Is it safe to link gcc 6, gcc 7, and gcc 8 objects?

Most of the language ABI changes seem like they wouldn't cause issues, but the calling convention change for empty class types in 12 might?

The change of the calling convention for empty classes can cause an issue on x86-64. Here's an example:

def.hpp:

struct Empty { };

struct Foo {
    char dummy[16];
    int a;

    Foo() : a(42) { }
};

void fn(Empty, Foo);

one.cpp:

#include "def.hpp"

int main() {
    fn(Empty(), Foo());
}

two.cpp:

#include <stdio.h>
#include "def.hpp"

void fn(Empty e, Foo foo) {
    printf("%d\n", foo.a);
}

Now, if you compile these with G++ 8 with differing ABIs of 11 and 12, for example:

g++ -c -fabi-version=11 one.cpp
g++ -c -fabi-version=12 two.cpp
g++ one.o two.o

the resulting a.out won't print the expected 42.

The reason is that the old ABI (11) reserves space for Empty() on the stack, but the new ABI (12) doesn't. So the address of foo will differ between the caller and the callee side.

(Note: I've included Foo::dummy so Foo gets passed using the stack instead of registers. If Foo was passed using registers, there would be no problem.)

Most of them alter mangling in minor ways, which could cause some undefined references while linking, or just some code bloat due to identical source code producing two equivalent symbols with different names, so won't be merged by the linker.

the calling convention change for empty class types in 12 might?

Yes, definitely. If you have non-final parameters that are empty types then that affects the ABI for the function, and differences can lead to undefined behaviour (in practice, accessing junk on the stack, or parameters getting the wrong values).