Parameter Passing

COP 3402 Software SystemsSpring 2014

Parameter Passing

• Passing parameter to procedures (instead of using variables) simplifies some types of programs, i.e., recursive procedures.

int a, b;procedure sum;int c;begin

c := a + b;write c;

end;begin

a = 2;b = 3;call sum;

end.

procedure sum(a,b);int c;begin

c := a + b;write c;

end;begin

call sum(2,3);end.

Parameter Passing

• To implement parameter passing we must modify:– Grammar– Activation Record– Parsing– Code generation

GRAMMAR CHANGES

Grammar Changesprogram ::= block "." . block ::= const-declaration var-declaration procedure-declaration statement.const-declaration ::= ["const" ident "=" number {"," ident "=" number} ";"].var-declaration ::= [ "int "ident {"," ident} “;"].procedure-declaration ::= { "procedure" ident ";" block ";" }statement ::= [ ident ":=" expression

| "call" ident| "begin" statement { ";" statement } "end" | "if" condition "then" statement ["else" statement]| "while" condition "do" statement| "read" ident| "write" expression| e ] .

...

Grammar Changesprogram ::= block "." . block ::= const-declaration var-declaration procedure-declaration statement.const-declaration ::= ["const" ident "=" number {"," ident "=" number} ";"].var-declaration ::= [ "int "ident {"," ident} “;"].procedure-declaration ::= { "procedure" ident param-block ";" block ";" }.param-block ::= "(" [ ident { "," ident} ] ")".param-list ::= "(" [ expression { "," expression} ] ")".statement ::= [ ident ":=" expression

| "call" ident param-list| "begin" statement { ";" statement } "end" | "if" condition "then" statement ["else" statement]| "while" condition "do" statement| "read" ident| "write" expression| e ] .

...

Activation Record Changes

• The AR must be expanded to include the Parameter Slots.

Return Value

Static Link

Dynamic Link

Return Address

Int Slots

…

Return Value

Static Link

Dynamic Link

Return Address

Param Slots

…

Int Slots

…

PARSING CHANGES

PROC-DECL Procedure

procedure PROC-DECL (level);begin while TOKEN = "procedure" do begin GET_TOKEN();

if TOKEN <> IDENT then ERROR (missing procedure declaration);ENTER(procedure, ident);GET_TOKEN();if TOKEN <> ";" then ERROR (procedure declaration must end with ;);GET_TOKEN();BLOCK(level+1);if TOKEN <> ";" then ERROR (no ; at the end of block);GET_TOKEN();

end;end;

procedure-declaration ::= { "procedure" ident ";" block ";" }.

PROC-DECL Procedure

procedure PROC-DECL (level);begin while TOKEN = "procedure" do begin GET_TOKEN();

if TOKEN <> IDENT then ERROR (missing procedure declaration);GET_TOKEN();num_params = PARAM-BLOCK(level);ENTER(procedure, ident, num_params);if TOKEN <> ";" then ERROR (procedure declaration must end with ;);GET_TOKEN();BLOCK(level+1, num_params);if TOKEN <> ";" then ERROR (no ; at the end of block);GET_TOKEN();

end;end;

procedure-declaration ::= { "procedure" ident param-block ";" block ";" }.

PARAM-BLOCK Procedureprocedure PARAM-BLOCK(level);begin

int params = 0;if TOKEN <> "(" then ERROR();GET_TOKEN();if TOKEN == IDENT then begin

params++;param_addr = 4 + params;ENTER(int, ident, param_addr);GET_TOKEN();while TOKEN == "," do begin

GET_TOKEN(); if TOKEN <> IDENT then ERROR();params++;param_addr = 4 + params;ENTER(int, ident, param_addr);GET_TOKEN();

endendif TOKEN <> ")" then ERROR();GET_TOKEN();

return params;end

param-block ::= "(" [ ident { "," ident} ] ")".

STATEMENT Procedureprocedure STATEMENT(level);…

else if TOKEN == "call" then beginGET_TOKEN();IF TOKEN <> IDENT then ERROR();GET_TOKEN();

end…

statement ::= … | "call" ident | …

STATEMENT Procedureprocedure STATEMENT(level, ar_size);…

else if TOKEN == "call" then beginGET_TOKEN();IF TOKEN <> IDENT then ERROR();GET_TOKEN();PARAM-LIST(level, ar_size);

end…

statement ::= … | "call" ident param-list | …

PARAM-LIST Procedureprocedure PARAM-LIST(level, ar_size);begin

if TOKEN <> "(" then ERROR();GET_TOKEN();if TOKEN <> ")" then begin

EXPRESSION();while TOKEN == "," do begin

GET_TOKEN(); EXPRESSION();

endendif TOKEN <> ")" then ERROR();GET_TOKEN();

end

param-list ::= "(" [ expression { "," expression} ] ")".

CODE GENERATION CHANGES

BLOCK Procedure

procedure BLOCK(level, num_params);begin

ar_size = 4 + num_params;jmpaddr = gen(JMP, 0, 0); if TOKEN = “const” then CONST-DECL(level);if TOKEN = “var” then VAR-DECL(level, &ar_size);if TOKEN = “procedure” then PROC-DECL(level);code[jmpaddr].addr = NEXT_CODE_ADDR;gen(INC, 0, ar_size);STATEMENT(level, ar_size);gen(OPR, 0, 0);

end;

PARAM-LIST Procedureprocedure PARAM-LIST(level, ar_size);Begin

int param_count = 0;if TOKEN <> "(" then ERROR();GET_TOKEN();if TOKEN <> ")" then begin

EXPRESSION();param_count++;gen(STO, curreg, 0, ar_size+4+param_count);curreg--;while TOKEN == "," do begin

GET_TOKEN(); EXPRESSION();param_count++;gen(STO, curreg, 0, ar_size+4+param_count);curreg--;

endendif TOKEN <> ")" then ERROR();GET_TOKEN();return param_count;

end

PARAM-LIST must initialize the parameters before we call the procedure.

EXPRESSION() gets the value for the parameter…

…and here we store the value in the parameter.

PARAM-LIST Procedure

Why???

procedure PARAM-LIST(level, ar_size);Begin

int param_count = 0;if TOKEN <> "(" then ERROR();GET_TOKEN();if TOKEN <> ")" then begin

EXPRESSION();param_count++;gen(STO, curreg, 0, ar_size+4+param_count);curreg--;while TOKEN == "," do begin

GET_TOKEN(); EXPRESSION();param_count++;gen(STO, curreg, 0, ar_size+4+param_count);curreg--;

endendif TOKEN <> ")" then ERROR();GET_TOKEN();return param_count;

end

PARAM-LIST Procedure

Return ValueStatic Link

Dynamic LinkReturn Address

Param Slots…

Int Slots…

Return ValueStatic Link

Dynamic LinkReturn Address

Param Slots…

Int Slots…

Current AR

Next AR(not created yet!)

procedure PARAM-LIST(level, ar_size);Begin

int param_count = 0;if TOKEN <> "(" then ERROR();GET_TOKEN();if TOKEN <> ")" then begin

EXPRESSION();param_count++;gen(STO, curreg, 0, ar_size+4+param_count);curreg--;while TOKEN == "," do begin

GET_TOKEN(); EXPRESSION();param_count++;gen(STO, curreg, 0, ar_size+4+param_count);curreg--;

endendif TOKEN <> ")" then ERROR();GET_TOKEN();return param_count;

end

Call Code Generation

procedure STATEMENT(level, ar_size);begin

…else if TOKEN = "call" then begin

GET_TOKEN();if TOKEN <> IDENT then ERROR (missing identifier);i = find(TOKEN);

if i == 0 then ERROR (); if symboltype(i) <> PROCEDURE then ERROR();

param_count = PARAM-LIST(level, ar_size);if param_count <> symbolparams(i) then ERROR();gen(CAL, 0, 0, symboladdr(i));

GET_TOKEN();end

…

Call Code Generation

procedure STATEMENT(level, ar_size);begin

…else if TOKEN = "call" then begin

GET_TOKEN();if TOKEN <> IDENT then ERROR (missing identifier);i = find(TOKEN);

if i == 0 then ERROR (); if symboltype(i) <> PROCEDURE then ERROR();

param_count = PARAM-LIST(level, ar_size);if param_count <> symbolparams(i) then ERROR();gen(CAL, 0, 0, symboladdr(i));

GET_TOKEN();end

…

We must verify that the amount of parameters is exactly the same amount that the procedure declared.