You got this backwards. If x86 had had POPCNT from the beginning but C did not have a standard popcount function, you would still need to express the popcount i

You got this backwards. If x86 had had POPCNT from the beginning but C did not have a standard popcount function, you would still need to express the popcount in source as in this question, so the compiler would still need to recognize the idiom. There would be no need for this sort of pattern matching if C had had a standard popcount function from the beginning, regardless of widespread hardware support.