Lesson 13

CDT301 – Compiler Theory, Spring 2011Teacher: Linus Källberg

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Outline

• Overview of Trac42VM• Activation records

OVERVIEW OF TRAC42VM

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4

Overview of Trac42VM

• A stack• A code memory area• Registers:

– PC: Points to the next instruction to execute– SP: Points to the top of the stack– FP: Points to the current activation record

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Memory arrangement

• Memory is partitioned into different sections– Executable code of a program*– Stack – e.g. local variables*– Static – global and static memory– Heap – dynamically allocated memory

• The stack and the heap grow and shrink during runtime

* Used in Trac42VM

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Lab 3 vs. lab 1

• No named program points, i.e., labels or functions– We use absolute code addresses

• No names on variables or (an no @)– We use relative stack addresses

• New data types: int, bool, and string– Need different operations– Need different amount of stack space

Data types in Trac42VMData type SizeInt 1Bool 1String 100

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Instructions in Trac42VM

• PUSHINT, PUSHBOOL, PUSHSTRING• RVALINT, RVALBOOL, RVALSTRING• ASSINT, ASSBOOL, ASSSTRING• EQINT, EQBOOL, EQSTRING,• LTSTRING, LESTRING• WRITEINT, WRITEBOOL, WRITESTRING• LINK, UNLINK 8

ACTIVATION RECORDS

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Function calls and the stack

• Conveys information:– To called function: arguments– From called function: return value– Program state necessary to resume execution after

the call (e.g., the return-to address)• The information held for one function call:

activation record (or stack frame)– The frame pointer register points to the current

activation record– One activation record for each active function

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(Possible) layout of anactivation record

Actual arguments

Return address

Frame pointer

Local variables

Temporary variables

Return value

Top

Bottom

FP-relative addresses

int funcA() { int x = funcB(23); return x + funcC(11, 17);}

int funcB(int x) { if (x == 0) return funcC(12, 13); return x * funcB(x – 1);}

int funcC(int x, int y) { return x + y;}

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Example

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Caller’srecord

42

Start here

42

42FP

SP

int twice(int x){3 int y;4 y = 2 * x;5 return y;}

7 a = twice(13);8 x = ...

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Example

Return value

Caller’srecord

?? 41

42

int twice(int x){3 int y;4 y = 2 * x;5 return y;}

7 a = twice(13);8 x = ...41

42FP

SP

Example

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Actual arguments

Return value

Caller’srecord

13 40

41

42

int twice(int x){3 int y;4 y = 2 * x;5 return y;}

7 a = twice(13);8 x = ...40

42FP

SP

Example

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Actual arguments

Return address

Return value

Caller’srecord

13

8 *

42

int twice(int x){3 int y;4 y = 2 * x;5 return y;}

7 a = twice(13);8 x = ...39

42FP

SP

40

39

41* does not map to high-level view

Example

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Actual arguments

Return address

Frame pointer

Return value

Caller’srecord

13

8

42

42

int twice(int x){3 int y;4 y = 2 * x;5 return y;}

7 a = twice(13);8 x = ...38

38FP

SP

40

39

38

41

Example

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Actual arguments

Return address

Frame pointer

Local variables

Return value

Callersrecord

13

8

42

??

42

int twice(int x){3 int y;4 y = 2 * x;5 return y;}

7 a = twice(13);8 x = ...37

38FP

SP

40

39

38

37

41

Example

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Actual arguments

Return address

Frame pointer

Local variables

Return value

Callersrecord

13

8

42

26

42

int twice(int x){3 int y;4 y = 2 * x;5 return y;}

7 a = twice(13);8 x = ...37

38FP

SP

40

39

38

37

41

Example

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Actual arguments

Return address

Frame pointer

Local variables

Return value

Callersrecord

13

8

42

26

26

42

int twice(int x){3 int y;4 y = 2 * x;5 return y;}

7 a = twice(13);8 x = ...37

38FP

SP

40

39

38

37

41

Example

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Actual arguments

Return address

Return value

Callersrecord

13

8

26

42

int twice(int x){3 int y;4 y = 2 * x;5 return y;}

7 a = twice(13);8 x = ...39

42FP

SP

40

39

41

Example

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Actual arguments

Return value

Callersrecord

13

26

40

41

42

int twice(int x){3 int y;4 y = 2 * x;5 return y;}

7 a = twice(13);8 x = ...40

42FP

SP

Example

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Return value

Callersrecord

26 41

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int twice(int x){3 int y;4 y = 2 * x;5 return y;}

7 a = twice(13); /*-> 26 */8 x = ...41

42FP

SP

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Calling conventions – responsibility

Calling function• Reserve return value space• Push actual arguments (in

reverse order)• Push the return-to address• Call the function• Pop actual arguments

Called function• Save and re-set FP• Reserve space for local

variables• Assign the return value• Restore FP• Return to caller

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In the caller

• Reserve space for the return value

• Evaluate and push each argument onto the stack

• Call the function and push the return-to address at the same time

• Pop the arguments

DECL size (or PUSH)

BSR address

POP size

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In the callee

• Push FP and set FP to a stack address that is fixed throughout the function’s execution

• Reserve space for all the local variables

• Do computations…

• Restore FP to the value it had when entering this function

• Return to the caller

LINK

DECL size

UNLINK

RTS

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The tasks of the compiler

• Calculate the addresses relative to FP:– Variables– Parameters – Return value– Generate instructions using proper addresses

• Generate code using the offsets

Offset calculation

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Actual arguments

Return address

Frame pointer

Local variables

Return value

Callersrecord

13

8

42

26

26

42

int twice(int x){3 int y;4 y = 2 * x;5 return y;}

7 a = twice(13);8 x = ...37

38FP

SP

What are the offsets for y, x and the return value?

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39

38

37

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A complete exampleRun the code below and assume that

initially, FP = 101 and SP = 100.

int twice(int x) {

int y;

y = 2 * x;

return y;

}

void trac42() {

int a;

a = twice(13);

}29

save FPy&y2x2 * xy = 2 * x&@y@ = yrestore FPreturnrestore FPreturn

save FPa&a@arg. 13call twicepop arg.a = @restore FPreturn

2 [twice] 3 LINK 4 DECL 1 5 LVAL -1(FP) 6 PUSHINT 2 7 RVALINT 2(FP) 8 MULT 9 ASSINT10 LVAL 3(FP)11 RVALINT -1(FP)12 ASSINT13 UNLINK14 RTS15 UNLINK16 RTS17 [trac42]18 LINK19 DECL 120 LVAL -1(FP)21 DECL 122 PUSHINT 1323 BSR 224 POP 125 ASSINT26 UNLINK27 RTS

Exercise (1)Show the stack contents as they are

just before instruction 26 if initially, FP = 201 andSP = 200.

int abs(int x) {if (x < 0)

x = -x;return x;

}

void trac42() {int a;a = abs(-7);

}

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2 [abs] 3 LINK 4 RVALINT 2(FP) 5 PUSHINT 0 6 LTINT 7 BRF 13 8 LVAL 2(FP) 9 RVALINT 2(FP)10 NEG11 ASSINT12 BRA 1313 LVAL 3(FP)14 RVALINT 2(FP)15 ASSINT16 UNLINK17 RTS18 [trac42]19 LINK20 DECL 121 LVAL -1(FP)22 DECL 123 PUSHINT 724 NEG25 BSR 226 POP 127 ASSINT28 UNLINK29 RTS

save FPx0x < 0jump to else&xx-xx = -xjump past else&@x@ = xrestore FPreturn

save FPa&a@7arg. -7call abspop the arg.a = @restore FPreturn

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Exercise (2)

Revert this to Trac42 code (invent names of identifiers as needed).

Hint 1: The WRITEINT instruction does not pop the written value

Hint 2: Do a trace first!

2 [id] 3 LINK 4 RVALINT 2(FP) 5 RVALINT 3(FP) 6 ADD 7 WRITEINT 8 POP 1 9 UNLINK 10 RTS 11 [trac42] 12 LINK 13 DECL 1 14 LVAL -1(FP) 15 PUSHINT 42 16 ASSINT 17 PUSHINT 27 18 RVALINT -1(FP) 19 PUSHINT 99 20 ADD 21 BSR 2 22 POP 1 23 POP 1 24 UNLINK 25 RTS

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Exercise (3)Translate this to Trac42VM code.

void print(string s1, int x, string s2, int y){ write s1; write x; write s2; write y;}

void trac42 (){ print("One: ", 1, "Two:", 2);}

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Parameter passing

• Call by value – push a copy of the value– Used in e.g. C and Trac42

• Call by reference – push a pointer to it– E.g. Java objects and references in C++– Usually combined with call by value

• Other:– Call by name – similar to macros in C

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Local functionsvoid trac42(void) { int x; foo(int y) { if (y == 0) return; x *= 2; foo(y – 1); } x = 1; foo(2);}• How is x found on the stack?• How is y found on the stack, not confusing it with the y of the (recursive)

caller?

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Local functions

Actual arguments

Return address

Frame pointer

Local variables

Return value

Access link

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Conclusion

• An activation record holds information on the stack necessary in function calls

• The frame pointer points to the activation record of the current active function

• Local variables, actual arguments, and the return value are accessed through the frame pointer