[WIP] Fix incorrect LLVM Linkage enum

[mattico]

The Linkage enum in librustc_llvm got out of sync with the version in LLVM and it caused two variants of the #[linkage=""] attribute to break.

Fixes #33992

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[alexcrichton]

Ah the test looks like it may fail for two reasons:

• It's gonna be compiled as an executable, and lacking a main function will cause that to fail
• Are you sure all those linkage types are supported on all platforms? This could perhaps be a codegen test where we just test the LLVM IR output

[alexcrichton]

Thanks for the PR though! The fix looks fine to me :)

[brson]

Since this enum has changed it will break compatibility with any older versions of LLVM right? Should we also change the configure script to reject older LLVM versions?

[alexcrichton]

Oh I was assuming that we just never had the right definition. If this changed in LLVM over time, though, then we should define our own enum with our own discriminants and then translate in C++ to the LLVM symbolic names.

[mattico]

@alexcrichton Good points. I'll update the PR once I actually get the tests to run locally...

[mattico]

Turns out the build failures I was having were due to this change. I was basing the enum values off of the github master branch of the llvm repo and that is very different from the submodule included in the rust distribution. After double-triple-checking that the enum values line up between my local rust and the local rust's llvm, I get the following error about 100 times:

Only global variables can have appending linkage!
i1 (%"option::Option<usize>"*, %"option::Option<usize>"*)* @"_ZN70_$LT$core..option..Option$LT$T$GT$$u20$as$u20$core..cmp..PartialEq$GT$2ne17hdb2b03f033fe475fE"
Only global variables can have appending linkage!
void (%"char::EscapeDefault"*, %"char::EscapeDefault"*)* @"_ZN54_$LT$I$u20$as$u20$core..iter..traits..IntoIterator$GT$9into_iter17h46cca4884daa3d2bE"
Only global variables can have appending linkage!
i8 (%"fmt::Formatter"*, float*, i1)* @_ZN4core3fmt23float_to_decimal_common17h1cc684f615bc0ea2E
Only global variables can have appending linkage!
<snip>
Only global arrays can have appending linkage!
i64* @const27542
Only global arrays can have appending linkage!
i8* @const27560
Only global arrays can have appending linkage!
i8* @const27561
Only global arrays can have appending linkage!
i32* @const27562
Only global arrays can have appending linkage!
i32* @const27563
Only global arrays can have appending linkage!

I can't find anywhere in the stdlib or compiler that AppendingLinkage is used, so this is very puzzling to me.

It seems this change is not as simple as I had thought.

[alexcrichton]

Oh that may just be a case that's OK to elide, appending linkage I think is a pretty special LLVM-ism

[mattico]

@alexcrichton I'll try that.

You mentioned translating the rust enum to the LLVM enum in C++, would that code go in librustllvm?

[alexcrichton]

@mattico yeah, I was thinking something like:

// src/librustc_llvm/lib.rs
pub enum RustLinkage {
    External,
    // ...
}

void SetLinkage(LLVMValue V, unsigned RustLinkage) {
    switch (RustLinkage) {
        case 0:
            SetLLVMLinkage(v, LLVMExternalLinkage);
            break;
        // ...
    }
}

(or something like that)

[alexcrichton]

Closing due to inactivity, but feel free to reopen with comments addressed!