Why was there no standard operator for this in C since the 1970s? You got various bit shifts and bitwise logic operators, so why not this? std::popcount exists in C++ now since C++20, but that took a while if this instruction was in CPU's that long.
1961: IBM Stretch
1964: CDC 6000
1975: Cray-1
Reading https://en.wikipedia.org/wiki/IBM_7030_Stretch, a grand total of 9 IBM Stretch’s were sold.
In comparison, the CDC 600 was a wild success, with “100+” sales (https://en.wikipedia.org/wiki/CDC_6600)
The Cray-1 scores similarly with “over 100” (https://en.wikipedia.org/wiki/Cray-1)
C is from 1972 and started life for the PDP-7 and was soon ported to the PDP-11, of which “around 600,000” were sold (https://en.wikipedia.org/wiki/PDP-11; I guess most of these will have been sold late in the 1970s and early 1980s, but I think it’s a safe bet they were almost immediately more common than those 3 machines)