CSE425

1. Introduction to
Programming Languages
Dr. M. A. Rouf

2. CSE-3821 Dr. M. A. Rouf, Dept. of CSE, DUET, Gazipur 2
Compilation vs. Translation
• Translation: does a ‘mechanical’ translation of the source
code
• No deep analysis of the syntax/semantics of the code
• Compilation: does a thorough understanding and translation
of the code
• A compiler/translator changes a program from one language
into another
• C compiler: from C into assembly
• An assembler then translates it into machine language
• Java compiler: from Java code to Java bytecode
• The Java interpreter then runs the bytecode

3. Compilation Steps of C Program/ High-level
Program
CSE-3821 Dr. M. A. Rouf, Dept. of CSE, DUET,
Gazipur
Static linking
3
1. Scanner
2. Parser
3. Semantic analysis
4. Intermediate code generation
5. Machine-independent code improvement
(optional)
1. Target code generation
2. Machine-specific code improvement (optional)

4. CSE-3821 Dr. M. A. Rouf, Dept. of CSE, DUET, Gazipur 4
Example of Compilation Process
Library
Object
Code

5. CSE-3821 Dr. M. A. Rouf, Dept. of CSE, DUET, Gazipur 5
Code Optimization
Scanner
(lexical
analysis)
Parser
(syntax
analysis)
Code
Optimizer
Semantic
Analysis
(IC generator)
Code
Generator
Symbol
Table
Source
language
tokens Syntactic
structure
Syntactic/semantic
structure
Target
language

6. CSE-3821 Dr. M. A. Rouf, Dept. of CSE, DUET, Gazipur 6
Peephole Optimizations
•Constant Folding
x := 32 becomes x := 64
x := x + 32
•Unreachable Code
goto L2
x := x + 1  Unneeded Code
L2:
•Flow of Control Optimizations
goto L1 becomes goto L2
…
L1: goto L2

7. CSE-3821 Dr. M. A. Rouf, Dept. of CSE, DUET, Gazipur 7
Peephole Optimizations
•Algebraic Simplification
x := x + 0  unneeded
•Dead code
void example1() {
int x = 10;
return; // Program exits here
x = 20; // Dead code (never executed)
}
Reduction in strength
Ex1: x := x * 2  x := x + x
Ex2: Y=X^2 -> Y=X*X;

8. CSE-3821 Dr. M. A. Rouf, Dept. of CSE, DUET, Gazipur 8
Basic Block Level
• Common Subexpression elimination
• Constant Propagation
• Dead code elimination
• Plus many others such as copy propagation, value numbering, partial
redundancy elimination, …

9. CSE-3821 Dr. M. A. Rouf, Dept. of CSE, DUET, Gazipur 9
Simple example: a[i+1] = b[i+1]
•t1 = i+1
•t2 = b[t1]
•t3 = i + 1
•a[t3] = t2
•t1 = i + 1
•t2 = b[t1]
•t3 = i + 1  no longer live
•a[t1] = t2
Common Expression can be Eliminated

10. CSE-3821 Dr. M. A. Rouf, Dept. of CSE, DUET, Gazipur 10
Control Flow Graph- CFG
CFG = < V, E, Entry >, where
V = vertices or nodes, representing an instruction or basic
block (group of statements).
E = (V x V) edges, potential flow of control
Entry is an element of V, the unique program entry
Two sets used in algorithms:
• Succ(v) = {x in V| exists e in E, e = v x}
• Pred(v) = {x in V| exists e in E, e = x v}
1 2 3 4 5

11. CSE-3821 Dr. M. A. Rouf, Dept. of CSE, DUET, Gazipur 11
Constant Propagation
b = 5
c = 4*b
c > b
d = b + 2
e = a + b
b = 5
c = 20
c > 5
d = 7
e = a + 5
e = a + b
t
f
t
f
b = 5
c = 20
20 > 5
d = 7
e = a + 5
t
f

12. CSE-3821 Dr. M. A. Rouf, Dept. of CSE, DUET, Gazipur 12
Constant Propagation
b = 5
c = 20
20 > 5
d = 7
e = a + 5
t
f
b = 5
c = 20
d = 7
e = a + 5

13. CSE-3821 Dr. M. A. Rouf, Dept. of CSE, DUET, Gazipur 13
Copy Propagation
b = a
c = 4*b
c > b
d = b + 2
e = a + b
b = a
c = 4*a
c > a
d = a + 2
e = a + a
e = a + b

14. CSE-3821 Dr. M. A. Rouf, Dept. of CSE, DUET, Gazipur 14
Data Dependencies
• Flow Dependencies – Read After Write (RAW)
x := 4;
y := x + 1
• Output Dependencies – Write After Write (WAW)
x := 4;
x := y + 1;
•Antidependencies – Write After Read (WAR)
y := x + 1;
x := 4;