The register hint is a bit of a semi-unrelated sidetrack, which you would only ever use in a really hot location when proven necessary by profiling + inspection

The register hint is a bit of a semi-unrelated sidetrack, which you would only ever use in a really hot location when proven necessary by profiling + inspection of the generated code, and the compiler is more often than not smarter than you anyway so it's rarely necessary. It's not relevant to GPs point about regular locals being stored on the stack. See https://en.wikipedia.org/wiki/C_syntax#Storage_class_specifiers https://en.wikipedia.org/wiki/C_syntax#Storage_class_specifi... for a better overview.