Bug 188146 – Use static global variables instead of static NeverDestroyed inside function if possible

• Status: RESOLVED
• Resolution: FIXED
• Component: New Bugs

[Yusuke Suzuki]

Use constexpr constructors more instead of NeverDestroyed

[Yusuke Suzuki]

Created attachment 346028 [details]
Patch

[Darin Adler]

Comment on attachment 346028 [details]
Patch

Change seems fine.

Bug title is a bit inaccurate.

Having a constexpr constructor does not affect whether NeverDestroyed is needed. It does affect whether we can have a global outside a function. If the constructor is not constexpr it needs to be inside a function so it gets run the first time the function is installed rather than at load time.

What affects the need for NeverDestroyed is whether the class has a trivial destructor. If it does then NeverDestroyed is not needed.

In all these cases in this patch we changed both things but they are independent.

You can have a static without NeverDestroyed if the destructor is trivial. You can have a global outside a function that uses NeverDestroyed as long as the entire constructor is all constexpr. (Might need to fix NeverDestroyed implementation to be sure it’s constructors are constexpr.)

[Darin Adler]

“Called” not “installed”

[Yusuke Suzuki]

(In reply to Darin Adler from comment #2)
> Comment on attachment 346028 [details]
> Patch
> 
> Change seems fine.
> 
> Bug title is a bit inaccurate.
> 
> Having a constexpr constructor does not affect whether NeverDestroyed is
> needed. It does affect whether we can have a global outside a function. If
> the constructor is not constexpr it needs to be inside a function so it gets
> run the first time the function is installed rather than at load time.
> 
> What affects the need for NeverDestroyed is whether the class has a trivial
> destructor. If it does then NeverDestroyed is not needed.
> 
> In all these cases in this patch we changed both things but they are
> independent.

Right, changed.

> 
> You can have a static without NeverDestroyed if the destructor is trivial.
> You can have a global outside a function that uses NeverDestroyed as long as
> the entire constructor is all constexpr. (Might need to fix NeverDestroyed
> implementation to be sure it’s constructors are constexpr.)

Currently, we cannot have constexpr NeverDestroyed, since it allocates an instance on aligned storage with placement `new`.
This type of code cannot be achieved inside constexpr constructor unfortunately :(

[Yusuke Suzuki]

Committed r234347: <https://trac.webkit.org/changeset/234347>

[Radar WebKit Bug Importer]

<rdar://problem/42710939>