1. Lecture 12 Intermediate Code Generation Translating Expressions
Compiler Design
Lecture 12
Intermediate Code Generation
Translating Expressions

2. Phases of Compiler Source program Intermediate-code Generator
Front End
Lexical Analyzer (Scanner)
Non-optimized Intermediate Code
Tokens
Back End
Intermediate-code Optimizer
Syntax Analyzer (Parser)
Optimized Intermediate Code
Parse tree
Target-code Generator and Optimizer
Semantic Analyzer
Abstract Syntax Tree w/ Attributes
Target machine code

3. Many Source and Target Languages - Problem
Common front end S2 T2
Common back end
Sn-1
Tm-1 Sn Tm
How many compiler needed?
N * M
e.g. n= 10, m = 20  200 compiler !

4. Many Source and Target Languages - Solution
Intermediate Language
Sn-1
Tm-1 Sn Tm
How many compiler needed?
N (front ends) + M (back ends)
e.g. n= 10, m = 20  30 compiler only

5. Intermediate Language (IR) Benefits
Generate code for several machines
One front-end
Several back-ends
Several high-level languages for one machine
Several front-ends
One back-end
Intermediate language can be interpreted by small program written in machine code
Example: Java and JVM
Code optimization can be done in IR
Written only once for all compilers
The intermediate form may be more compact than machine code.

6. Choosing Intermediate Language (IR)
Intermediate Language should be:
Easy to translate from a high-level language.
Easy to translate from the intermediate language to machine code.
Suitable for optimization
Trade of between first two contradictory issues
High-level IR makes it hard for back-ends
Low-level IR makes it hard for front-ends
Solution
use 2 IR; high-level and low-level
And a translator between them

7. Some Intermediate Representations
Abstract Syntax Tree (AST)
Direct Acyclic Graph (DAG)
Postfix Notation
Three-Address Code
The most common
Set of instruction
Each has at most 3 addresses in memory

8. The three-address code is an intermediate form, which consists of a sequence of assembly-like instructions with three operands per instruction. Each operand can act like a register.

9. 1- Each three-address assignment instruction has at most one operator on the right side.
Thus, these instructions fix the order in which operations are to be done.
2- The compiler must generate a temporary name to hold the value computed by a three-address instruction.
3-Some "three-address instructions" have fewer than three operands.

10. Grammar for an Example IR
binary operator
A transfer from memory
relational operator
(=, ≠ <, >, ≤ or≥ ).
basics_lulu.pdf

11. Example High-level langue Translated to IR: Not optimized! z*(x-y)+w
t1 := z
t5 := x
t6 := y
t2 := t5 − t6
t3 := t1 * t2
t4 := w
t0 := t3 + t4
Not optimized!
No problem, can be optimized later
Why? To simplify translation to IR

12. Translating Expressions
The main complication:
Expression language is tree-structured
IR is flat: sequential
The result of every operation should be stored in a temporary variable
Every argument of operation should be stored also in a temporary variable

13. Syntax Directed Definition for Expression Translation
done at compile-time
At run-time
154
The function transop translates the name of an operator in the expression language into the name of the corresponding operator in the intermediate language.

14. Expression Example Idz*idx-idy Exp Exp1 - Exp2 id Exp * Exp id id t3=z
.place1 = newvar() = t1, .place2 = newvar() = t2
.code1 = exp1.code(t1),
.code2 = exp2.code(t2),
Exp
.code = [t3=z, t4=x,t1 = t3 * t4, t2=y, t0 = t1 – t2]
Exp Exp2
.place1 = newvar() = t3,
.place2 = newvar() = t4
.code = [t2 = y]
Exp * Exp id
.code = [t3=z, t4=x, t1 = t3 * t4]
.code = [t3 = z]
.code = [t4 = x] id id
Idz*idx-idy
t3=z
t4=x
t1 = t3 * t4
t2=y
t0 = t1 – t2

15. Translating Expression without Inherited Attributes
(Three-Address Code)
top denote the current symbol table
Function top.get retrieves the entry
Attribute E.addr denotes the address that will hold the value of E.
new Temp() creates t1 , t2 ,….
gen(x '=‘ y '+' z) represents the three-address instruction x = y + z. Expressions appearing in place of variables like x, y, and z are evaluated when passed to gen
Compilers Principles, Techniques, and Tools, 2-Ed - Ahuo, Ulman
379
402
Advantage: no code generated for variable access (last rule)
Disadvantage: not good with side effects e.g. x-(x=3)

16. a = b + - c

17. Expression Example 3 idx-(idx=3) x*z – (x=3) Exp Exp1 - Exp2 id
.place = newvar() = t0
.code1 = exp1.code = ‘’,
.code2 = exp2.code = [t1=3, x = t1]
Exp
.code = [t1 = 3, x = t1, t0 = x – x]
Exp Exp2
.place = x
.code = ‘’
.place = x
.code = [t1=3, x = t1] id
Id = Exp
.place = newvar() = t1
.code = [t1 = 3]
num
idx-(idx=3)
Always gives 0
t1 = 3
x = t1
t0 = x – x
first argument takes the new value!
x*z – (x=3)
t1 = x * z old value, correct
t2 = 3
x = t2
t0 = t1 – x new value

18. Expression Example 2 idx-(idx=3) Exp Exp1 - Exp2 id Id = Exp numt0
.place1 = newvar() = t1, .place2 = newvar() = t2
.code1 = exp1.code(t1),
.code2 = exp2.code(t2)
Exp
.code = [t1=x, t3=3,x=t3,t2=x,t0=t1-t2]
Exp Exp2
.code = [t1 = x]
.place1 = newvar() = t3
.code = [t3 = 3, x=t3, t2=x] id
Id = Exp
.code = [t3 = 3]
num
idx-(idx=3)
t1=x old value
t3=3
x=t3
t2=x new value
t0 = t1 – t2 t1 holds the old value of x  correct

19. Translating Function Call

20. Example 4
3+f(x-y,z)
Will be:

21. References
Basics of Compiler Design, Torben Ægidius Mogensen. Published through lulu.com, 2008
Compilers: Principles, Techniques and Tools, Alfred V. Aho, Monica S. Lam, Ravi Sethi, and Jeffrey D. Ullman. 2nd Edition, Addison-Wesley, 2007 (sections: 6.4)