Finally rsqrt() made its way into C23 [https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf], and a couple more years later it has been implemented in glibc 2.42 [https://sourceware.org/glibc/manual/2.42/html_node/Exponents-and-Logarithms.html#index-rsqrt].