Lesson 7

CDT301 – Compiler Theory, Spring 2011Teacher: Linus Källberg

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Outline

• Stack machine code• Generating stack machine code using SDT

STACK MACHINE CODE

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Stack machine code

• Values are kept on a stack• Operations pop operands and push the

result– Similar to postfix notation

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Examples of stack machines

• Early architectures (around 1960)• Java Virtual Machine• Floating-point part of x86• Adobe's PostScript• Trac42VM

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Trac42VM code

• x + y:RVALINT xRVALINT yADD

• z = x * y:LVAL zRVALINT xRVALINT yMULTASSINT

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Trac42VM codeif (x < 0) y = 0 - x;else y = x;

RVALINT x PUSHINT 0 LTINT BRF label1 LVAL y PUSHINT 0 RVALINT x SUB ASSINT BRA label2[label1] LVAL y RVALINT x ASSINT[label2] …

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Trac42VM

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Instruction(s) FunctionPUSHINT x Push the constant xDECL l Declare a variable named l. For function return

values, the reserved name @ can be used.RVALINT l Push the contents (the “r-value”) of variable lLVAL l Push the address (the “l-value”) to the variable lPOP n Pop n elementsASSINT Pop a value x, pop a stack address a, and then store

x to aNOT Pop a value x, then push 1 if x == 0, otherwise 0

Trac42VM

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Instruction(s) FunctionADD, SUB, MULT, DIV, EQINT, LTINT, LEINT

Pop two operands, perform the operation (the right operand is the first popped), and then push the result. The boolean operators return 0 for false and 1 for true.

[l] Adds the code label lBRA l Jump to the label lBRF l Pop a value, and if 0 jump to the label lBSR l Jump to the subroutine labelled lRTS Return from the current subroutine

Exercise (1)What does this code store

in x?Tip: “run” it with some

different values on a and b, and then draw conclusions.

LVAL x RVALINT a RVALINT b LTINT BRF label1 RVALINT b BRA label2[label1] RVALINT a[label2] ASSINT

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Exercise (2)Write Trac42VM code for this Trac42 program:

while (i < 10) ++i;

Hint: the code above is equivalent to loop_start:

if (i >= 10) goto loop_exit; i = i + 1; goto loop_start;loop_exit: …

(If labels and gotos were allowed in Trac42, that is)11

Making function calls

1. Declare room for the return value:DECL @

2. Evaluate the arguments– Leave the results on the stack

3. Jump to the subroutine:BSR the_function

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In the function

• Symbolic names for the parameters• Returning from the function:

1. Evaluate the return value2. Store it:

LVAL @ASSINT

3. Return from the subroutine:RTS 13

After the function call

1. Pop the arguments2. (Pop the return value)

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Function call example 1int squared(int x) { return x*x;}

int trac42(void) { int four; four = squared(2); return 0;}

[squared] LVAL @ RVALINT x RVALINT x MULT ASSINT RTS[trac42] DECL four LVAL four DECL @ PUSHINT 2 BSR squared POP 1 ASSINT LVAL @ PUSHINT 0 ASSINT RTS

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Function call example 2int squared(int x) { return x*x;}

int trac42(void) { squared(3 + 3); return 0;}

[squared] LVAL @ RVALINT x RVALINT x MULT ASSINT RTS[trac42] DECL @ PUSHINT 3 PUSHINT 3 ADD BSR squared POP 1 POP 1 LVAL @ PUSHINT 0 ASSINT RTS

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Exercise (3)Starting from [trac42], “execute” the code here to the right.Source program:

int squared(int x) { return x*x;}

int trac42(void) { int ten; ten = squared(1 + 2) + 1; return 0;}

[squared] LVAL @ RVALINT x RVALINT x MULT ASSINT RTS[trac42] DECL ten LVAL ten DECL @ PUSHINT 1 PUSHINT 2 ADD BSR squared POP 1 PUSHINT 1 ADD ASSINT LVAL @ PUSHINT 0 ASSINT RTS

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GENERATING STACK MACHINE CODE USING SDT

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Trac42VM code for expressionsexpr → expr + num { print(“PUSHINT “);

print(num.value); print(“\nADD\n”) }

| expr – num { print(“PUSHINT “); print(num.value); print(“\nSUB\n”) }

| num { print(“PUSHINT “); print(num.value); print(“\n”) }

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Trac42VM code for expressionsexpr → num { print(“PUSHINT “);

print(num.value); print(“\n”) } rest

rest → + num { print(“PUSHINT “); print(num.value); print(“\nADD\n”) } rest

| - num { print(“PUSHINT “); print(num.value); print(“\nSUB\n”) } rest

| ε20

A recursive descent parser

void expr() {if (lookahead == NUM) {

int num_value = attribute;match(NUM);printf(“PUSHINT %d\n“, num_value);rest();

}else error();

}21

A recursive descent parservoid rest() {

int num_value;switch (lookahead) {case '+':

match('+');num_value = attribute;match(NUM);printf(“PUSHINT %d\n”, num_value);printf(“ADD\n”);rest();break;

case '-':...analogous ...

default: break; // Epsilon}

}

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Generating labels

if_stmt → if ( cond ) block if_tailif_tail → ε

| else block

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Generating labels• Format of translated if-else statement:

Evaluate condition BRF “else” label “True” block BRA “after” label[“else” label] “Else” block[“after” label]

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Generating labelsvoid if_stmt() { char* else_label, * after_label;

match(IF); match( '(' ); cond(); match( ')' ); // Generate new labels else_label = get_new_label(); after_label = get_new_label();

// "True" block printf(" BRF %s\n", else_label); // Jump past "true" block if false block(); printf(" BRA %s\n", after_label); // Jump past "else" block // "Else" block printf("[%s]\n", else_label); // Generate "else" label if_tail(); // This might or might not generate any code printf("[%s]\n", after_label); // Generate "after" label}

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Exercise (4)Given is the following translation scheme.

expr → expr * factor { print("MULT\n") } | factor

factor → ! factor { print("NOT\n") } | num { print("PUSHINT ");

print(num.value); print("\n") }

a) Rewrite it to remove all left recursion.b) Draw the parse tree for “1 * 2 * !3”, with the semantic

actions embedded (with dashed edges).c) Traverse the tree and “execute” the semantic actions.

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Conclusion

• Stack machine code• Generating stack machine code using SDT

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Next time

• Bottom-up parsing

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