Dead code elimination is a compiler optimization technique that removes code that can never be executed, such as unreachable code, or code with outputs that are never used. Removing dead code shrinks program size and reduces runtime. It works by analyzing the control flow and data dependencies of a program to identify code that is deemed dead. Both static and dynamic dead code elimination techniques exist.

1. Compiler Design
Dead Code Elimination

2. Dead Code Elimination
› Also known as DCE, dead code removal, dead code
stripping, or dead code strip.
› Compiler optimization process to remove code.

3. What Is Dead Code?!!!
› Dead code is one or more than one code statements,
which are:
• Code that can never be executed (unreachable code).
• Or if executed, their output is never used.
• Code that only affects dead variables (written to, but never read again).

4. Benefits of Dead Code Elimination
› Two benefits :
1. It shrinks program size.
2. Reduces its running time.

5. Example of Dead Code
int global;
Void function()
{
int i;
i=1; //dead store//
global=1; //dead store//
global=2;
return;
global=3; //unreachable//
}

6. Dead Code Elimination
› The code fragment after dead code elimination.
int global;
void function()
{
global=2;
return;
}

7. Dead Code Elimination via Preprocessor
int main(void)
{
int a = 5;
int b = 6;
int c;
c = a * (b >> 1);
if (0) { /* DEBUG */
printf("%dn", c);
}
return c;
}

8. Dynamic Dead Code Elimination
› The techniques used to dynamically detect demand,
identify and resolve dependencies, remove conditionally
dead code, and recombine the remaining code at load
or runtime.
› Remove dead code through dead code elimination
at compile time.
› Very rare with languages compiled ahead-of-time.
› language implementations doing just-in-time compilation.

9. Partially Dead Code
› Value computed by partially dead statement is
sometimes used and sometimes not used.

10. Another Example of Partial Dead Code

11. REFERENCE
› http://www.tutorialspoint.com/compiler_design/compile
r_design_code_optimization.htm
› https://en.wikipedia.org/wiki/Dead_code_elimination
› http://www.compileroptimizations.com/category/dead_c
ode_elimination.htm