Run-time-omgivningar

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Idag: Run-time-omgivningar

ALSU-07 avsnitt 1.6
ALSU-07 avsnitt 7.1-7.4
(ASU-86 7.1-7.3, 7.6)
En text om skräpsamling: The Very Basics of Garbage Collection.
(KP kapitel 4)

7 Run-Time Environments

What happens when the program is executed. Especially how names in the source program, like a variable called x, is translated to memory locations.

Några grundläggande begrepp

ALSU-07, bl a 1.6 Programming Language Basics och delar av kapitel 7
(ASU-86: 7.1 Source Language Issues)

Procedure (including functions)
Procedure definition
Procedure name
Procedure body
Procedure call (Swedish: proceduranrop)
Formal parameters = formal arguments = formals:
x and y in int f(int x, int y) { ... }
Actual parameters = actual arguments = actuals
3 + 2 and n in f(3 + 2, n)

Activation
Activation record (Swedish: aktiveringspost) = frame
Control stack
Stack pointer, stack top (often kept in a register)
Lifetime
Recursive
Control tree
Scope (Swedish: räckvidd?)
Local / non-local
Data object
Environment (Swedish: omgivning)
Binding (of a name to a storage location)

7.1 Storage organization

ALSU-07 fig. 7.1 (ASU-86 fig. 7.7): Typical subdivision of run-time memory into code and data areas.

The stack is cheaper than the heap.

Nyare: Stacken växer nerifrån, dvs från höga adresser mot lägre.
Växer stacken uppåt eller neråt på din dator? Testa med programmet adresser.c:

#include <stdlib.h>
#include <stdio.h>
#include <limits.h>

int g;

void f(int n) {
  int x;
  printf("x i f(%d) finns på adressen %p = %lu\n",
	 n, (void*)&x, (long unsigned)&x);
  if (n < 3)
    f(n + 1);
}

int main(void) {
  int x;
  void* p;
  printf("En void-pekare är %d bitar.\n",
	 sizeof(void*) * CHAR_BIT);
  printf("x i main finns på adressen %p = %lu\n",
	 (void*)&x, (long unsigned)&x);
  f(1);
  printf("Den globala variabeln g finns på adressen %p = %lu\n",
	 (void*)&g, (long unsigned)&g);
  p = malloc(100);
  printf("Den malloc-allokerade arean p finns på adressen %p = %lu\n",
	 (void*)p, (long unsigned)p);
  printf("Funktionen f finns på adressen %p = %lu\n",
	 (void*)f, (long unsigned)f);
  return 0;
}

Resultat på en dator (en PC med Intel x86-processor):

En void-pekare är 32 bitar.
x i main finns på adressen 0xbf9d1c14 = 3214744596
x i f(1) finns på adressen 0xbf9d1bf4 = 3214744564
x i f(2) finns på adressen 0xbf9d1bd4 = 3214744532
x i f(3) finns på adressen 0xbf9d1bb4 = 3214744500
Den globala variabeln g finns på adressen 0x8049838 = 134518840
Den malloc-allokerade arean p finns på adressen 0x804a008 = 134520840
Funktionen f finns på adressen 0x80483c4 = 134513604

Resultat på en annan dator (en Sun med Sparc-processor):

En void-pekare är 32 bitar.
x i main finns på adressen ffbff964 = 4290771300
x i f(1) finns på adressen ffbff8ec = 4290771180
x i f(2) finns på adressen ffbff874 = 4290771060
x i f(3) finns på adressen ffbff7fc = 4290770940
Den globala variabeln g finns på adressen 20b4c = 133964
Den malloc-allokerade arean p finns på adressen 20b60 = 133984
Funktionen f finns på adressen 106cc = 67276

Activation records

ALSU-07 fig. 7.5 (ASU-86 fig. 7.8): A general activation record.

Or, use registers for actual parameters and for the returned value.

Compile-time layout of local data

All this is determined at compile-time. Each variable, such as the local variable i, has a fixed offset in the activation record. Fast to access, if a stack top pointer is kept in a register. The compiler generates code to access i as (e. g.) SP - 24. Tre sätt att allokera data (= bestämma var variablerna ska finnas i minnet):

Static allocation
Stack allocation (a form of dynamic allocation)
Heap allocation (a form of dynamic allocation)

Static allocation

The compiler can decide the size and address of each data object. If activation records are static => recursion difficult (i. e., manage your own stack in a static data area). Example: FORTRAN.

(But don't learn the FORTRAN details in this chapter for the exam!)

7.2 Stack Allocation

ALSU-07 fig. 7.6 (ASU-86 fig. 7.13): Downwards-growing stack allocation of activation records

Call sequence (Sw: anropskonventioner), return sequence

What should the caller (= the callING procedure) do, and what should the callee (=the callED procedure) do?
E. g., allocated the activation record, save registers.
Various call sequences exist.
In general, let the callED do as much as possible. (Just code generated for that once, instead of for each call to it!)

ALSU-07 fig. 7.7 (ASU-86 fig. 7.14): Division of tasks between caller and callee.

Example:

The callER evaluates actuals. (And stores them?)
The callER stores a return address and the old SP in the new activation record, and increments SP.
The callED procedure saves registers and other status information.
The callED procedure initializes local data + begins to execute.

The other way around when returning.

Variable-length data

Can be stored at the end of the activation record, with just pointers to it among the local variables.

Dangling references

Possible with malloc + bad free, but also with stack allocation. Example:

char* getstr() {
  char s[100];
  fgets(s, sizeof s, stdin);
  return s;
}

Heap allocation

malloc + free (or new + delete)

But also: activation records, if they have to be retained!

7.3 Access to non-local data on the stack

Lexical scope = static scope
Dynamic scope

1.6.6 Parameter passing

Can be skipped, but you may want to learn the difference between:

Call by value
Call by reference
Copy-restore
Call by name
Macro expansion

(ASU-86 7.6) Symboltabeller

Very simple, from ASU-86 chapter 2:

struct entry {
  char *lexptr; 
  int  token;    
};

#define SYMMAX 100
struct entry symtable[SYMMAX];
int lastentry = 0;

Additionally:

Type information
Size (in memory)
Location (= where in memory) (unless we are generating assembly language)
Several entries with the same lexeme!
(Example: many procedures can have a local variable named i.)
= scope!

"Built during analysis, used during synthesis,"

We make a difference between:

identifer (such as x)
name (such as the name x of this variable, and the name x of that procedure)

Operations to handle scope in a block-structured language:

begin_scope()
find_name(x) - in the current block, and then the next outside, etc
add_name(x)
end_scope()

We can use a stack! (Push each new scope.)
But hash tables are much more efficient.

7.4 Heap management

Om hur minnet på heapen ("den allmänna, dynamiska allolkeringsplatsen i processens adressrymd") hanteras, med allokering och avallokering. Datorns minneshierarki. Lokalitet. Fragmentering.

7.5-7.8 Skräpsamling

Läs i mån av intresse. Kursinnehållet finns i The Very Basics of Garbage Collection.

Skräpsamling (eng: garbage collection)

The Very Basics of Garbage Collection

Några viktiga begrepp:

Dynamically allocated memory (C: malloc, C++: new)
Manual memory management (C: free, C++: delete)
Garbage
Why is manual memory management hard?
Automatic garbage collection
Live data / dead data
Reachable data / unreachable data
Root set
Mark and sweep: mark bit, marking phase, sweep phase
Reference counting
Generational collection
Incremental collection
Conservative collection. For C and C++: Boehm (http://www.hpl.hp.com/personal/Hans_Boehm/gc/)

Old program (grows and crashes):

#include <stdlib.h>

int main() {
  while (1)
    malloc(100);
  return 0;
}

New program (works Ok):

#include <stdlib.h>
#include "gc.h"

int main() {
  while (1)
    GC_malloc(100);
  return 0;
}

C++: Several ways. One is to inherit from class gc:

class A: public gc {...};
A* a = new A;       // a is collectable.

Kursen Kompilatorer och interpretatorer | Föreläsningar: 1 2 3 4 5 6 7 8 9 10 11 12

Thomas Padron-McCarthy (Thomas.Padron-McCarthy@tech.oru.se) 20 september 2007