* [Bug c++/112666] Missed optimization: Value initialization zero-initializes members with user-defined constructor
2023-11-22 8:58 [Bug c++/112666] New: Missed optimization: Value initialization zero-initializes members with user-defined constructor paisanafc at gmail dot com
@ 2023-11-22 11:40 ` redi at gcc dot gnu.org
2023-11-23 15:05 ` paisanafc at gmail dot com
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From: redi at gcc dot gnu.org @ 2023-11-22 11:40 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112666
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(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.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug c++/112666] Missed optimization: Value initialization zero-initializes members with user-defined constructor
2023-11-22 8:58 [Bug c++/112666] New: Missed optimization: Value initialization zero-initializes members with user-defined constructor paisanafc at gmail dot com
2023-11-22 11:40 ` [Bug c++/112666] " redi at gcc dot gnu.org
@ 2023-11-23 15:05 ` paisanafc at gmail dot com
2023-11-23 15:08 ` paisanafc at gmail dot com
` (3 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: paisanafc at gmail dot com @ 2023-11-23 15:05 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112666
--- Comment #2 from Francisco Paisana <paisanafc at gmail dot com> ---
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.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug c++/112666] Missed optimization: Value initialization zero-initializes members with user-defined constructor
2023-11-22 8:58 [Bug c++/112666] New: Missed optimization: Value initialization zero-initializes members with user-defined constructor paisanafc at gmail dot com
2023-11-22 11:40 ` [Bug c++/112666] " redi at gcc dot gnu.org
2023-11-23 15:05 ` paisanafc at gmail dot com
@ 2023-11-23 15:08 ` paisanafc at gmail dot com
2023-11-23 15:37 ` paisanafc at gmail dot com
` (2 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: paisanafc at gmail dot com @ 2023-11-23 15:08 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112666
Francisco Paisana <paisanafc at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |FIXED
Status|UNCONFIRMED |RESOLVED
--- Comment #3 from Francisco Paisana <paisanafc at gmail dot com> ---
This ended up being a misinterpretation of the C++ standard.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug c++/112666] Missed optimization: Value initialization zero-initializes members with user-defined constructor
2023-11-22 8:58 [Bug c++/112666] New: Missed optimization: Value initialization zero-initializes members with user-defined constructor paisanafc at gmail dot com
` (2 preceding siblings ...)
2023-11-23 15:08 ` paisanafc at gmail dot com
@ 2023-11-23 15:37 ` paisanafc at gmail dot com
2023-11-23 16:34 ` redi at gcc dot gnu.org
2023-11-23 16:35 ` sjames at gcc dot gnu.org
5 siblings, 0 replies; 7+ messages in thread
From: paisanafc at gmail dot com @ 2023-11-23 15:37 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112666
--- Comment #4 from Francisco Paisana <paisanafc at gmail dot com> ---
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."
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug c++/112666] Missed optimization: Value initialization zero-initializes members with user-defined constructor
2023-11-22 8:58 [Bug c++/112666] New: Missed optimization: Value initialization zero-initializes members with user-defined constructor paisanafc at gmail dot com
` (3 preceding siblings ...)
2023-11-23 15:37 ` paisanafc at gmail dot com
@ 2023-11-23 16:34 ` redi at gcc dot gnu.org
2023-11-23 16:35 ` sjames at gcc dot gnu.org
5 siblings, 0 replies; 7+ messages in thread
From: redi at gcc dot gnu.org @ 2023-11-23 16:34 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112666
--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(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.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug c++/112666] Missed optimization: Value initialization zero-initializes members with user-defined constructor
2023-11-22 8:58 [Bug c++/112666] New: Missed optimization: Value initialization zero-initializes members with user-defined constructor paisanafc at gmail dot com
` (4 preceding siblings ...)
2023-11-23 16:34 ` redi at gcc dot gnu.org
@ 2023-11-23 16:35 ` sjames at gcc dot gnu.org
5 siblings, 0 replies; 7+ messages in thread
From: sjames at gcc dot gnu.org @ 2023-11-23 16:35 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112666
Sam James <sjames at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|FIXED |INVALID
^ permalink raw reply [flat|nested] 7+ messages in thread