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Comment by cakoose | Hacker News Reader

> I think as a straightforward, easily correct transition from 2000 globals, a giant structure isn't an awful idea.

Agree.

> It's not like the globals were organized before!

Using a struct with 2000 fields loses some encapsulation.

When a global is defined in a ".c" file (and not exported via a ".h" file), it can only be accessed in that one ".c" file, sort of like a "private" field in a class.

Switching to a single struct would mean that all globals can be accessed by all code.

There's probably a way to define things that allows you to regain some encapsulation, though. For example, some spin on the opaque type pattern: https://stackoverflow.com/a/29121847/163832

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No that is what a static in a .c file is for.

A plain global can be accessed from other compiled units - agreed with no .h entry it is my=uch more error prone e.g. you don't know the type but the variables name is exposed to other objects

Wouldn't those statics also be slated for removal with this change?

At most they'd be determined to be read only constants that are inlined during constant folding. This includes most integral sized / typed scalar values that fit into registers for the most part, and nothing you've taken the address of either - those remain as static data.

I think there might be a terminology mix-up here. In C, a global variable with the `static` keyword is is still mutable. So it typically can't be constant-folded/inlined.

The `static` modifier in that context just means that the symbol is not exported, so other ".c" files can't access it.

A static variable in C is mutable in the same sense that a local variable is, but since it's not visible outside the current compilation unit the optimizer is allowed to observe that it's never actually modified or published and constant fold it away.

Check out the generated assembly for this simple program, notice that kBase is folded even though it's not marked const: https://godbolt.org/z/h45vYo5x5

It is also possible for a link-time optimizer to observe that a non-static global variable is never modified and optimize that away too.

But the Postgres mailing list is talking about 2000 global variables being a hurdle to multi-threading. I doubt they just didn't realize that most of them can be optimized into constants.

Yea. Just about none of them could be optimized to constants because, uh, they're not constant. We're not perfect, but we do add const etc to TU level statics/globals that are actually read only. And if they are actually read only, we don't care about them in the context of threading anyway, since they wouldn't need any different behaviour anyway.

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