Does the compiler optimize references to constant variables?

Question

When it comes to the C and C++ languages, does the compiler optimize references to constant variables so that the program automatically knows what values are being referred to, instead of having to peek at the memory locations of the constant variables? When it comes to arrays, does it depend on whether the index value to point at in the array is a constant at compile time?

For instance, take a look at this code:

int main(void) {
    1:  char tesst[3] = {'1', '3', '7'};
    2:  char erm = tesst[1];
}

Does the compiler "change" line 2 to "char erm = '3'" at compile time?

Answer 1

It mostly depends on the level of optimization and which compiler you are using.

With maximum optimizations, the compiler will indeed probably just replace your whole code with char erm = '3';. GCC -O3 does this anyway.

But then of course it depends on what you do with that variable. The compiler might not even allocate the variable, but just use the raw number in the operation where the variable occurs.