Undocumented parameters for __builtin_cpu_supports()?

[Martin Reinecke]

Compiling and running the following code on a CPU with BMI2 support
prints "BMI2 detected" as one would expect:

#include <stdio.h>

int main(void)
  {
  if (__builtin_cpu_supports("bmi2"))
    printf("BMI2 detected\n");
  return 0;
  }

However, "bmi2" is not documented in the list of arguments at
https://gcc.gnu.org/onlinedocs/gcc-8.2.0/gcc/x86-Built-in-Functions.html#x86-Built-in-Functions

Is this list supposed to be complete? If yes, I can open a PR. If not,
is there any other way to obtain the full list of allowed parameters?

Cheers,
  Martin