Now, because this article got a little long, as per a friend's suggestion, here's a table of contents:

Optimization gives us optimal programs

Branch weights and the CPU's branch predictor

-O3 produces much faster code than -O2

Javascript interpreters JIT at runtime because they don't know which paths are hot ahead of time

If you have a compiler, you don't need an interpreter

The middle-end is target/platform-independent

The compiler optimizes for data locality

-O0 gives you fast compilation

Templates are slow to compile

Separate compilation is always worth it

Why does link-time optimization (LTO) happen at link-time?

Inlining is useful primarily because it eliminates a call instruction

The role of the inline keyword

The best production compiler to study is LLVM

Undefined behavior only enables optimizations

The compiler can "simply" define undefined behavior 99% correctness rate is ok