> Most people believe that modern compilers generate better-optimized assembly code than humans, but look at this example from AVR-GCC 5.4.0 with -O2 optimization level

Modern compilers? GCC 5.1.0 was released in 2015, and 5.4.0 is a bugfix release. Why would someone blog about its poor code generation without bothering to try the latest release?

Besides, the AVR is an exceptional architecture: an 8-bit RISC with sizeof(int) == sizeof(short) == 2. Neither GCC nor C were designed for 8-bit processors.