https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95384
--- Comment #4 from Barry Revzin ---
Here's another example of the same kind of issue
(https://godbolt.org/z/KWr9rMssj):
template
struct tagged_union {
tagged_union(T t) : index(0), a(t) { }
tagged_union(U u) : index(1), b(u) { }
union {
T a;
U b;
};
char index;
};
struct X { int i; };
struct Y { int j; };
tagged_union as_tagged_union(X x) {
return x;
}
template
struct tagged_union_wrapped : tagged_union {
using tagged_union::tagged_union;
};
auto as_tagged_union2(X x) {
return tagged_union_wrapped(x);
}
this on -O3 emits:
as_tagged_union(X):
mov eax, edi
ret
as_tagged_union2(X):
mov DWORD PTR [rsp-8], edi
mov BYTE PTR [rsp-4], 0
mov rax, QWORD PTR [rsp-8]
ret
If you change the index member from 'char' to 'int', causing the tail padding
to disappear, as_tagged_union2 improves to the same code gen as
as_tagged_union.
This is relevant for std::variant performance. std::variant behaves like
tagged_union_wrapped, whereas if you drop down to the implementation
details and directly use _Variant_storage_alias, that behaves like
tagged_union for these purposes.
