Optimization auto-setting _FORTIFY_SOURCE ??

[Patrick Herbst via Gcc-help <gcc-help@xxxxxxxxxxx>]

Using GCC 9.4, I've noticed setting -O1 will set
_FORTIFY_SOURCE=2

I'm able to see this by running
gcc -c -E -dM -O1 - < /dev/null | grep FORTIFY_SOURCE

But running the same command without -O1 doesn't set it.

Is this documented?  is this expected behavior?  I could not find any
reference to how optimization leads to setting this macro.

Any tips on where I can look, or what i'm missing to see this connection?

Thanks!