A lot of code can be pessimized by golfing instruction counts, hurting instruction-level parallelism and microcode optimizations by introducing false data dependencies.
Compilers outperform humans here almost all the time.
However, that doesn't mean that looking at the generated asm / even writing some is useless! Just because you can't globally outperform the compiler, doesn't mean you can't do it locally! If you know where the bottleneck is, and make those few functions great, that's a force multiplier for you and your program.
I'm going to be annoying and nerd-snipe you here. It's, generally, really easy to beat the compiler.
Can you explain what this phrase means?
It means that the shorter sequence of instructions is not necessarily faster, and can in fact make the CPU stall unnecessarily.
The fastest sequence of instructions is the one that makes the best use of the CPU’s resources.
Compilers make mistakes too and they can output very erroneous code. But that’s a different topic.
"Compilers can do all these great transformations, but they can also be incredibly dumb"
-Mike Acton, CPPCON 2014