve a space in the stack for the variable by
generating the following code:
push<type> 0
lab label: specifies a label that can be the target of a branch. label is a string with no more than eight
alpha characters.
compiler actions: Create an entry in the symbol table. The entry will be of the form <key, value>,
where the key will be the concatenation of the function label of the function being compiled and
label and the value will be the offset within the generated program code generated that the label
appears.
subr cnt flabel: declares an flabel that is the start of a subroutine, where flabel is a string with no more
than eight alpha characters. The main procedure always has the flabel of main. No two functions
share the same flabel. cnt is the number of arguments subr takes and is an integer.
compiler actions: Create an entry in the symbol table (see above). The entry will be of the form <key,
value>, where the key will be the flabel and the value will be the offset within the program code
generated that the label appears and the value of cnt. Note that variables and flabels may have the
same string since the variable is always part of a function, and the function name is part of the
variable’s key.
ret: a subroutine return. Pop all local variables off of the stack, pop the return value off of the stack
and into the PC (program counter). The next statement to be executed will be the one indicated by the
updated PC.
compiler actions: Generate the following code:
pushi 0
bc.popa // 0 means pop everything and keep nothing
bc.ret // jmp to the location at the top of the stack – the return
// address
The previous code from 0.18 can be made to work, but Sara says the code above leads to a
simpler implementation. I’ve included the previous code below (italicized) for reference.
bc.pop <count of local variables added to the stack + count
of arguments>
bc.ret // jmp to the location at the top of the stack – the
return // address
retr var: a subroutine return that returns a variable value. Pop all local variables off of the stack, pop
the return value off of the stack and into the PC (program counter). The next statement to be executed
will be the one indicated by the updated PC.
compiler actions: Generate the following code:
pushi (var)
bc.pushv<type> // type is the var type
pushi 1
bc.popa // pop all local variables off of the stack but return
variable
bc.swp
bc.ret // jmp to the location at the top of the stack – the return
// address
The previous code from v0.18 can be made to work, but Sara says the code above leads to a
simpler implementation. I’ve included the previous code below (italicized) for reference.
bc.pop <count of local variables added to the stack + count
of arguments>
pushi (var)
bc.pushv<type> // type is the var
type bc.swp
bc.ret // jmp to the location at the top of the stack – the
return // address
print<type> literal: prints the literal. Remember to increment the byte code offset in memory by the
length of the literal in bytes.
compiler actions: Generate the following code:
bc.push<type> literal
bc.print<type>
printv var: prints the value of the variable
compiler actions: Generate the following code:
bc.pushi (var)
bc.pushv<type> // type is the var
bc.print<type>
jmp label: jump to the statement immediately following the label. The jmp statement pops the offset at
the top of the stack off of the stack when it performs a jump.
compiler actions: Generate the following code:
pushi (label)
bc.jmp
jmpc label: jump to the statement immediately following the label if the top of the stack has a
1, otherwise do nothing. Typically used after a compe, complt or compgt
compiler actions: Generate the following code:
pushi (label)
bc.jmpc
cmpe: Let t be a pointer to the top of the stack, the result of this is *t = *(t-1) == *t. 1 will be at the
top of the stack if the comparison is true, 0 otherwise. The stack depth decreases by one at the end of
this operation.
compiler actions: Generate the following code:
bc.cmpe
cmplt: Let t be a pointer to the top of the stack, the result of this is *t = *(t-1) < *t. 1 will be at the top
of the stack if the comparison is true, 0 otherwise. The stack depth decreases by one at the end of this
operation.
compiler actions: Generate the following code:
bc.cmplt
cmpgt: Let t be a pointer to the top of the stack, the result of this is *t = *(t-1) >*t. 1 will be at the top
of the stack if the comparison is true, 0 otherwise. The stack depth decreases by one at the end of this
operation.
compiler actions: Generate the following code:
bc.cmpgt
call cnt vara 0 vara2 … varan-1 flabel: jump to the subroutine specified by flabel, i.e., jump to the offset
that is in the symbol table for the label. The address of the next instruction after the call is pushed onto
the stack. cnt is the number of arguments passed to the subroutine, and is an integer.
compiler actions:
If flabel is not in the symbol table, add it. You will not be able to add the location of flabel yet. Add
the argument count, cnt, to the symbol table. If flabel is in the symbol table check that cnt is the same
as the cnt in the symbol table. If not, print an error and terminate the compiler.
Generate the following code:
bc.pushi PC // compute the next instruction byte offset after the
bc.pushi 6+2*n+x+1 // call. add the current instruction (PC)+8
/ non-arg push instructions + 2*n arg push
/ instructions +x positions taken by the arg values
/ and (flabel) value pushed
/ + 1 to skip past the last position to the
/ next instruction after the call.
/ 8+n+x+1 can be determined at compile time,
/ and the resulting integer variable pushed
/ note that n may be zero. Note the extra space for
/ the return argument varr which sits in the old stack
/ frame immediately before the arguments are pushed
bc.add
bc.pushi (var0)
bc.pushv<type> // push the arguments (n pushes)
bc.pushi (var1)
bc.pushv<type>
. . .
bc.pushi (varn-1)
bc.pushi (flabel)
bc.pushi n+1
bc.call
callr cnt varr vara 0 vara2 … varan-1 flabel: jump to the subroutine specified by flabel, i.e., jump to the
offset that is in the symbol table for the label. cnt is the number of arguments passed to the subroutine,
and is an integer. Space is reserved on the stack to hold the return value. After the call returns, the
value in this stack location is moved to varr. The address of the next instruction after the call is pushed
onto the stack. NOTE: varr must the same type for all calls and the type is set by the first call.
compiler actions: Generate the following code:
If flabel is not in the symbol table, add it. You will not be able to add the location of flabel yet. Add
the argument count, cnt, to the symbol table. If flabel is in the symbol table check that cnt is the same
as the cnt in the symbol table. If not, print an error and terminate the compiler.
bc.pushi PC // compute the next instruction byte offset after the
bc.pushi 8+2*n+x+1 // call. Add the current instruction (PC)+8
/ non-arg push instructions + 2*n arg push
/ instructions +x positions taken by the arg values
/ and (flabel) value pushed
/ + 1 to skip past the last position to the
/ next instruction after the call.
/ 8+n+x+1 can be determined at compile time,
/ and the resulting integer variable pushed
/ note that n may be zero. Note the extra space for
/ the return argument varr which sits in the old stack
/ frame immediately before the arguments are pushed
bc.add
// compute the stack depth added by arguments,
// (flabel) (n args + 1 for (flabel)
bc.pushi (var0)
bc.pushv<type> // push the arguments (n pushes)
bc.pushi (var1)
bc.pushv<type>
. . .
bc.pushi (varn-1)
bc.pushv<type>
bc.pushi (flabel)
bc.pushi n+1 // compute the stack depth added by arguments
bc.call
pushi (varr)
bc.popv
push<type> val: push the val onto the stack. <type> specifies the type, either an s, i or f for short, int
and float, respectively. The type of the operand to be pushed is inferred from the format of val.
compiler actions: Generate the following code:
bc.push<type> val
push<type> var: push the value of the variable whose name is given by var onto the stack. <type>
specifies the type, either an s, i or f for short, int and float, respectively. The type of the operand to be
pushed is inferred from the type of the variable.
compiler actions: Generate the following code:
bc.pushs<type> (var)
popm val: pop the top entry val entries from the stack. The values in the stack are lost. val will be an
integer.
compiler actions: Generate the following code:
bc.pushi val
bc.popm
popv var: pop the current top of the stack and put the value into the variable var.
compiler actions: Generate the following code:
pushi (var)
bc.popv
peek var val: var = stack[sp+val]. The types of the stack entry and the variable var must be the same.
sp+val should be a valid stack entry. The primary use is to examine arguments. If there are n
arguments then peek var, k-n get’s the value of the kth argument. val is typically negative compiler
actions: Generate the following code:
pushi (var)
pushi val
bc.peek<type>
poke val var: stack[sp+val] = var. The types of the stack entry and the variable var must be the same.
sp+val should be a valid stack entry. The primary use is to change the value of arguments. compiler
actions: Generate the following code:
pushi (var)
pushi val
bc.poke<type>
swp: Swap top two stack elements, i.e., t = stack[sp]; stack[sp] = stack[sp-1]; stack[sp-1] = t.
compiler actions: Generate the following code:
bc.swp
add: add the top two elements of the stack and push the result onto the stack. The stack depth
decreases by one at the end of this operation.
compiler actions: Generate the following code:
bc.add
sub: subtract the top two elements of the stack and push the result onto the stack. Thus, if t is a
pointer to the top of the stack, the result of this is *t = *(t-1) – *t. The stack depth decreases by one at
the end of this operation.
compiler actions: Generate the following code:
bc.sub
mul: multiply operand op1 and op2, and push the value onto the stack. he stack depth decreases by one
at the end of this operation.
compiler actions: Generate the following code:
bc.mul
div: Divide the top two elements of the stack and push the result onto the stack. Thus, if t is a pointer
to the top of the stack, the result of this is *t = *(t-1) / *t. The stack depth decreases by one at the end
of this operation.
compiler actions: Generate the following code:
bc.div
Opcode values and meanings are found in the interpreter document.

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