Compiler Design Dead Code Elimination

Dead Code Elimination

› Also known as DCE, dead code removal, dead code stripping, or dead code strip.

›  Compiler optimization process to remove code.

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).

Benefits of Dead Code Elimination

› Two benefits :1.  It shrinks program size.2.  Reduces its running time.

Example of Dead Codeint global;Void function() {

int i;i=1; //dead store//global=1; //dead store//global=2;return;global=3; //unreachable//

}

Dead Code Elimination› The code fragment after dead code elimination.

int global;void function() {

global=2;return;

}

Dead Code Elimination via Preprocessorint main(void) {

int a = 5; int b = 6;

int c; c = a * (b >> 1); if (0) { /* DEBUG */ printf("%d\n", c);

} return c; }

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.

Partially Dead Code› Value computed by partially dead statement is

sometimes used and sometimes not used.

Another Example of Partial Dead Code

REFERENCE› http://

www.tutorialspoint.com/compiler_design/compiler_design_code_optimization.htm

› https://en.wikipedia.org/wiki/Dead_code_elimination› http://www.compileroptimizations.com/category/

dead_code_elimination.htm

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