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 certain optimizations. I was trying to look at the assembly listing generated with g++ (g++ -c -g -O2 -Wa,-ahl=file.s file.c) to possibly see what is going on under the hood, but the output is too cryptic to me. What techniques do people use to tackle this problem, and are there any good references on how to interpret the assembly listings of optimized code or articles specific to the GCC toolchain that talk about this problem?

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!)