- Imperative Language Constructs - Lecture Compilers SS 2009
Dr.-Ing. Ina Schaefer
Software Technology Group TU Kaiserslautern
Ina Schaefer Translation to Target Language 1
Content of Lecture
1. Introduction: Overview and Motivation 2. Syntax- and Type Analysis
2.1 Lexical Analysis
2.2 Context-Free Syntax Analysis
2.3 Context-Dependent Syntax Analysis 3. Translation to Target Language
3.1 Translation of Imperative Language Constructs 3.2 Translation of Object-Oriented Language Constructs 4. Selected Aspects of Compilers
4.1 Intermediate Languages 4.2 Optimization
4.3 Command Selection 4.4 Register Allocation 4.5 Code Generation 5. Garbage Collection
6. XML Processing (DOM, SAX, XSLT)
Outline
1. Language Constructs of Procedural Languages 2. Assembly and Machine Languages
3. Translation of Variables and Data Types 4. Translation of Expressions
5. Translation of Statements
6. Translation of Procedures and Local Objects
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Translation to Target Language
Focus:
• Differences between source languages and target languages/target machines
• Most important translation techniques for different programing paradigms (procedural/object-oriented)
Educational Objectives:
• Overview of imperative and procedural language constructs
• Typical language constructs of assembler languages
• Translation techniques for procedural language constructs
• Translation of object-oriented language constructs
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Translation of Procedural Languages
• Language constructs of procedural programing languages
• Language constructs of assembly languages
• Translation of variables and data types
• Translation of expressions
• Translation of statements (control structures)
• Translation of procedures
Language Constructs of Procedural Languages
Procedural Languages
From a conceptional and semantical view point, procedural languages have the following constructs:
• Domains with operations (often typed)
! pre-defined: int, boolean, ...
! user-defined: records, classes, ...
! implicitly defined: field types, address types, function types
• Variables
! simple and compound types
! global, local, statically/dynamically allocated
! define memory state
• Expressions
! computation of values with implicit intermediate results
! possibly in combination with execution control and state modification
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Language Constructs of Procedural Languages
Procedural Languages (2)
• Statements
! simple and combined statements
! define execution control and state modification
• Procedures
! abstraction of parametrized statements
! may be recursive
! may be nested
Modules usually do not have a semantic meaning and are only
relevant for translation in name analysis and for binding and loading.
Nested Procedures
Example from [Wilhelm, Maurer; Fig. 2.9]
Übersetzung geschachtelter Prozeduren
Geschachtelte/lokale Prozeduren werden z.B.
von Pascal und Ada unterstützt
Beispiel: (geschachtelte Prozeduren)
von Pascal und Ada unterstützt.
proc P(a) var b
Abb. 2.9)
var b var c proc Q
var a proc R
elm/Maurer,
var b begin ... b ...
... a ...
c
mt aus Wilhe
... c ...
end begin ... a ...
... b ...
spiel stamm
... call Q ...
end proc S
var a begin
(das Beis
begin ... a ...
... call Q ...
end begin
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... a ...
... call Q ...
end
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Assembly and Machine Languages
Assembly and Machine Languages
Assembly languages have the following language constructs:
• Finite sequences of bits of various length: byte, word, halfword, ...
• Global memory
! register, flags (addressing by name)
! indexed, mostly word addressed main memory
• Instructions
! load, store
! arithmetic and boolean operations
! execution control (jumps, procedures)
! simple, not combined statements
! possibly complex addressing of operands
• Initialization instructions
Assembly and Machine Languages
The MIPS Assembler
MIPS - Microprocessor without interlocked pipeline stages
• RISC Architecture, originally 32 bit (since 1991 64bit)
• developed by John Hennessy (Stanford) starting 1981
• MARS Simulator
http://courses.missouristate.edu/KenVollmar/MARS/
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Assembly and Machine Languages
MIPS Architecture
• Arithmetic-Logic Unit (ALU)
• Floating-Point Unit (FPU)
• 32 Registers (inkl. stack pointer, frame pointer, global pointer, return address)
• Main memory, 230 memory words (4 byte)
• 5-stage pipeline
MIPS Commands
• Arithemic
add add $s1, $s2, $s3 $s1 = $s2 + $s3 subtract sub $s1, $s2, $s3 $s1 = $s2 - $s3 add immediate addi $s1, $s2, c $s1 = $s2 + c multiply mul $s1, $s2, $s3 $s1 = $s2 * $s3
(lower 32 bits in $s1)
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Assembly and Machine Languages
MIPS Commands (2)
• Data Transfer
load word lw $s1, c($s2) $s1 = Memory[$s2 + c]
store word sw $s1, c($s2) Memory[$s2 + c] = $s1 load immediate li, $s1, c $s1 = c
load half lh $s1, c($s2) $s1 = Memory[$s2 + c]
store half sh $s1, c($s2) Memory[$s2 + c] = $s1 load byte lb $s1, c($s2) $s1 = Memory[$s2 + c]
store byte sb $s1, c($s2) Memory[$s2 + c] = $s1
Assembly and Machine Languages
MIPS Commands (3)
• Logical
and and $s1, $s2, $s3 $s1 = $s2 & $s3 or or $s1, $s2, $s3 $s1 = $s2 | $s3 nor nor $s1, $s2, $s3 $s1 = ¬ ( $s2 | $s3 ) and immediate andi $s1, $s2, c $s1 = $s2 & c
or immediate ori $s1, $s2, c $s1 = $s2 | c shift left logical sll $s1, $s2, c $s1 = $s2 « c shift right logical sll $s1, $s2, c $s1 = $s2 » c
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Assembly and Machine Languages
MIPS Commands (4)
• Conditional Branches
branch on equal beq $s1, $s2, label if ($s1 == $s2) goto label branch on not equal bne $s1, $s2, label if ($s1 != $s2)
goto label set on less than slt $s1, $s2, $s3 if ($s2< $s3)
$s1 := 1 else $s1 := 0 set o.l.t. immediate slti $s1, $s2, c if ($s2< c)
$s1 := 1 else $s1 := 0
• Unconditional Branches
jump j label goto label jump register jr $ra goto $ra
jump and link jal label $ra = PC + 4; goto label
Adressing in MIPS
• Immediate: Operand is a constant, e.g. 25
• Register: Operand is a register, e.g. $s2
• Base or Displacement Addressing: Operand is a memory location whose address is the sum of the register and a constant, e.g. 8($sp)
• PC relative: Address is the sum of PC and a constant
• Pseudodirect Addressing: Jump address is the 26 bit of the instruction with the upper bits of the PC
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Assembly and Machine Languages
MIPS Registers
• $zero: the constant 0
• $at: assembler temporary
• $v0, $v1: values for function results and expression evaluation
• $a0 - $a3: arguments
• $t0 - $t9: temporaries
• $s0 - $s7: saved temporaries
• $k0, $k1: reserved for OS kernel
• $gp: global pointer
• $sp: stack pointer
• $fp: frame pointer
• $ra: return address
Assembly and Machine Languages
Syscalls for MARS/SPIM Simulators
How to use System Calls:
• load service number into register $v0
• load argument values, if any into $a0, $a1, $a2
• issue call instruction syscall
• retrieve return values, if any Example:
li $v0, 1 # print integer
add $a0, $t0, $zero # load value into $a0 syscall
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Assembly and Machine Languages
List of System Services
Service Code in $v0 Arguments
print integer 1 $a0 = integer to print
print string 4 $a0 = address of
null-terminated string to print exit (terminate execution) 10
print character 11 $a0 = character to print
exit2 (terminate with value) 17 $a0 = termination result
Example: Translation to MIPS
The example illustrates the MIPS assembler and typical translation tasks.
Code quality is not considered.
Source Code in C:
Das Beispiel soll zum einen die MI-Assemblersprachep p demonstrieren, zum anderen aber auch Übersetzungs- probleme veranschaulichen. Auf die Qualität des Zielprogramms wurde kein Wert gelegt.
char a[3], b[3];
int i;
char res;
Quellprogramm in C:
char res;
void main() { i:= 2;
res := 1;
res : 1;
while( -1 < i ) { if( res ) {
res = (a[i]==b[i]);
res (a[i] b[i]);
i = i-1;
} else {
i = i-1;
} } } }
Den Prozeduraufruf von main vernachlässigen
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Den Prozeduraufruf von main vernachlässigen wir bei diesem einführenden Beispiel.
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Assembly and Machine Languages
MIPS Program
# sp + 0 : 1
# sp + 4 : res
# sp + 5 : base address of a
# sp + 8 : base address of b
addi $sp, $sp, -12 # make space for the variables li $t1, 2
sw $t1, 0($sp) # set i to 2 li $t1, 1
sb $t1, 4($sp) # set res at sp +4 loop:
lw $t2, 0($sp) # load i into $t2 li $t3, -1 # load -1 into $t3 slt $t0, $t3, $t2 # $t3 < $t2 ?
beq $t0, $zero, exit # if i < -1 goto exit lw $t1, 4($sp) # load res from stack
Assembly and Machine Languages
MIPS Program (2)
add $t4, $sp, 5 # base address of array a add $t4, $t4, $t2 # add offset/ array index lb $t0, 0($t4) # load a[i]
add $t4, $sp, 8 # base address of array b add $t4, $t4, $t2 # add offset/ array index lb $t1, 0($t4) # load b[i]
beq $t0, $t1, equal # if a[i] == b[i]
sb $zero, 4($sp) # set res to 0
j after
equal:
add $t3, $zero, 1
sb $t3, 4($sp) # set res after:
subi $t2, $t2, 1 # i = i-1
sw $t2, 0($sp) # store i to $sp +4
j afterif # goto end of if statement
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Assembly and Machine Languages
MIPS Program (3)
elseif:
subi $t2, $t2, 1 # i = i-1
sw $t2, 0($sp) # store i to $sp +4 afterif:
j loop # return to loop
exit:
addi $sp, $sp, 12 # reset stack pointer li $a0, 1 # terminated successfully li $v0, 17
syscall
Translation to MIPS
Remarks:
The example illustrates typical translation tasks:
• Translation of data types, memory management, addressing
• Translation of Expressions, Management of intermediate results, mapping of operations of the source language to operations of the target language
• Translation of statements by implementation with jumps
• Bad code quality with simple systematic approach
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Assembly and Machine Languages
Translation Process
Concrete Syntax
SL
Concrete Syntax
MIPS
AST SL AST
MIPS Lexical and
Context-Free Analysis
Context- Dependent
Analysis
Translator Code Generator
Assembly and Machine Languages
MIPS Abstract Syntax
Prog * Code Code =
ADD (reg reg reg) | ADDI (reg reg const) | SUB (reg reg reg) | MUL (reg, reg, reg) |
AND (reg reg reg) | OR (reg reg reg) | NOR (reg reg reg) | ANDI (reg reg const) | ORI (reg reg const) |
SLL (reg reg const) | SRL (reg reg const) | BEQ (reg reg label) | BNE (reg reg label) | SLT (reg reg reg) | SLTI (reg reg const) | JUMP (label) | JR (reg) | JAL (label) |
STORE (reg const reg) | LOAD (reg const reg) | LOADI (reg const) | STOREB (reg const reg) | LOADB (reg const reg) |
LABEL (name)
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Translation of Variables and Data Types
Translation of Variables and Data Types
Compiler
Programing Language
Assembly Language
named variables complex types
addresses of memory regions index and offset computation
Translation of Variables and Data Types (2)
The translation of variables and data types comprises:
• handling of basis data types
• conversion of data types (e.g. int → float)
• memory organisation
• translation of arrays
• translation of records and classes
• implementation of dynamic objects
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Translation of Variables and Data Types
Basis Data Types
Often, there is good support of basis data types of source language on the target machine:
• int, long → 4 byte word with integer arithmetic
• float, double → accordingly
Potentially, data types have to be encoded:
• boolean → 1 byte or 4 byte words
Problem: If target machine does not comply to requirements of source language, e.g.
• floating point arithmetic is not handled according to IEEE standard
• overflows are not dealt with correctly (cmp. Java FP-strict expressions)
• operations for conversion are missing on target machine
Translation of Variables and Data Types
Memory Layout
The conceptional memory layout of most imperative programing languages and target machines is similar. (Details depend on OS and machine)
dynamic variables, objects, ...
intermediate results, procedure-local values, objects with
restricted scope
OS kernel
global values
low addresses
high addresses
global, static variables, constants, ...
heap
stack
program
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Translation of Variables and Data Types
Translation of Arrays
Efficient translation of Arrays is important for many tasks.
One-dimensional static arrays
• Allocate memory in the region for global values (starting at $gp)
• Address computation with base address of array, index of array element and size of element type
Consider the array declaration T tarray[57]:
• $gp contains the base adress for the global memory region
• Let Rrel contain the relative address of the array tarray in the global memory region
• Let Ri contain the index i of the array component
If k = sizeof(T), then the address of tarray[i] is $gp+ Rrel + k ∗Ri.
Translation of Arrays (2)
Computation in MIPS LI $ti, k
MUL $ti, Ri, $ti ADD $ti, R_rel, $ti ADD $ti, $gp, $ti LOAD $ti, 0, $ti
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Translation of Variables and Data Types
More Translation of Arrays
Multi-dimensional static arrays
Consider as example the Pascal declaration var a:array[-5..5][1..9] of integer;
which corresponds to 99 integer variables:
a[-5, 1] ... a[-5,9]
...
a[5,1] ... a[5,9]
Matrix is stored in rows in memory. Storing rows is more efficient than storing columns as second index is often incremented in inner loops.
Translation of Variables and Data Types
Further Translation of Arrays(2)
Translation of Access to a[E1,E2]:
Assume results of evaluating E1 and E2 are stored in $t1 and $t2.
As ais a static array, we know the dimensions at compile time.
a[$t1,$t2] is the r-th component of a linear array with r = ($t1 −(−5)∗ ((9− 1) +1) + ($t2− 1)
= 9 ∗$t1 +45+ $t2− 1
= 9 ∗$t1 + $t2+44
Result: Store the address of the 44-th component as base address of the array in symbol table. Then it suffices to add 9∗$t1+ $t2 to base address.
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Translation of Variables and Data Types
Further Translation of Arrays(2)
Code example for Access to a[E1,E2]:
[Code for E1 -> $t1]
[Code for E2 -> $t2]
LOADI ($t3, 9)
MUL ($t1, $t1, $t3) ADD ($t1, $t1, $t2) LOADI ($t2, 4)
MUL ($t1, $t1, $t2) ADDI ($t1, $t1, relA) ADD ($t1, $t1, $gp) LOAD ($t1, 0, $t1)
where relA = offset(a) + 44
General Translation of Arrays
General Array Declaration of Dimension k
var a: array [u1..o1], ...., [uk..uk] of T;
Storing Rows yields the following adress for accessing a[R1, ..., Rk]:
r = (R1 −u1)∗size(array[u2..o2, ...,uk..ok]of T) + (R2 −u2)∗size(array[u3..o3, ...,uk..ok]of T)
+ . . .
+ (Rk − uk)∗ size(T)
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Translation of Variables and Data Types
General Translation of Arrays (2)
For i = 1, . . . ,k −1, it holds that
size(i) := size(array[u{i +1}..o{i +1}, ...,uk..ok]of T) size(k) = size(t)
This implies
size(i −1) = size(i)∗(oi − ui +1) Simplification yields:
r =
!k i=1
Ri ∗size(i)−
!k i=1
ui ∗size(i)
At runtime, only first summand has to be computed for which code has to be generated.
Translation of Variables and Data Types
Code Generation for Array Access
Abstract Syntax of Source Language:
Einfache Codeerzeugung für Feldzugriff:
Beispiel:
ArrayAccess ( UsedId uid, IndexExps ies ) UsedId ( Ident id )
IndexExps = IndexExpElem | IndexExp
IndexExpElem ( IndexExp ie, IndexExps ies )p ( p , p ) IndexExp ( ... )
Symboltabelle
Register, in dem Ergebnis steht ( Reg(Ri) ) Adressierung des Feldelements
Code für den Unterbaum
Liste der Größen zu jeder Felddimension
Relativadresse zur Adressierung eines Feldes a:
relA = offset(a) - !"kui * size(i)
I=1
lkupRA: Ident x SymTab ! Adresse lk SZL Id t S T b ! I tLi t
I=1
lkupSZL: Ident x SymTab ! IntList
Zur Konkatenation von Codelisten benutzen wir “+“, die Erzeugung einer einelementigen Liste aus einem
El t h ib i l [ ]
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Element e schreiben wir als [e] .
Ina Schaefer Translation to Target Language 39
Translation of Variables and Data Types
Code Generation for Array Access (2)
Attribution:
Einfache Codeerzeugung für Feldzugriff:
Beispiel:
ArrayAccess ( UsedId uid, IndexExps ies ) UsedId ( Ident id )
IndexExps = IndexExpElem | IndexExp
IndexExpElem ( IndexExp ie, IndexExps ies )p ( p , p ) IndexExp ( ... )
Symboltabelle
Register, in dem Ergebnis steht ( Reg(Ri) ) Adressierung des Feldelements
Code für den Unterbaum
Liste der Größen zu jeder Felddimension
Relativadresse zur Adressierung eines Feldes a:
relA = offset(a) - !"
kui * size(i)
I=1
lkupRA: Ident x SymTab ! Adresse lk SZL Id t S T b ! I tLi t
I=1
lkupSZL: Ident x SymTab ! IntList
Zur Konkatenation von Codelisten benutzen wir “+“, die Erzeugung einer einelementigen Liste aus einem
El t h ib i l [ ]
Element e schreiben wir als [e] .
Symbol Table
Result Register Ri
Address of Array Element Code for Subtree
List of Sizes for each Array Dimension Relative Address for Array a
Ina Schaefer Translation to Target Language 40
Code Generation for Array Access (3)
Operations for Attribution:
• lkupRA: Ident × SymTab → Address
• lkupSZL: Ident × SymTab → IntList
• + : List Concatenation, for an element e, [e] is the list containing only e.
In the following, the SymTab attribute is only explicitly given where it is required.
Ina Schaefer Translation to Target Language 41
Translation of Variables and Data Types
Code Generation for Array Access (4)
Das Symboltabellenattribut ist nur angegeben, wo es gebraucht wird. R0 enthält die Basisadresse des Speicherbereichs, in dem das Feld gespeichert ist.
ArrayAccess
UsedId IndexExps
Bdispx(Reg(R0),_,_)
UsedId IndexExps
lkupRA(_,_) lkupSZL(_,_)
IndexExpElem Ident
IndexExpElem
_ + rest(_) first(_)
_ +
[ Mult2(W,Imm(_),_) ] + [ Add2(W,_,_) ]
IndexExps IndexExp IndexExp
ADD(Ri,Ri, $gp) ADD(Ri, Ri,RA)
RA Ri
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Translation of Variables and Data Types
Code Generation for Array Access (5)
Um die Attributierungsbilder übersichtlicher zu gestalten, können Bezeichner für Attributwerte benutzt werden:
IndexExpElem rest(_) first(_)
CL + CR +
[ Mult2(W,Imm(_),RL) ] + [ Add2(W,RL,RR) ]
IndexExps
IndexExp RL CL RR CR
Zur Laufzeit braucht wieder nur der erste Summand berechnet werden. Dafür muss also Code generiert werden. Bei der schrittweisen Berechung kann auch eine Bereichsprüfung für das Feld vorgenommen werden.
Bemerkungen:
• Bei der Berechnung von Feldindizes gibt es häufig eine großes Potential für Optimierungen.
• Für die Übersetzung dynamischer Felder muss die Adressierung geeignet verallgemeinert werden
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die Adressierung geeignet verallgemeinert werden.
(siehe z.B. Wilhelm/Maurer, Abschnitt 2.6.2).
CL + CR +
[LOADI (RT, FI)] + [MUL (RL, RL, RT) ] + [ADD (RR, RR, RL) ]
FI
During stepwise computation also array bounds can be checked.
Ina Schaefer Translation to Target Language 43
Translation of Variables and Data Types
Array Access
Remarks:
• Computation of array indices offers great potential for optimizations.
• For translation of dynamic arrays, addressing has to be generalized appropriately. (cf. Wilhelm/Maurer, Sect. 2.6.2)
Translation of Records
Translation of Records is similar to translation of arrays:
• Determine size and memory layout
• Compute adresses for selection of record components and pointer dereferencing
• Translation of record operations, e.g. assignments to record components
Recommended Reading: Wilhelm, Maurer, Section 2.6.2
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Translation of Variables and Data Types
Implementation of Dynamic Objects
Dynamic objects = dynamically allocated variables and objects in sense of OO programing
Dynamic objects are stored on the heap:
• number of dynamic objects is not know at compile time, objects are created at runtime
• dynamic objects have a designated lifetime which disallows handling with stack
Memory representation and addressing of components is similar to static records.
Translation of Variables and Data Types
Implementation of Dynamic Objects (2)
Example:
Implementierung dynamischer Objekte
Dynamische Objekte werden hier als Sammelbegriff für Dynamische Objekte werden hier als Sammelbegriff für dynamisch allozierte Variable und Objekte im Sinne der OO-Programmierung verwendet.
Dynamische Objekte werden auf der Halde verwaltet:
Dynamische Objekte werden auf der Halde verwaltet:
• Ihre Anzahl ist im Allg. zur Übersetzungszeit nicht
bekannt. Deshalb werden sie erst zur Laufzeit erzeugt.
• Sie haben eine Lebensdauer die eine kellerartigeSie haben eine Lebensdauer, die eine kellerartige Behandlung im Allg. nicht zulässt.
Beispiel: (dynamische Objekte) Beispiel: (dynamische Objekte)
typedef struct listelem { int head;
struct listelem* tail; }* list;
# define listelemSIZE sizeof(struct listelem{
int h; struct listelem* t;}) list append( int i list l ) {
list append( int i, list l ) {
list lvar = (list) calloc(1,listelemSIZE);
lvar->head = i;
lvar->tail = l;
return lvar;
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} ...
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Translation of Variables and Data Types
Dynamic Memory Management
Dynamic memory management
• is handled by runtime environment
• can be supported by compiler
• can partially be handled by user program
Runtime environment provides operations for dynamic memory management:
• for the programmer, e.g. in C malloc, calloc, realloc, free
• for the compiler as in Pascal, Java, Ada
• no memory deallocation by programer possible, but garbage collection by runtime environment e.g. in Java
Dynamic Memory Management (2)
General Problem: Provide memory blocks of different sizes from a linear memory and reuse memory after it has been freed
Simple memory management by linear list of free memory areas Structure of free memory area of variable length:
user data size
header
free used
used free used
freelist
Ina Schaefer Translation to Target Language 49
Translation of Variables and Data Types
Dynamic Memory Management (3)
List of free memory areas:
user data size
header
free used
used free used
freelist
Procedure to allocate and deallocate memory:
• Allocate memory
! Search memory area B of appropriate size
! Update references:
• If area has exactly required size, remove it from list.
• Else update header of area, create header for rest of free memory and add this area instead of the old area to list.
Translation of Variables and Data Types
Dynamic Memory Management (4)
! Return pointer to memory cell after header (size information has to be kept.)
! If no memory area of required size is found, new memory has to be requested from the OS
• Free memory
! Find header for memory area to be freed by pointer to this area
! If previous or next memory areas are free, join the areas
! Add resulting memory area to list
Ina Schaefer Translation to Target Language 51
Translation of Variables and Data Types
Dynamic Memory Management (5)
Remarks:
• If program writes over assigned memory area, references or size information can be destroyed with bad consequences.
• If memory cannot be allocated in bytes, alignment restrictions have to be obeyed.
• For practical use the above principle can be improved by
! non linear search
! search for exact memory areas, avoiding defragmentation
! support for joining memory areas after deallocation
Translation of Expressions
Difficulties for translation of expressions
• Management of intermediate results on stack or in registers
• Translation of source language operations
! no counterpart in target language
! addressing
! context-dependent (Boolean expression as condition is handled differently as Boolean expression in an assignment.)
Ina Schaefer Translation to Target Language 53
Translation of Expressions
Translation of Expressions (2)
Abstract Syntax of Expressions:
Hier demonstrieren wir die generellen Probleme anhand eines kleinen Beispiels, das die direkte Übersetzung von Ausdrücken demonstriert.
Fortgeschrittene Techniken werden in Kapitel 3 behandelt.
B i i l ( i f h A d k üb t ) Beispiel: (einfache Ausdrucksübersetzung)
Wir betrachten die Ausdruckssyntax aus dem MI-Übersetzungsbeispiel in Abschnitt 3.1.2:
Exp = ArtihmExp | Relation | IntConst
| CharConst | ArrayAccess | Var ArithmExp = Add | Sub
Add, Sub ( Exp left, Exp right ) Relation = Lt | Eq
Relation Lt | Eq
Lt, Eq ( Exp left, Exp right ) IntConst ( Int i )
CharConst ( Char c )
ArrayAccess ( UsedId uid, Exp e ) i
Var ( UsedId uid ) UsedId ( Ident id )
Wir treffen folgende Entwurfsentscheidungen:
Zwischenergebnisse werden auf dem Keller verwaltet
• Zwischenergebnisse werden auf dem Keller verwaltet.
• Vergleiche werden durch Sprünge implementiert:
- Subtrahiere die beiden Werte auf dem Keller.
- In Abhängigkeit des Ergebnisses springe einen In Abhängigkeit des Ergebnisses springe einen Befehl an der 1 kellert bzw. der 0 kellert.
Dazu sind entsprechende Marken zu generieren.
Ina Schaefer Translation to Target Language 54
Translation of Expressions
Translation of Expressions (3)
Design Decisions:
• Intermediate results are stored on stack.
• Comparisons are implemented by jumps:
! compare values on stack
! dependent on result, jump to command pushing 1 or pushing 0
! generate associated labels
Ina Schaefer Translation to Target Language 55
Translation of Expressions
Translation of Expressions (4)
Attribution:
Attributdeklarationen:
Relativadresse einer Variable oder eines Feldes Typ eines Ausdrucks ( int, char, int[ ], char[ ] ) Code für den Unterbaum vom Typ CodeList
eindeutige Marke für Ausdruck vom Typ String
Attributierung für das Code-Attribut:
Add
CL + CR +
[ Add2(W Postinc(SP) Regdef(SP) ]
tt but e u g ü das Code tt but
Exp
[ Add2(W,Postinc(SP),Regdef(SP) ]
CL CR
Exp
Lt
CL + CR + M
[ Sub2( W, Postinc(SP), Regdef(SP) ] + [ Jlt( Label( “PUSH1_“ + M ) ) ] + [ Move( W, Imm(0), Regdef(SP) ) ] + [ Jump( Label( “ENDREL_“ + M )) ] + [ Label( “PUSH1 “ + M ) ] +
Exp
[ Label( PUSH1_ + M ) ] + [ Move( W, Imm(1), Regdef(SP) ) ] + [ Label( “ENDREL_“ + M ) ]
CL CR
Exp
Exp Exp
( Die Attributierungen für Sub und Eq sind entsprechend. ) Relative Address of Variable or Array
Type of Expression (int, char, int[], char[]) Code for Subtree of Type CodeList
Unique Label for Expression of Type String
Ina Schaefer Translation to Target Language 56
Translation of Expressions
Translation of Expressions (5)
Relativadresse einer Variable oder eines Feldes Typ eines Ausdrucks ( int, char, int[ ], char[ ] ) Code für den Unterbaum vom Typ CodeList
eindeutige Marke für Ausdruck vom Typ String
Attributierung für das Code-Attribut:
Add
CL + CR +
[ Add2(W Postinc(SP) Regdef(SP) ]
tt but e u g ü das Code tt but
Exp
[ Add2(W,Postinc(SP),Regdef(SP) ]
CL CR
Exp
Lt
CL + CR + M
[ Sub2( W, Postinc(SP), Regdef(SP) ] + [ Jlt( Label( “PUSH1_“ + M ) ) ] + [ Move( W, Imm(0), Regdef(SP) ) ] + [ Jump( Label( “ENDREL_“ + M )) ] + [ Label( “PUSH1 “ + M ) ] +
Exp
[ Label( PUSH1_ + M ) ] + [ Move( W, Imm(1), Regdef(SP) ) ] + [ Label( “ENDREL_“ + M ) ]
CL CR
Exp
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Exp Exp
( Die Attributierungen für Sub und Eq sind entsprechend. ) CL +
CR +
[LOAD (R2, 0, $sp) ADD ($sp, $sp, 4) LOAD (R1, 0, $sp) ADD (R1, R1, R2) STORE (R1, 0, $sp)]
Ina Schaefer Translation to Target Language 57
Translation of Expressions
Translation of Expressions (6)
Attributdeklarationen:
Relativadresse einer Variable oder eines Feldes Typ eines Ausdrucks ( int, char, int[ ], char[ ] ) Code für den Unterbaum vom Typ CodeList eindeutige Marke für Ausdruck vom Typ String Attributierung für das Code-Attribut:
Add
CL + CR +
[ Add2(W Postinc(SP) Regdef(SP) ] tt but e u g ü das Code tt but
Exp
[ Add2(W,Postinc(SP),Regdef(SP) ]
CL CR
Exp
Lt
CL + CR + M
[ Sub2( W, Postinc(SP), Regdef(SP) ] + [ Jlt( Label( “PUSH1_“ + M ) ) ] + [ Move( W, Imm(0), Regdef(SP) ) ] + [ Jump( Label( “ENDREL_“ + M )) ] + [ Label( “PUSH1 “ + M ) ] +
Exp
[ Label( PUSH1_ + M ) ] + [ Move( W, Imm(1), Regdef(SP) ) ] + [ Label( “ENDREL_“ + M ) ]
CL CR
Exp
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Exp Exp
( Die Attributierungen für Sub und Eq sind entsprechend. ) CL + CR +
[LOAD (R2, 0, $sp) ADD($sp, $sp, 4) LOAD (R1, 0, $sp) SLT (R1, R1, R2)
BEQ (R1, $zero, “PUSH_0_”+M) LOADI (R1, 1)
STORE (R1, 0, $sp) JUMP (“ENDREL_”+M) LABEL(“PUSH_0_”+M) LOADI (R1, 0)
STORE (R1, 0, $sp) LABEL (“ENDREL_”+M)]
Translation of Expressions
Translation of Expressions (7)
IntConst
[ Move( W, Imm( ), Predec(SP) ] [ Move( W, Imm(_), Predec(SP) ] Int
Var TV
if TV = int then
[ Move( W, Bdisp(Reg(R0), RA), Predec(SP) ] else // TV = char
else // TV char
[ Conv( Bdisp(Reg(R0), RA), Predec(SP) ] UsedId RA
ArrayAccess TV
ArrayAccess
CR + [ Move( W, Regdef(SP), Reg(R1) ] + if TV = int then
[ Move(W, Bdispx( Reg(R0), Reg(R1), RA),
[ ( p ( g( ) g( ) )
Regdef(SP) ] else // TV = char
[ Conv( Bdispx( Reg(R0), Reg(R1), RA), Regdef( SP ) ]
Beachte: Die Attributierung von Var und ArrayAccess
UsedId RA CR
Exp
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Beachte: Die Attributierung von Var und ArrayAccess erzeugt Code zum Kellern des Werts vom Ausdruck, nicht für die Adressierung des Zugriffs.
[LOADI (Ri, int) ] + [SUB ($sp, $sp, 4)] + [STORE (Ri, 0, $sp)]
if TV = int then
[SUB ($sp, $sp, 4) LOADI(R1,RA) ADD (RI, RI, $gp) LOAD(R2, 0, RI)
STORE (R2, 0, $sp) ] else // TV = char
[SUB ($sp,$sp,1) LOADI(R1,RA) ADD (RI, RI, $gp) LOAD(R2, 0, RI)
STOREB (R2, 0, $sp) ]
Ina Schaefer Translation to Target Language 59
Translation of Expressions
Translation of Expressions (8)
IntConst
[ Move( W, Imm( ), Predec(SP) ] [ Move( W, Imm(_), Predec(SP) ] Int
Var TV
if TV = int then
[ Move( W, Bdisp(Reg(R0), RA), Predec(SP) ] else // TV = char
else // TV char
[ Conv( Bdisp(Reg(R0), RA), Predec(SP) ] UsedId RA
ArrayAccess TV
ArrayAccess
CR + [ Move( W, Regdef(SP), Reg(R1) ] + if TV = int then
[ Move(W, Bdispx( Reg(R0), Reg(R1), RA),
[ ( p ( g( ) g( ) )
Regdef(SP) ] else // TV = char
[ Conv( Bdispx( Reg(R0), Reg(R1), RA), Regdef( SP ) ]
Beachte: Die Attributierung von Var und ArrayAccess
UsedIdRA CR
Exp
Beachte: Die Attributierung von Var und ArrayAccess erzeugt Code zum Kellern des Werts vom Ausdruck, nicht für die Adressierung des Zugriffs.
[LOADI (Ri, int) ] + [SUB ($sp, $sp, 4)] + [STORE (Ri, 0, $sp)]
if TV = int then [SUB ($sp, $sp, 4) LOADI(R1,RA) ADD (RI, RI, $gp) LOAD(R2, 0, RI) STORE (R2, 0, $sp) ] else // TV = char [SUB ($sp,$sp,1) LOADI(R1,RA) ADD (RI, RI, $gp) LOAD(R2, 0, RI) STOREB (R2, 0, $sp) ]
Ina Schaefer Translation to Target Language 60
Translation of Expressions
Translation of Expressions (9)
[ Move( W, Imm( ), Predec(SP) ] [ Move( W, Imm(_), Predec(SP) ] Int
Var TV
if TV = int then
[ Move( W, Bdisp(Reg(R0), RA), Predec(SP) ] else // TV = char
else // TV char
[ Conv( Bdisp(Reg(R0), RA), Predec(SP) ] UsedId RA
ArrayAccess TV
ArrayAccess
CR + [ Move( W, Regdef(SP), Reg(R1) ] + if TV = int then
[ Move(W, Bdispx( Reg(R0), Reg(R1), RA),
[ ( p ( g( ) g( ) )
Regdef(SP) ] else // TV = char
[ Conv( Bdispx( Reg(R0), Reg(R1), RA), Regdef( SP ) ]
Beachte: Die Attributierung von Var und ArrayAccess
UsedIdRA CR
Exp
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Beachte: Die Attributierung von Var und ArrayAccess erzeugt Code zum Kellern des Werts vom Ausdruck, nicht für die Adressierung des Zugriffs.
CR +
[LOAD (R1, 0, $sp) LOADI (R2, RA) ADD (R1, R1, R2) ADD (R1, R1, $gp)] + if TV = int then
[LOAD (R2, 0, RI) STORE (R2, 0, $sp)]
else // TV = char [LOADB (R2 0, RI) STOREB (R2, 0, $sp)]
Ina Schaefer Translation to Target Language 61
Translation of Expressions
Improvements
• Improvement of generated code by
! Storage of intermediate results in registers
! Context-dependent optimizing instruction selection
! Avoiding redundant computations by evaluating common subexpressions only once
• Improvement of translation technique by usage of intermediate language
Translation of Statements
Translation of Statements
Most statements can be translated by translation schemes with jumps:
Verbesserungen:
• des erzeugten Codes durch
Verwaltung von Zwischenergebnissen in Registern - Verwaltung von Zwischenergebnissen in Registern - kontextabhängige, optimierende Befehlsauswahl - Vermeidung redundanter Berechnungen durch
einmalige Auswertung gemeinsamer Teilausdrücke Ü
3 1 5 Übersetzung von Anweisungen
• der Übersetzungstechnik durch Benutzung einer Zwischensprache
Für die meisten Anweisungen lassen sich relativ leicht Übersetzungsschemata mittels Sprüngen angeben:
3.1.5 Übersetzung von Anweisungen
While
[ Label( “BEGWHILE_“ + M ) ] + CE +
[ Cmp( W Imm(0) Postinc(SP) ) ] + M
[ Cmp( W, Imm(0), Postinc(SP) ) ] + [ Jeq( Label( “ENDWHILE_“+M) ) ] + CS +
[ Jump(Label( “BEGWHILE_“+M)) ] + [ Label( “ENDWHILE_“ + M ) ]
Schwieriger ist die gute Übersetzungen von switch-
Exp
( )
CE CS
Stat
© A. Poetzsch-Heffter, TU Kaiserslautern
g g g
Anweisungen und die effiziente Berücksichtigung von nicht-strikten Ausdrücken.
[LABEL (“BEGWHILE_”+M)] + CE +
[LOAD (R1, 0, $sp) ADD ($sp, $sp, 4)
BEQ (R1, $zero, “ENDWHILE_”+M)] + CS +
[JUMP (“BEGWHILE_”+ M)] + [LABEL (“ENDWHILE_”+M)]
Ina Schaefer Translation to Target Language 63
Translation of Statements
More Complex Translation of Statements
More complex is a good translation of switch-statements and efficient handling of non-strict expressions.
We consider the translation of non-strict Boolean expressions as an example of an optimizing translation and for the usage of context information.
Example: Abstract Syntax
Wir demonstrieren hier die Übersetzung nicht-strikter boolescher Ausdrücke:
• als Beispiel für eine optimierende Übersetzung
• um die Verwendung von Kontextinformation zu illustrieren.
Beispiel: (Verwendung ererbter Information)
Stat = While | IfThenElse | ...
BExp = And | Or | Not | StrictExp
Beispiel: (Verwendung ererbter Information)
Wir betrachten folgendes Sprachfragment:
BExp And | Or | Not | StrictExp While ( BExp c, Stat b )
IfThenElse ( BExp c, Stat then, Stat else ) And, Or ( BExp left, BExp right )
Not ( Bexp e ) StrictExp ( Exp e ) Ein Programmfragment dazu:
if( (B1 || B2) && ! B3 ) { while( !(B4 || B5) ) A1
Wobei A1 und A2 Anweisungen sind und B1 bis B5 while( !(B4 || B5) ) A1
} else { A2 }
Wobei A1 und A2 Anweisungen sind und B1 bis B5 strikte Ausdrücke. Wie in C und Java sind die
booleschen Ausdrücke || und && nicht-strikt, d.h. z.B.
dass bei Auswertung von B1 und B2 zu false, B3 nicht mehr ausgewertet werden braucht und darf!
nicht mehr ausgewertet werden braucht und darf!
Außerdem sollen Sprungkettenvermieden werden,
Ina Schaefer Translation to Target Language 64
Translation of Statements
More Complex Translation of Statements (2)
A program fragment:
nicht-strikter boolescher Ausdrücke:
• als Beispiel für eine optimierende Übersetzung
• um die Verwendung von Kontextinformation zu illustrieren.
Beispiel: (Verwendung ererbter Information)
Stat = While | IfThenElse | ...
BExp = And | Or | Not | StrictExp
Beispiel: (Verwendung ererbter Information)
Wir betrachten folgendes Sprachfragment:
BExp And | Or | Not | StrictExp While ( BExp c, Stat b )
IfThenElse ( BExp c, Stat then, Stat else ) And, Or ( BExp left, BExp right )
Not ( Bexp e ) StrictExp ( Exp e )
Ein Programmfragment dazu:
if( (B1 || B2) && ! B3 ) { while( !(B4 || B5) ) A1
Wobei A1 und A2 Anweisungen sind und B1 bis B5
while( !(B4 || B5) ) A1 } else {
A2 }
Wobei A1 und A2 Anweisungen sind und B1 bis B5 strikte Ausdrücke. Wie in C und Java sind die
booleschen Ausdrücke || und && nicht-strikt, d.h. z.B.
dass bei Auswertung von B1 und B2 zu false, B3 nicht mehr ausgewertet werden braucht und darf!
12.06.2007
© A. Poetzsch-Heffter, TU Kaiserslautern
224nicht mehr ausgewertet werden braucht und darf!
Außerdem sollen Sprungketten vermieden werden, d.h. Sprünge zu unbedingten Sprungbefehlen.
where
• A1, A2 are statements
• B1 – B5 are strict expressions
Ina Schaefer Translation to Target Language 65
Translation of Statements
More Complex Translation of Statements (3)
In C and Java, we have that || and && are non-strict, i.e. if B1 and B2 evaluate to false, B3 may not be evaluated.
Further, jump cascades should be avoided, i.e. jumps to other unconditional jumps.
Idea for Attribution:
For each boolean expression, compute
• Label for true case (Attribute: %)
• Label for false case (Attribute: &)
• Information of type bool in which case to jump (Attribute: ')
Translation of Statements
More Complex Translation of Statements (4)
Further Attributes:
Idee der Attributierung:
Ermittele zu jedem booleschen Ausdruck:
• das Sprungziel für den true-Fall (Attribut ),
• das Sprungziel für den false-Fall (Attribut ),
• die Information vom Typ bool, in welchem Fall
Weitere Attributdeklarationen:
yp ,
zu springen ist (Attribut ).
Code für den Unterbaum vom Typ CodeList
Weitere Attributdeklarationen:
eindeutige Marke für jede Anweisung und jeden Booleschen Ausdruck vom Typ String
IfThenElse M
“THEN“ + M
CB +
[ Label( “THEN“ + M ) ] + CT +
[ Jump( Label( “END“+M))] + [ Label( “ELSE“ + M ) ] + false
THEN + M
“ELSE“ + M
[ Label( ELSE + M ) ] + CE +
[ Label( “END“ + M ) ]
CB C C
false
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BExp CB CT
Stat CE
Stat Code for subtree of type CodeList
Unique label for each statement and for each boolean expression of type String
Ina Schaefer Translation to Target Language 67
Translation of Statements
More Complex Translation of Statements (5)
Idee der Attributierung:
Ermittele zu jedem booleschen Ausdruck:
• das Sprungziel für den true-Fall (Attribut ),
• das Sprungziel für den false-Fall (Attribut ),
• die Information vom Typ bool, in welchem Fall
Weitere Attributdeklarationen:
yp ,
zu springen ist (Attribut ).
Code für den Unterbaum vom Typ CodeList Weitere Attributdeklarationen:
eindeutige Marke für jede Anweisung und jeden Booleschen Ausdruck vom Typ String
IfThenElse M
“THEN“ + M
CB +
[ Label( “THEN“ + M ) ] + CT +
[ Jump( Label( “END“+M))] + [ Label( “ELSE“ + M ) ] + false
THEN + M
“ELSE“ + M
[ Label( ELSE + M ) ] + CE +
[ Label( “END“ + M ) ]
CB C C
false
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BExp CB CT
Stat CE
Stat
More Complex Translation of Statements (6)
While M
[ Label( “BEGW“ + M ) ] + CB +
[ Label( “BODY“ + M ) ] + CS +
[ Jump( Label( “BEGW“+M))] +
“BODY“ + M
“ENDW“ + M
BExp
[ Jump( Label( BEGW +M))] + [ Label( “ENDW“ + M ) ]
CB CS
Stat false
p
Not
BExp not(_)
And M
“BER“ + M CL +
false
CL +
[ Label( “BER“ + M ) ] + CR
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BExp CL BExp CR
Ina Schaefer Translation to Target Language 69
Translation of Statements
More Complex Translation of Statements (7)
While M
[ Label( “BEGW“ + M ) ] + CB +
[ Label( “BODY“ + M ) ] + CS +
[ Jump( Label( “BEGW“+M))] +
“BODY“ + M
“ENDW“ + M
BExp
[ Jump( Label( BEGW +M))] + [ Label( “ENDW“ + M ) ]
CB CS
Stat false
p
Not
BExp not(_)
And M
“BER“ + M CL +
false
CL +
[ Label( “BER“ + M ) ] + CR
BExp CL BExp CR
Ina Schaefer Translation to Target Language 70
Translation of Statements
More Complex Translation of Statements (8)
While M
[ Label( “BEGW“ + M ) ] + CB +
[ Label( “BODY“ + M ) ] + CS +
[ Jump( Label( “BEGW“+M))] +
“BODY“ + M
“ENDW“ + M
BExp
[ Jump( Label( BEGW +M))] + [ Label( “ENDW“ + M ) ]
CB CS
Stat false
p
Not
BExp not(_)
And M
“BER“ + M CL +
false
CL +
[ Label( “BER“ + M ) ] + CR
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BExp CL BExp CR
Ina Schaefer Translation to Target Language 71
Translation of Statements
More Complex Translation of Statements (9)
Or M
“BER“ + M CL +
true
BER M CL +
[ Label( “BER“ + M ) ] + CR
BExp CL BExp CR
StrictExp
CE +
[ Cmp( W, Imm(1), Postinc(SP) ) ] + TT FT JI
[ p( ( ) ( ) ) ]
( if JI then
[ Jeq( Label( TT) ) ] else
[ Jne( Label( FT) ) ] )
Exp CE
Bemerkung:
Falls nicht-strikte und strikte boolesche Ausdrücke gemischt sind, wird die Codegenerierung komplexer.
Beispiel: a = ( b && f(c) ) + g;
Ina Schaefer Translation to Target Language 72
Translation of Statements
More Complex Translation of Statements (10)
Or
“BER“ + M CL +
true
BER M CL +
[ Label( “BER“ + M ) ] + CR
BExp CL BExp CR
StrictExp
CE +
[ Cmp( W, Imm(1), Postinc(SP) ) ] + TT FT JI
[ p( ( ) ( ) ) ]
( if JI then
[ Jeq( Label( TT) ) ] else
[ Jne( Label( FT) ) ] )
Exp CE
Bemerkung:
Falls nicht-strikte und strikte boolesche Ausdrücke gemischt sind, wird die Codegenerierung komplexer.
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Beispiel: a = ( b && f(c) ) + g;
CE +
[LOAD (R1, 0, $sp) ADD ($sp, $sp, 4)] + if JL then
[BNE (R1, $zero, LABEL(TT))]
else
[BEQ (R1, $zero, LABEL(FT)]
Ina Schaefer Translation to Target Language 73
Translation of Statements
More Complex Translation of Statements (11)
Remarks:
If non-strict and strict Boolean expressions are mixed, code generation becomes more complex.
Example: a = ( b && f(c)) + g ;
Recommended Reading:
• Wilhelm, Maurer: Sec. 2.4, pp. 12 –16
Translation of Procedures and Local Objects
Translation of Procedures and Local Objects
Most procedural languages support recursion, procedure-local variables and nested procedures. In the following, we consider
• Translation of recursive procedures
• Translation of local variables
• Translation of nested procedures
We do not consider the translation of procedures as parameters.
Ina Schaefer Translation to Target Language 75
Translation of Procedures and Local Objects
Procedures
The declaration of a procedure consists of
• the name of the procedure
• the declaration of the formal parameters
• the declaration of local variables
• the body of the procedure
Each dynamic call of a procedure corresponds to a procedure incarnation.
Analogy:
• Procedure declaration → procedure incarnation
• Class declaration → object/class instance
Procedure Call Tree
The runtime behaviour of a procedural program can be described by a procedure call tree.
Example (C-Program):
Das Laufzeitverhalten eines prozeduralen Programms lässt sich durch den Prozeduraufrufbaumbeschreiben.
Beispiel: (Prozeduraufrufbaum)
Wir betrachten folgendes C-Programm:
int even(int n){return n==0?1:odd(n-1);}
int odd (int n){return n==0?0:even(n-1);}
i i (){ (2)? (1) dd(1) }
int main(){return even(2)?even(1):odd(1);}
main even odd
even
odd even
Bemerkung:
Bemerkung:
• Der Prozeduraufrufbaum ist eine abstrakte Beschreibung des Laufzeitverhaltens und damit abhängig von den Eingabewerten des Programms.
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• Zu jedem Ausführungszeitpunkt gibt es einen aktiven Pfad in dem Baum.
Ina Schaefer Translation to Target Language 77
Translation of Procedures and Local Objects
Procedure Call Tree (2)
Remarks:
• The procedure call tree is an abstract description of the runtime behaviour and depends on the inputs of the program.
• For each execution point, there is an active path in the tree.
Translation of Procedures and Local Objects
Translation of Recursive Procedures
Main Tasks:
• Parameter passing on entry, return of result at exit of procedure
• Addressing of parameters
• Handling of recursion Main Idea:
For each procedure incarnation, a stack frame is allocated. The stack frame contains:
• the current parameters
• the return address
• the register contents of the caller
• further information
Ina Schaefer Translation to Target Language 79
Translation of Procedures and Local Objects
Stack Frame
Structure of stack frame
For procedure with result, also memory has to be allocated. (Where?)