[Bug c++/112666] Missed optimization: Value initialization zero-initializes members with user-defined constructor
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112666 Sam James changed: What|Removed |Added Resolution|FIXED |INVALID
[Bug c++/112666] Missed optimization: Value initialization zero-initializes members with user-defined constructor
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112666 --- Comment #5 from Jonathan Wakely --- (In reply to Francisco Paisana from comment #4) > One last thing, I might have misread this as well. > > > "Zero-initialization is performed in the following situations: > > ... > > 2) As part of value-initialization sequence [...] for members of > > value-initialized class types that have no constructors." > > I was interpreting it as "members that have no ctors of classes are > zero-init". I don't even know what that would mean. > However, this could be also read as "members of classes, where > the classes have no ctor, are zero-init." This is the correct reading. N.B. cppreference is not the standard. It's usually an accurate paraphrasing of the standard, but it's not gospel.
[Bug c++/112666] Missed optimization: Value initialization zero-initializes members with user-defined constructor
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112666 --- Comment #4 from Francisco Paisana --- One last thing, I might have misread this as well. > "Zero-initialization is performed in the following situations: > ... > 2) As part of value-initialization sequence [...] for members of > value-initialized class types that have no constructors." I was interpreting it as "members that have no ctors of classes are zero-init". However, this could be also read as "members of classes, where the classes have no ctor, are zero-init."
[Bug c++/112666] Missed optimization: Value initialization zero-initializes members with user-defined constructor
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112666 Francisco Paisana changed: What|Removed |Added Resolution|--- |FIXED Status|UNCONFIRMED |RESOLVED --- Comment #3 from Francisco Paisana --- This ended up being a misinterpretation of the C++ standard.
[Bug c++/112666] Missed optimization: Value initialization zero-initializes members with user-defined constructor
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112666 --- Comment #2 from Francisco Paisana --- Jonathan Wakely, thanks a lot for your clarification. I finally got it. In summary, we established that: 1. if a type T (in my case C) has no user-defined ctor, it will be zero-initialized. 2. and for that T, "each non-static data member ... is zero-initialized." For others that might fall into the same trap as me, the important detail in 2 is that members are "zero-initialized" and not "value-initialized". If non-static data members were value-initialized (not the case!), then my original comment would have been true based on the clause (see https://en.cppreference.com/w/cpp/language/zero_initialization): "Zero-initialization is performed in the following situations: ... 2) As part of value-initialization sequence [...] for members of value-initialized class types that have no constructors." I wonder if there is a way to forbid the members of a class type from ever being zero-initialized in C++. In any case, we can mark this issue as solved.
[Bug c++/112666] Missed optimization: Value initialization zero-initializes members with user-defined constructor
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112666
--- Comment #1 from Jonathan Wakely ---
(In reply to Francisco Paisana from comment #0)
> The struct "C" which is just "B" and an int is much slower at being
> initialized than B when value initialization (via {}) is used. However, my
> understanding of the C++ standard is that members with a user-defined
> default constructor do not need to be zero-initialized in this situation.
I think that's not quite right. Types with a user-provided default constructor
will not be zero-initialized when value-init is used. B does have a
user-provided default constructor, so value-init for an object of type B does
not perform zero-init first.
But that applies when constructing a complete B object, not when constructing a
member subobject.
C does not have a user-provided default constructor, so value-initialization
means:
"- the object is zero-initialized and the semantic constraints for
default-initialization are checked, and if T has a non-trivial default
constructor, the object is default-initialized;"
So first it's zero-initialized, which means:
"- if T is a (possibly cv-qualified) non-union class type, its padding bits
(6.8.1) are initialized to zero bits and each non-static data member, each
non-virtual base class subobject, and, if the object is not a base class
subobject, each virtual base class subobject is zero-initialized;"
This specifically says that *each non-static data member ... is
zero-initialized." So the B subobject must be zero-initialized. That's not the
same as when you value-init a B object.
> Looking at the godbolt assembly output, I see that both `A a{}` and `C c{}`
> generate a memset instruction, while `B b{}` doesn't. Clang, on the other
> hand, seems to initialize C almost as fast as B.
I don't know whether Clang considers the zero-init to be dead stores that are
clobbered by B() and so can be eliminated, or something else. But my
understanding of the standard is that requiring zero-init of B's members is
very intentional here.
> This potentially missed optimization in gcc is particularly nasty for
> structs with large embedded storage (e.g. structs that contain C-arrays,
> std::arrays, or static_vectors).
Arguably, the problem here is that B has a default ctor that intentionally
leaves members uninitialized. If you want to preserve that behaviour in types
that contain a B subobject, then you also need to give those types (e.g. C in
your example) a user-provided default ctor.
