> Okay, wait. Every modern CPU has a popcount instruction
What do you consider a "modern CPU"? Atom chips sold less than a decade ago didn't support popcnt. AMD shipped some C-series chips without support for it as recently as 2012. The low-end chips were likely to be sold later without refreshes to newer features in some cases, and those are also likely the ones to be repurposed for small and cheap x86 devices later. I wouldn't want the default compilation settings without specifying CPU extensions to use an extension that might not exist on my target platform.
What do you consider a "modern CPU"? Atom chips sold less than a decade ago didn't support popcnt. AMD shipped some C-series chips without support for it as recently as 2012. The low-end chips were likely to be sold later without refreshes to newer features in some cases, and those are also likely the ones to be repurposed for small and cheap x86 devices later. I wouldn't want the default compilation settings without specifying CPU extensions to use an extension that might not exist on my target platform.