by common sub expression we mean- the expression which is already computed but during compilation it again appears for computation.
in constant folding - the expression which is having a constant value at the time of compilation it can be referred to its respective value or we can say that it can be replaced with its similar value.
like we can use 3.14 for 22/7 in the circumference of a circle.
this is what has been asked in question - that The identification of common sub-expression and replacement of run time computations by compile-time computations is known as what??
its constant folding where the replacement is done during compilation time.