Comment by jcranmer

Example compiler explorer view: https://godbolt.org/z/b5z3q1616

Clang does the sensible thing with UB and just returns poison (a form of undefined value) in both cases, which manifests as do nothing on x86-64 and load a zero value on i386, because you need to push something on the stack and fldz is one of the cheapest ways to push something. Meanwhile, gcc is in both cases for the UB variant returning a + a + a + a;

FWIW, going back through older gcc versions, it seems i386 gcc stops implementing 'add the arguments' in version 11.1, although it's not until 15.1 that it has a sensible assembly for 'a + a + a + a'. The x86-64 gcc version is broken in 4.0 (where it stops copying the register arguments to the stack when va_start isn't called, I guess). Then it's adding xmm0 to the top 3 values on the stack until 11.1, when it's adding 'a + a + a + a', although not sensibly until version 15.1.