08. run-time support

Laszlo Böszörmenyi Compilers

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Compilers

8. Run-time Support


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Run-Time Environment • A compiler needs an abstract model of the run-

time environment of the compiled code • It must generate code for cooperation with it • The run-time environment communicates with the

operating system and maybe with the hardware • Main tasks Storage management Handling of run-time errors

Provide information for symbolic debugging Real Time (RTD) resp. Post Mortem Debugger (PMD)

Hardware extensions Emulated instructions, virtual registers


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Design questions for “imperative” languages

• Are procedures recursive? • Are functions (procedures returning a value) supported? • What should happen with local names (variables) of a

procedure after return? • Can non-local names be referenced? • What kind of parameter passing modes and return-types are

supported? • Can be procedures passed as parameters or returned as a

function value? • Is dynamic memory management desirable? • Is unused memory to be de-allocated explicitly or is a

garbage-collector required? • … further questions for not “procedure-oriented” languages


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Kinds of Storage • Static storage Code (immutable) Static instance (or module) variables

• Semi-dynamic storage (stack) Allocated on procedure activation

and de-allocated on return • Dynamic storage (heap) Storage allocated explicitly at run time

E.g. new in Java, malloc in C De-allocated explicitly

E.g. free in C Or implicitly, e.g. in Java and C#

By a garbage collector

Code Static Data

Heap ↓

free ↑

Stack

Usual subdivison of run-time memory

Heap and stack may be co-managed by a VMM


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Activation Tree and Control Stack • The activation tree describes the flow of control

over the procedure call-chains 1. Each call is represented by a node 2. The root represents the activation of the main

procedure 3. Node a is a parent of b iff the flow of control goes from

a to b (a is calling b) 4. Node a is to the left to b iff a terminates before b

• Control-Stack That set of the active procedures of a call-chain Push at call (activation) Pop at return


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Sketch of Quicksort class sort { …int Arr[11]; // Array to be sorted: Arr[1].. Arr[9] void readarray (); { int i; … } // Reads 9 integers in Arr[i]; 1 ≤ i ≤ 9 // Let assume: -9999 < Arr[i] < 9999 int partition (int m, int n); { … // Partitions Arr[m..n] over a separator value V: } // Arr[m..p-1] < V and Arr[p+1..n] ≥ V; returns p void quicksort (int m, int n); { int i; if (n > m) // As long not sorted (left-right partitions distinct) i = partition(m, n); // Partition quicksort(m, i-1); // Call quicksort recursively on the left quicksort(i+1, n) // and on the right partition } } main () { readarray(); // Initialize Arr Arr[0] := -9999; Arr[10] := 9999; // Sentinel values (accelerate tests) (-∞ and +∞) quicksort(1, 9) // Initial call of quicksort on 9 elements } // class sort


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Activation tree and control stack for quicksort

Activation tree

Control stack at q(2,3)


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Activation record of a procedure • At call: push activation record

1. Reserve place for return values If not returned in a register

2. Push actual parameters 3. Push actual status (registers)

Temporary results of an expression 4. Push return address

PC (program counter) 5. Set access (static) link

Points to surrounding scope 6. Set control (dynamic) link

Points to the caller’s local data 7. Local and temporary variables

Variable-length data: indirection: A pointer to stack or heap

• On return: pop in similar steps

Returned values Actual parameters

… Saved status

Return address

Local variables …

Temp. variables …

Caller

Callee

static link dynamic link

Set by callee push


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Stack of activation records - Example

Code for readarray Code for partition Code for quicksort

Code for main

Arr[0] .. Arr[10] parameter m: 1 parameter n: 9 status … local variable i parameter m: 1 parameter n: 3 status … local variable i parameter m: 2 parameter n: 3 status … local variable i Control stack at q(2,3)


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Scoping • Names are valid in a certain scope • Static scoping

The validity area is defined by the place in program text Nested block can access outer names (via access link)

• Dynamic scoping Validity is defined by actual state of variables Dynamic binding of a variable to a type – e.g. class membership

E.g. ((Student)person).matrNum valid, if person is instance of Student

• Mapping of names of variables to storage areas The compiler generates relative addresses (offsets) Run-time environment maps these to storage address During execution values are assigned to variables Name → Offset → Storage → Value XYZ → 38 → 100038 → 25 (int XYZ:= 25)


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Points to the valid set of q’s locals

Static (lexical scoping) program sort(input, output); n = 0 var Arr : array [1 .. 10] of integer; x: integer; procedure readarray; n = 1 var i: integer; begin … Arr … end {readarray} ; procedure exchange (i, j: integer); n = 1 begin x := Arr[i]; Arr[i] : = Arr[j]; Arr[j] := x end {exchange}; procedure quicksort(m, n: integer); n = 1 var k, v : integer; function partition(y, z: integer): integer; n = 2 var i, j : integer ; begin … k, v, …. exchange(i, j); … end {partition} ; begin ... end {quicksort} ; begin … end {sort}.

n: nesting level

Arr[1] .. Arr[10] x q(1, 9) access link k, v q(1, 3) access link k, v p(1, 3) access link i,j e(1, 3) access link

Which k and v?


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Fast static scoping (Display) • Display d[i] points to the actual

activation record at nesting level i Nesting level known at compilation Fast, but predefined length of d

• At call of a procedure Store d[i] in the activation record

Arr[1] .. Arr[10] x q(1, 9) saved d[1] k, v q(1, 3) saved d[1] k, v p(1, 3) saved d[2] i,j e(1, 3) saved d[1]

Let d[i] point to the new activation record

• Before returning Restore d[i]

d[0] d[1] d[2] null …


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Parameter passing modes • Call by value

1. Value of actual parameter is computed 2. Value is assigned to formal parameter

As an initialized local variable

• Call by reference The address of the actual parameter is passed Assignment to the formal parameter effects the act. par.

• Call by name (textual replacement, as a “macro”) Out-dated mode

• Copy – restore (used in remote procedure calls) Call by value and use locally Before return assign formal par. to act. par.


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Parameter passing - Examples VAR i: INTEGER; a: ARRAY [1 .. 2] of INTEGER; PROCEDURE P (x: INTEGER); (* … P (VAR x: INTEGER);*) BEGIN i := i + 1; x := x + 2; END P; . . . BEGIN a[1] := 10; a[2] := 20; i := 1; P(a[i]) …

Call – by – value: a = (10, 20) Call – by – reference: a = (12, 20) Call – by – name: a = (10, 22) Copy – restore: a = (12, 20) (same as ref. – not always!)

a[1]: 10 a[2]: 20


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Dynamic Storage Allocation • Explicit allocation of variable-length blocks External (global) fragmentation Allocation methods

First-fit (fast), Best-fit, Worst-fit

• Explicit allocation of fixed-length blocks Continuous runs for large areas Internal fragmentation

• Compromise: Buddy algorithm Variable length of blocks is limited to power of 2

• Virtual memory management solves fragmentation Fixed-length blocks, linked together with hardware support


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Garbage Collection • Explicit de-allocation

is error-prone Memory leaks

Memory never released Bad in server code

Dangling references Pointing at released

memory – very bad! Java, C#, Modula-3,

ML, Prolog … use g.c. • Difficulties Careful memory usage Avoid too long pause

12 • •

15 • •

20 • •

• 9

• 7

59 • •

37 • •

• p • q

• r

Tree in use

List unused


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Reachability of Data • Root set Data accessed directly, without a pointer E.g. in Java the static field members + stack The root set is always reachable

• Transitive (recursive) reachability Data that can be reached via the reachable set is

reachable • Set of reachable data changes at Object allocations Parameter passing and return values Reference assignments Procedure returns


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Mark and Sweep • Mark all reachable data

function DFS(x) if x is a pointer pointing to the heap if record x is not marked Mark x for each field fi of record x DFS(x.fi)

• Sweep: release all unmarked data p:= first address in heap while p < last address in heap if record p is marked unmark p else let f1 be the first word of p (is unused, because free!) p.f1:= freeList; freeList:= p; p:= p + (size of record p)


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Price for mark and sweep • Let be

H: Heap size R: Size of all reachable blocks C1,C2: Number of required instructions for mark resp. sweep Cost = (C1*R + C2*H) / (H – R) (usually C1 > C2) If H >> R (which is desirable): Cost ~ C2 If H >> R not true

Try to get memory from the operating system

• Problem of the recursive algorithm Depth of recursion could be H in the worst case! Stack must be rather built explicitly on the heap itself

Traversed record points back to predecessor (pointer reversal) On return, the reversed pointers have to reset (reversed again) Needs only a few additional variables for the management


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Mark and Sweep - Example


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Reference counting • Each memory block has a reference counter If a new reference to the block is set: increment If a reference is deleted: decrement If reference counter == 0, the block is garbage

• Assignments must be tracked p:= q

pprev.refC--; q.refC++ Makes code slow

2 • •

• p

3 • •

• q

ref. counter

With data flow analysis Counter ops can be reduced Complex task in compiler

1 • •

• p

4 • •

• q

• Cycles remain undetected


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Copying collectors • Memory is partitioned into 2 semispaces A and B Memory is allocated in A If end reached

Used blocks are copied into B A is now fully free

Role of A and B swaps after each run • Disadvantage Half of the memory remains unused Addresses must be changed at run-time

Especially bad, if memory address is used as a hash value (e.g. in legacy C code)


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Short-Pause Garbage Collection • Simple gc “stops the world”

Especially bad for real-time applications E.g. during watching a movie

• Partial collection We collect only a little bit Generational garbage collection

Many generations of memory areas Only the oldest generation is collected Very efficient, if not too much cross-generation references exist

• Incremental collection The reachability analysis is broken into small pieces The collector may oversee garbage

But must never collect non-garbage! Typically runs in an own thread in the background

08. run-time support

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