P0732R0 and “trivially comparable”

Someone in SG14 asks, what do we all think about Jeff Snyder’s P0732R0 “Class Types in Non-Type Template Parameters”. Executive summary: I think it’s great.

First, the problem statement:

template<class T, T value>
int foo() {
    return 42;
}

This function works great when called as foo<int, 7>() or foo<char, 'A'>(), but it fails to compile when invoked as foo<float, 3.14>() or foo<std::string, "hello world">(). Why is that?

Let’s look at how the compiler treats foo in each case.

_Z3fooIiLi7EEiv:
  movl $42, %eax
  ret
_Z3fooIcLc65EEiv:
  movl $42, %eax
  ret

Notice that the name foo is getting name-mangled based on its template parameters. This is what ensures that foo<int, 7> and foo<int, 8> end up with distinct names; and, contrariwise, ensures that foo<int, 7> has the same name no matter which translation unit we’re in. Notice that the name-mangling scheme for these primitive types is very simple: we just name the type, and then express its value as a decimal integer. (If the value is negative, we prefix the character n instead of -, since the latter isn’t a valid identifier character.)

Notice also that no matter whether we write foo<int, 7> or foo<int, 3+4>, the compiler takes care of making sure that the name is mangled into exactly the same, unique, unambiguous representation of the value we were trying to express (namely, “seven”).

Converting a value into an unambiguous representation in a certain alphabet is closely related to serialization, marshalling, or what Python calls pickling. Different applications of serialization require different guarantees from the process. In the case of C++ template mangling, we require very stringent guarantees:

  • Obviously, foo<6> must be a different function from foo<7>. We must have mangle(v1) != mangle(v2) whenever v1 != v2.

  • Vice versa, foo<7> and foo<3+4> must be the same function. We must have mangle(v1) == mangle(v2) whenever v1 == v2.

Okay, fine, those weren’t very stringent. But they implicitly contain some interesting assumptions…

What does it mean to say “whenever v1 == v2”? We’re talking about the behavior of operator== there; but we’re also implicitly talking about all of value semantics. For example, we are implicitly assuming that v1 == v1.

For which C++ types does v1 sometimes not equal v1?

So that’s why C++ has gone through so many revisions and still doesn’t let you write f<float, 3.14>()! Because what would we do with f<float, some_nan_value>?

In fact, it gets worse. Are f<3.14f> and f<3.13f + 0.01f> the same entity, or not? Maybe it would depend on your platform. Maybe it would depend on your current rounding mode! And when we go to name-mangle the thing, how sure are we that each compiler involved will output exactly, let’s say, _Z3fooIfLf3p14EEiv, and not _Z3fooIfLf3p1400000001EEiv or _Z3fooIfLf3p139999984EEiv? Floating-point is kind of a hot mess. Floating-point template parameters would be too messy to live.

So what about P0732?

P0732 proposes that we permit template parameters of a user-defined type if and only if that type is what is known as trivially comparable. This is kind of analogous to C++11’s notion of trivially destructible (and so on). The type must have a defaulted comparison operator — this is new in C++2a, thanks to Herb Sutter’s P0515 “Consistent comparison” — and that defaulted comparison operator must essentially be equivalent to memcmp.

A type is trivially comparable if it is:

  • a scalar type for which the <=> operator returns either std::strong_ordering or std::strong_equality, or

  • a non-union class type for which

    • there is an operator<=> that is defined as defaulted within the definition of the class,
    • operator<=> returns either std::strong_ordering or std::strong_equality,
    • the type of each of its bases is trivially comparable, and
    • the type of each of its non-static data members is either trivially comparable or an array of such a type (recursively).

Notice that according to P0515R3, all floating-point types have an operator<=> that returns std::partial_ordering. So structs containing floating-point members will not be trivially comparable (because memcmp would do the wrong thing for them).

Now, one thing that P0732 probably gets wrong is that it forgets about padding bytes. So for example it would consider the following user-defined type to be trivially comparable:

struct Oops {
    char ch;
    // 3 padding bytes of indeterminate value
    int i;
};

This is not a problem for name-mangling, because name-mangling always happens in a context where the compiler can see the struct definition, enumerate the members one by one, and quietly skip over the padding bytes (whose values will be indeterminate and thus must not contribute to the mangling). However, the padding bytes do prevent us from using memcmp at runtime to compare the values of two struct Oopses. The topic of one of my C++Now 2018 talks will be on ways to speed up vector<T>::operator==, std::hash<T>, and so on, by doing the fast thing when is_trivially_comparable<T> can be detected at compile time. This approach would fall flat on its face if we permitted is_trivially_comparable<Oops>.

Now, maybe all we need is two distinct names — is_memberwise_comparable for P0732’s purposes, and is_memcmp_comparable for vector::operator==’s purposes. But it would be really really nice to get the two notions unified into a single concept. Because is_trivially_copyable already means “as if by memcpy,” I think it would be nice for is_trivially_comparable to mean “as if by memcmp.”

The problem with just saying that structs with padding aren’t trivially comparable, and otherwise letting P0732 go through as written, is that it would prevent us from using types such as Oops (or more realistically, std::pair<char, int>) as template parameters! Worse, it would end up being implementation-defined whether a certain type was usable as a template parameter or not; and adding or removing or reordering the members of a struct might unexpectedly make it unusable. Admittedly we have this problem already today, in that innocuous-looking changes could make a structure non-literal or non-trivially-copyable; but I don’t like the idea of making it worse.

Final thoughts

I would prefer that P0732 do something about padding bytes, rather than pretend that they’re okay. I’m not sure what to do about them, though.

Finally, I am mildly concerned that we’re adding so many ways for the compiler to “guess” at the triviality or “well-knownedness” of user-definable operations. For example, C++11 gave us trivially copyable; but recently the Clang project has (wisely!) added a new attribute [[trivial_abi]] whose raison d’etre is for the programmer to force-override the compiler’s “wrong” guess at trivial copyability — to say “I know this type looks non-trivial — and maybe it is — but trust me, it’s okay to pass it in registers.” With P0732 in C++2a, might we soon need an attribute [[trivially_comparable]] in order to say, “I know I provided a non-defaulted operator==, but trust me, it’s okay to use this type as a template parameter”?

Posted 2018-03-19