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Concept
Dead-code elimination
Summary
In compiler theory, dead-code elimination (DCE, dead-code removal, dead-code stripping, or dead-code strip) is a compiler optimization to remove dead code (code that does not affect the program results). Removing such code has several benefits: it shrinks program size, an important consideration in some contexts, and it allows the running program to avoid executing irrelevant operations, which reduces its running time. It can also enable further optimizations by simplifying program structure. Dead code includes code that can never be executed (unreachable code), and code that only affects dead variables (written to, but never read again), that is, irrelevant to the program.
Consider the following example written in C.
int foo(void) {
int a = 24;
int b = 25; /* Assignment to dead variable /
int c;
c = a * 4;
return c;
b = 24; / Unreachable code */
return 0;
}
Simple analysis of the uses of values would show that the value of b after the first assignment is not used inside foo. Furthermore, b is declared as a local variable inside foo, so its value cannot be used outside foo. Thus, the variable b is dead and an optimizer can reclaim its storage space and eliminate its initialization.
Furthermore, because the first return statement is executed unconditionally and there is no label after it which a "goto" could reach, no feasible execution path reaches the second assignment to b. Thus, the assignment is unreachable and can be removed.
If the procedure had a more complex control flow, such as a label after the return statement and a goto elsewhere in the procedure, then a feasible execution path might exist to the assignment to b.
Also, even though some calculations are performed in the function, their values are not stored in locations accessible outside the scope of this function. Furthermore, given the function returns a static value (96), it may be simplified to the value it returns (this simplification is called constant folding).
Most advanced compilers have options to activate dead-code elimination, sometimes at varying levels.
Official source
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