Compiler Explorer
Godbolt is great for comparing compiler versions (and compilers).For example, you can see gcc's progression of efficiency for c atomics with https://godbolt.org/z/brsoEr. If you increment the gcc version number, you will see the (very slow) mfence disappear, and xchg show up.Then there is Clang at O3: If an int falls in the forest, and there is no one around, was it ever incremented? No. The function turns into a bare ret.