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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

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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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