Anonymous allocations in statics get duplicated for multiple codegen units

[RalfJung]

According to analysis performed by @bjorn3, this code actually leads to two allocations containing 42, i.e., FOO and BAR point to different things. The linker later merges the two, so the issue is currently not directly observable. However, it becomes observable when things are mutable.

I think this is a bug. When a static is defined as pub static BAR: &i32 = crate::a::FOO;, IMO we should guarantee that BAR and FOO have the same value. I see no lee-way here for duplicating the memory they both point to.

Cc @oli-obk

[RalfJung]

IIUC #116564 is an attempt to fix this.

[RalfJung]

IIUC #116564 is an attempt to fix this.

I was wrong.

But I also don't think I understand this example. This is all in the same crate. We could evaluate the static FOO once, which will create two allocations, and then BAR points to the inner of these two. Where does the duplicate AllocId come from...?

[oli-obk]

Oh neat you found a single crate repro.

The issue is just that llvm treats alloc ids without a name as codegen unit local and thus generates one per unit. We need to give them a name, but we only want to give those a name that come from a static.

[RalfJung]

I didn't find it, you did. 😂

[RalfJung]

The issue is just that llvm treats alloc ids without a name as codegen unit local and thus generates one per unit. We need to give them a name, but we only want to give those a name that come from a static.

That example fails even with -Ccodegen-units=1 so multiple codegen units can't be the issue I think?

[RalfJung]

I think this happens because the codegen backend does not cache allocations. There is a function that codegens an allocation, and if the same AllocId is requested multiple times that just generates multiple LLVM allocations.

[oli-obk]

Ah good point. That won't help us cross crate tho

[oli-obk]

The issue is just that llvm treats alloc ids without a name as codegen unit local and thus generates one per unit. We need to give them a name, but we only want to give those a name that come from a static.

That example fails even with -Ccodegen-units=1 so multiple codegen units can't be the issue I think?

Yea, the modules are not needed: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=a447edb36a144bd8863b11f743a37937