> Why bother with this category? The code exercising it is buggy either way

Because it is an actual security vulnerability if you cross privilege boundaries (infoleaks/(K)ASLR bypass, etc.), and one people often miss at that.

Say you write:

    struct { long long a; char b; } foo; foo.a = 0; foo.b = 1; return foo;

You end up leaking 7 stack bytes here (due to padding).

GCC's `-ftrivial-auto-var-init=pattern` currently initializes all unknown-value stack variables with 0xFEFEFEFE(...). This is usually an invalid fp value, invalid offset and invalid virtual address, allowing crashes to happen. This is a good thing.

Regarding performance, there is an attribute to opt out (both for the standard C++26 feature and the GCC option that is a subset of it)