gcc: Differences between -march=native and -march=<specific arch>

In order to compare which flags various -march settings will enable, I am comparing the outputs of the following commands, as detailed in this SO answer:

$ gcc -Q -march=native         --help=target
$ gcc -Q -march=skylake-avx512 --help=target

Please note, for the avoidance of doubt, the detected arch output from using -march=native is skylake-avx512.

$ gcc -Q -march=native --help=target | grep march
  -march=                           skylake-avx512

Most of the flags the two -march variants output match exactly.

However, there are a few differences:

$ diff <(gcc -Q -march=native --help=target) <(gcc -Q -march=skylake-avx512 --help=target)
12c12
<   -mabm                               [enabled]
>   -mabm                               [disabled]
119c119
<   -mpku                               [disabled]
>   -mpku                               [enabled]
136c136
<   -mrtm                               [enabled]
>   -mrtm                               [disabled]
138c138
<   -msgx                               [disabled]
>   -msgx                               [enabled]

It is these differences which have prompted me to ask this question.

How does -march=native choose which instruction sets to enable and which to disable?

I have the following conjecture:

• -march=native will be using CPUID instructions to calculate supported instruction sets etc in order to detect the processor variant
• -march=foobar will use a hardcoded list of instruction sets which processor foobar supports.

If that is correct then I can see two possible ways this shakes out:

Option 1:

It is possible that -march=native may not get it 100% correct, whereas when a new processor is released, the table of supported instruction sets is updated, and is more likely to be correct.

Therefore we would expect -march=foobar to be the "more correct" flag.

Option 2:

-march=native will be using CPUID instructions to calculate supported instruction sets - and is therefore guaranteed to be correct, whereas -march=foobar will use a hardcoded list of instruction sets which may not be correct.

Therefore we would expect -march=native to be the "more correct" flag.

If Option 2 is correct, one could surmise that using -march=foobar could end up with an unsupported instruction set enabled - and if the program were to emit these instructions result in a crash.

I have thus far been unsuccessful in finding the answer as to whether either or any of the above is correct.

If I want to target a specific arch, be sure all (and only) supported instruction sets are enabled, and am unable to use -march=native, what is the best way to do this?