fipa-pure-const & fdse Compiler Options

fipa-pure-const

The fipa-pure-const option allows the compiler to "Discover which functions are pure or constant. Enabled by default at -O and higher."

To understand pure functions there are two requirements:

1. The function always evaluates the same result value given the same argument value(s). The function result value cannot depend on any hidden information or state that may change while program execution proceeds or between different executions of the program, nor can it depend on any external input from I/O devices.
2. Evaluation of the result does not cause any semantically observable side effect or output, such as mutation of mutable objects or output to I/O devices.

Pure functions examples are:

• sin(x), returning the sine of a number x
• length(s), returning the size of a string s

Impure functions would be functions that are not the same value if the conditions are the same, like the day of the week or a random().

Implication

for(int i = 0; i < fibonacci(100); ++i)




with the fipa-pure-const the loop becomes


for(int i = 0,end = fibonacci(100); i < end; ++i)


as the fibonacci() doesn't require to be calculated more than once.

fdse

The fdse option allows the compiler to "Perform dead store elimination (DSE) on RTL. Enabled by default at -O and higher."

Dead store is a variable that is assigned a value but is not read by any subsequent instruction. Dead store values waste processor time and memory.

Implication

function func(a, b) {
    var x;
    var i = 300;
    while (i--) {
        x = a + b; // dead store
    }
}

Removing dead store values can have the unintended consequence of not allowing 
overwrites to happen. 

Sources: 

https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
https://en.wikipedia.org/wiki/Pure_function
https://en.wikipedia.org/wiki/Constant_function
https://en.wikipedia.org/wiki/Dead_store