Common Subexpression Elimination

One very common optimization recognizes that the same expression appears in more than one place, and that performance can be improved by generating the code for that expression in one place. For example, if the same operation appears in both the if and the else branches of a block of code, the compiler can move that operation to just before the if statement. When it does so, it assigns line numbers to the instructions based on one of the previous occurrences of the expression. If the line numbers assigned to the common code correspond to one branch of an if structure, and the code actually always takes the other branch, the annotated source shows metrics on lines within the branch that is not taken.