Do compilers automatically optimise repeated calls to mathematical functions?

Question

Say I had this snippet of code:

#include 

// ...

float f = rand();
std::cout << sin(f) << \" \" << sin(f);

As

Answer 1

I'm fairly certain GCC marks sin with the non-standard pure attribute, ie __attribute__ ((pure));

This has the following effect:

Many functions have no effects except the return value and their return value depends only on the parameters and/or global variables. Such a function can be subject to common subexpression elimination and loop optimization just as an arithmetic operator would be.

http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html

And so there is a very good chance that such pure calls will be optimized with common subexpression elimination.

(update: actually cmath is using constexpr, which implies the same optimizations)