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Dead Code EliminationIn compiler theory, Dead code elimination is a technique used to reduce program size by removing code which can never be executed. Consider the following example written in C. void foo() { int a = 24; int b = 25; int c; c = a << 2; return; b = 24; } The variable b is assigned a value after a return statement, which makes it impossible to get to. That is, since code execution is linear, and there is no conditional expression wrapping the return statement, any code after the return statement cannot possibly be executed. (This would not be the case if there were a label after the return statement, which opens the possibility of there being a jump that places execution after the return statement.) Furthermore, if we elimate that assignment, than we can see that the variable b is never used at all, except for its declaration and inital assignment. Depending on the aggressiveness of the optimizer, the variable b might be eliminated entirely from the generated code. 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, the function does not return a value. Thus, it can be said that the function has no side effects and returns no value, and thus does no work. In that case, a highly aggressive optimizer might reduce this function to nothing more than a return call, or even eliminate it entirely. Most advanced compilers have options to activate dead code elimination, sometimes at varying levels. A lower level might only remove instructions which cannot be executed. A higher level might also not reserve space for unused variables. Yet a higher level might determine instructions or functions that serve no purpose and eliminate them. A common use of dead code elimination is as an alternative to optional code inclusion via a preprocessor. Consider the following code. int main() { int a = 5; int b = 6; int c; c = a * (b >> 1); if (0) { /* DEBUG */ printf("%d\n", c); } return c; } Because the expression 0 will always evaluate to false, the code inside the if statement can never be executed, and dead code elimination would remove it entirely from the optimized program. This technique is common in debugging to optionally activate blocks of code; using an optimizer with dead code elimination eliminates the need for using a preprocessor to perform the same task.
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