1. Lesson 7 CDT301 – Compiler Theory, Spring 2011 Teacher: Linus Källberg

2. 2 Outline Stack machine code Generating stack machine code using SDT

3. STACK MACHINE CODE 3

4. Stack machine code Values are kept on a stack Operations pop operands and push the result –Similar to postfix notation 4

5. Examples of stack machines Early architectures (around 1960) Java Virtual Machine Floating-point part of x86 Adobe's PostScript Trac42VM 5

6. Trac42VM code x + y: RVALINT x RVALINT y ADD z = x * y: LVAL z RVALINT x RVALINT y MULT ASSINT 6

7. Trac42VM code if (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] … 7

8. Trac42VM 8 Instruction(s)Function PUSHINT xPush the constant x DECL lDeclare a variable named l. For function return values, the reserved name @ can be used. RVALINT lPush the contents (the “r-value”) of variable l LVAL lPush the address (the “l-value”) to the variable l POP nPop n elements ASSINTPop a value x, pop a stack address a, and then store x to a NOTPop a value x, then push 1 if x == 0, otherwise 0

9. Trac42VM 9 Instruction(s)Function ADD, 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 l BRA lJump to the label l BRF lPop a value, and if 0 jump to the label l BSR lJump to the subroutine labelled l RTSReturn from the current subroutine

10. 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 10

11. 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

12. 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 12

13. In the function Symbolic names for the parameters Returning from the function: 1.Evaluate the return value 2.Store it: LVAL @ ASSINT 3.Return from the subroutine: RTS 13

14. After the function call 1.Pop the arguments 2.(Pop the return value) 14

15. Function call example 1 int squared(int x) { return x*x; } int trac42(void) { int four; four = squared(2); return 0; } [squared] LVAL @ RVALINT x MULT ASSINT RTS [trac42] DECL four LVAL four DECL @ PUSHINT 2 BSR squared POP 1 ASSINT LVAL @ PUSHINT 0 ASSINT RTS 15

16. Function call example 2 int squared(int x) { return x*x; } int trac42(void) { squared(3 + 3); return 0; } [squared] LVAL @ RVALINT x MULT ASSINT RTS [trac42] DECL @ PUSHINT 3 ADD BSR squared POP 1 LVAL @ PUSHINT 0 ASSINT RTS 16

17. 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 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 17

18. GENERATING STACK MACHINE CODE USING SDT 18

19. Trac42VM code for expressions expr→ 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”) } 19

20. Trac42VM code for expressions expr→ 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 | ε 20

21. 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

22. A recursive descent parser void 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 } 22

23. Generating labels if_stmt→ if ( cond ) block if_tail if_tail→ ε | else block 23

24. Generating labels Format of translated if-else statement: Evaluate condition BRF “else” label “True” block BRA “after” label [“else” label] “Else” block [“after” label] 24

25. Generating labels void 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 } 25

26. 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. 26

27. Conclusion Stack machine code Generating stack machine code using SDT 27

28. Next time Bottom-up parsing 28