Verifying compiler optimizations in gcc/g++ by analyzing assembly listings

Question

I just asked a question related to how the compiler optimizes certain C++ code, and I was looking around SO for any questions about how to verify that the compiler has performed

Answer 1

GCC's optimization passes work on an intermediary representation of your code in a format called GIMPLE.

Using the -fdump-* family of options, you can ask GCC to output intermediary states of the tree.

For example, feed this to gcc -c -fdump-tree-all -O3

unsigned fib(unsigned n) {
    if (n < 2) return n;
    return fib(n - 2) + fib(n - 1);
}

and watch as it gradually transforms from simple exponential algorithm into a complex polynomial algorithm. (Really!)