1. Code generation and data types
Translation and Address Calculation

2. Outline Intermediate codes Code generation, using attribute grammar
3-address code
P-code
Code generation, using attribute grammar
Data types
Primitive data types
Arrays and Strings
Records
Address calculation
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3. Intermediate Codes 3-address code P-code
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4. Intermediate Code Intermediate representation for programs
Why intermediate code
Reduce amount of work if optimization is done for intermediate code
Easy for retargeting compilers
Forms of intermediate code
Abstract syntax tree
Linearization of abstract syntax tree
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5. 3-address code x = y op z Labels can be assigned to a location.
x, y, and z are addresses of
Variables (perhaps temporaries)
Locations in programs
y and z must be differed from x.
Labels can be assigned to a location.
Operators can be:
Arithmetic operators (3- address)
Relational operators (3- address)
Conditional operators (2- address)
If_false … goto …
Jump (1-address)
Input/Output
Halt
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6. Example: 3-address code
read (x); if x>0 then { fact:=1; repeat { fact:=fact*x; x:=x-1; } until x==0; write(fact);
read x t1=x>0 if_false t1 goto L1 fact=1 label L2 t2=fact*x fact=t2 t3=x-1 x=t3 t4=x==0 if_false t4 goto L2 write fact label L1 halt
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7. P-code Code for a hypothetical stack machine For Pascal compilers
No variable name is required
Instructions
Load stack
Arithmetic and relational operators
Jumps
Operations are performed on topmost values on stack
0-address or 1-address instructions
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8. P-code Instructions Load: push stack
Load value
Load address
Load constant
Store: save top of stack in memory
Destructive store
Nondestructive store
Arithmetic operations
Add
Subtract
Multiply
Compare
Greater
Less
equal
Label
Jump
Unconditional jump
Conditional jump
I/O
Read
Write
Stop
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9. Example: P-code
read (x); loada x read if x>0 then loadv x loadc 0 greater jumpONfalse L1 { fact:=1; loada fact loadc 1 store repeat Label L2 { fact:=fact*x; loada fact loadv fact loadv x mult
x:=x-1; loada x loadv x loadc 1 sub store } until x==0; loadv x loadc 0 equ jumpF L2 write(fact); loadv fact write } Label L1 stop
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10. Code Generation Using Attribute Grammar
Chapter 6 Code Generation and Data Types
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11. Code Generation Using Synthesized Attributes
An attribute is created for the sequence of characters representing generated code.
An attribute grammar is written to generate the intermediate/target code.
The value of the attribute is passed from child nodes up to their parent node to construct a larger chunk of code.
Chapter 6 Code Generation and Data Types
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12. Attribute Grammar: P-code
Grammar Rules
Semantic Rules
exp1  id = exp2
exp1.code =
“loada” || id.strval ++ exp2.code ++ ”stn”
exp  aexp
exp.code = aexp.code
aexp1  aexp2 + factor
aexp1 .code =
aexp2 .code ++ factor.code ++ “add”
aexp  factor
aexp.code = factor.code
factor  ( exp )
factor.code = exp.code
factor  num
factor.code = “loadc ” || num.strval
factor  id
factor.code = “loadv ” || id.strval
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13. Generating P-code: Example
exp
loada x
loadv x
loadc 3
adi
stn
loadc 4
aexp
loada x
loadv x
loadc 3
adi
stn
loadc 4
aexp
factor +
loada x
loadv x
loadc 3
adi
stn
factor
num
loada x
loadv x
loadc 3
adi
stn
exp ) (
loadv x
loadc 3
adi
exp = id
loadv x
loadc 3
adi
aexp
loadc 3
loadv x
aexp +
factor
loadv x
factor
num id
Chapter 6 Code Generation and Data Types
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14. Attribute Grammar: 3-address code
Grammar Rules
Semantic Rules
exp1  id = exp2
exp1.name = exp2.name;
exp1.code = exp2.code ++ id.strval || = || exp2.name
exp  aexp
exp.name = aexp.name; exp.code = aexp.code
aexp1  aexp2 + factor
exp1.name = newtemp();
aexp1.code = aexp2.code ++ factor.code ++ aexp1.name || = || aexp2 .name || + || factor.name
aexp  factor
aexp.name=factor.name; aexp.code=factor.code
factor  ( exp )
factor.name =exp.name; factor.code = exp.code
factor  num
factor.name = num.strval; factor.code = “”
factor  id
factor.name = id.strval; factor.code = “”
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15. Generating 3-address Code: Example
t1=x+3
x=t1
t2=t1+4
aexp t1
t1=x+3
x=t1 4
aexp
factor + 4 t1
t1=x+3
x=t1
factor
num t1
t1=x+3
x=t1
exp ) ( t1
t1=x+3
exp id = t1
t1=x+3
aexp x 3 +
aexp
factor x
factor
num id
Chapter 6 Code Generation and Data Types
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16. Code Generation: Tree Traversal
procedure genCode (T:node)
{ if (T != null) then
{ generate code to prepare for code of left child;
genCode(T. leftChild);
generate code to prepare for code of right child;
genCode(T. rightChild);
generate code to implement the action of T; }
Chapter 6 Code Generation and Data Types
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17. Code Generation: P-code
gencode(T:Tree) { if (T is not null) { switch (T.type) case plusnode: { gencode(T.lchild); gencode(T.rchild); emitCode(“add”); } case asgnnode: { emitcode(“loada”,T.strval); emitcode(“stn”); }
case constnode: { emitcode(“loadc”,t.strval); } case idnode: { emitcode(“loadv”,t.strval); } default: { emitcode(“error”); } }
Chapter 6 Code Generation and Data Types
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18. Control Statements: code generation
If statements
While loops
Logical expressions
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19. Code Generation for If Statements
3-address code
P-code
IF ( E ) S1 ELSE S2
<code evaluating E
and assigning to t1>
if_false t1 goto L1
<code for S1>
goto L2
label L1
<code for S2>
label L2
<code evaluating E>
jumpF L1
<code for S1>
jump L2
label L1
<code for S2>
label L2
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20. Code Generation for While Loops
3-address code
P-code
WHILE ( E ) S
label L1 <code evaluating E and assigning to t1> if_false t1 goto L2 <code for S> goto L1 label L2
label L1 <code evaluating E> jumpF L2 <code for S> jump L1 label L2
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21. Generating Labels Forward jump For intermediate code
label L1
<code for E>
jumpF L2
<code for S>
jump L1
label L2
Forward jump
Label must be generated before defining the label
For intermediate code
Generate label at the jump instruction
Store the label until the actual location is found
For target code (assembly)
Leave the destination address in the jump instruction
When the destination address is found, go back and fill the address (backpatching)
Short jump or long jump?
Leave space enough for long jump
If only short jump is required, the extra space is filled with NOP.
Chapter 6 Code Generation and Data Types
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22. Code Generation for Logical Expressions
Data types and operators
Use Boolean data type and operators if included in the target/intermediate language
Use integer 0/1 and bitwise operators
Short-circuit evaluation
IF a AND b THEN S
If a is false,
the whole exp is false
there is no need to evaluate b
IF a OR b THEN S
If a is true,
the whole exp is true
Intermediate code for
IF a AND b THEN S
<code for a>
if_false a goto L1
<code for b>
if_false b goto L1
<code for S>
Label L1
IF a OR b THEN S
goto L2
if_false b goto L3
Label L2
Label L3
Chapter 6 Code Generation and Data Types
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23. Data Types Primitive data types Strings Arrays Records
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24. Primitive Data Types Integer Floating Point Decimal Boolean Character
Short, long, signed, unsigned
Floating Point
IEEE floating point format
Decimal
Boolean
Character
ASCII, Unicode
Chapter 6 Code Generation and Data Types
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25. Ordinal Types Integers
User-defined ordinal types: range of possible values can be associated with I+
Enumeration types
user-defined set of possible values.
Subrange types
Ordered contiguous subsequence of an ordinal type.
Chapter 6 Code Generation and Data Types
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26. Strings Can be either Operations Primitive type: Ada, FORTRAN, Basic
Array: Pascal, C, C++
Class: Java (String)
Operations
Assignment
Comparison
Catenation
Substring reference
Pattern Matching
Chapter 6 Code Generation and Data Types
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27. Descriptor for Strings
Descriptor is the information stored in a compiler.
Static string
Type: static string
Length
Address
Limited dynamic string
Type: dynamic string
Length: maximum and current length
Static string
length
address
Dynamic string
Max length
Current length
address
Chapter 6 Code Generation and Data Types
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28. Arrays Types of subscripts Implementation
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29. Subscripts Types of subscripts Range of subscripts Integers
FORTRAN, C, Java
Ordinal types
Enumeration types
Pascal, Ada
Static
C, FORTRAN
Java Arrays
Dynamic
Ada
FORTRAN ALLOCATBLE
Perl, JavaScript
Java ArrayList
Chapter 6 Code Generation and Data Types
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30. Implementation Number of subscripts Descriptor
Most languages have no limit, but some versions of FORTRAN have.
Descriptor
Type: Array
Element type
Index type
Number of dimensions
Index ranges, for each dimension
Address
Array
Element type
Index type
Number of dimensions
Index range 1 …
Index range n
Address
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31. Array: Address calculation
Row
Column 1 2 … j n i  m
Address of A[i, j] = s + ((i-1)n + j)*size(elementType)
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32. Records
heterogeneous aggregate of data elements in which the individual elements are identified by names.
Example:
record student
{ int id;
char firstname[40];
char lastname[40];
float grade; }
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33. Implementation Descriptor Address Calculation Type: record
For each field:
Name, type, offset
Address
Address Calculation
S + field offset
Record
Name
Type
Offset …
Address
Field 1
Field n
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34. Address Calculation Addressing operations Array reference
Record reference
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35. Addressing Operations
3-address code
P-code
Address of x
&x
Indirect address *x
Address of x
loada x
Indirect load
ind x
(load *(top+x))
Indexed address
ixa x
(load top*x+(top-1))
Chapter 6 Code Generation and Data Types
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36. Array References 3-address code P-code x=a[i] a[i]=x Find offset
t1=i*elesize(a)
t2=&a+t1
x=*t2
a[i]=x
*t2=x
x=a[i]
loada x
loada a
loadv i
ixa elesize(a)
ind 0
sto
a[i]=x
loadv x
Find offset
Find address
Find address
Find content
Find offset
Find address
Find address
Chapter 6 Code Generation and Data Types
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37. More Complex Array References
3-address code
P- code
a[i+1]=a[j+2]+3
t1=j+2
t2=t1*elesize(a)
t3=&a+t2
t4=*t3
t5=t4+3
t6=i+1
t7=t6*elesize(a)
t8=&a+t7
*t8=t5
a[i+1]=a[j+2]+3
loada a
loadv i
loadc 1
adi
ixa elesize(a)
loadv j
loadc 2
ind 0
loadc 3
sto
push &a[i+1]
t4=a[t1]
push a[j+2]
a[t6]=t5
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38. Record References 3-address code P-code x.j x.c x.i Address of a field
t1=&x+offset(x,j)
Content of a field
t2=*t1
Address of a field
loada x
loadc offset(x.j)
ixa 1
Content of a field
ind offset(x.j)
x.j
x.c
Offset of x.j
x.i
base address of x
Offset of x.c
Chapter 6 Code Generation and Data Types
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