[eichin]

They've been able to recognize this pattern since the 1960s (popcount is a very historically special case and not really a sign of complexity, since traditionally it was "if you write this exact code from the documentation, you'll get the machine instruction" and didn't imply any more general cleverness.)

[cperciva]

popcount is a very historically special case

To elaborate a bit on the specialness of popcount: It is a generally accepted belief in the computer architecture community that several systems included a popcount iNstruction Solely due to A request from a single "very good customer".

[LegionMammal978]

Yeah, once I tried writing out a dumber popcount loop (checking each bit) for better readability, and was annoyed to find that GCC and Clang didn't recognize it. I ended up looking into the source of both compilers to find that they only transform the more 'idiomatic' version.