Incremental compilation fails when a generic function uses a private symbol

[mjbshaw]

The way that incremental compilation works breaks code that uses private symbols. For example, the following code will fail to compile when using incremental compilation (e.g. the default cargo build), but will build successfully when incremental compilation is disabled (e.g. the default cargo build --release):

// I have reproduced this issue for the following targets:
// mips-unknown-linux-gnu
// x86_64-unknown-linux-gnu
// x86_64-apple-darwin
// aarch64-apple-ios
fn foo<T>() {
    // See m:<mangling> at https://llvm.org/docs/LangRef.html#data-layout
    // This is only reproducible when using ELF, Mips, or Mach-O mangling.
    #[cfg_attr(all(target_os = "linux", not(target_arch = "mips")),
               export_name = "\x01.L_this_is_a_private_symbol")]
    #[cfg_attr(any(target_os = "macos", target_os = "ios"),
               export_name = "\x01L_this_is_a_private_symbol")]
    #[cfg_attr(target_arch = "mips", export_name = "\x01$_this_is_a_private_symbol")]
    static X: usize = 0;

    // Use the static symbol in a way that prevents the optimizer from
    // optimizing it away.
    unsafe { std::ptr::read_volatile(&X as *const _); }
}

fn main() {
    foo::<usize>();
}

Output (for both cargo build and cargo build --release):

$ cargo build
   Compiling zzz v0.1.0 (file:///private/tmp/zzz)
LLVM ERROR: unsupported relocation of undefined symbol 'L_this_is_a_private_symbol'
error: Could not compile `zzz`.

To learn more, run the command again with --verbose.
$ cargo build --release
   Compiling zzz v0.1.0 (file:///private/tmp/zzz)
    Finished release [optimized] target(s) in 0.26s

I have reproduced this issue for the following targets:

• mips-unknown-linux-gnu
• x86_64-unknown-linux-gnu
• x86_64-apple-darwin
• aarch64-apple-ios

I'm interested in fixing this issue, and have been trying to better understand librustc_mir/monomorphize/partitioning.rs (which I assume is where the problem is). I'll update this issue if I've made any progress, but I would appreciate any guidance anyone might be able to offer.

These private symbols are useful for low-level FFI work (e.g. the linker on macOS/iOS will deduplicate selector names only if they have a private symbol name).

[mjbshaw]

I forgot to mention what the cause of the error is:

The private symbol is partitioned into a different codegen unit than the generic function, which means the generic function is attempting to reference a different module's private symbol, which obviously fails.

I believe the correct fix would be to modify the partitioning code so that private symbols and functions that reference them are always partitioned together into a codegen unit.

[michaelwoerister]

Yes, the partitioning code does not really handle setting explicit linkage very well yet.

Incremental compilation will create two compilation units for each Rust module, one for the generic code and one for the non-generic code. So I think we would need to treat private symbols the same as inline functions: instantiate them in every CGU they are accessed in. However, that could have surprising. Consider the following example:

mod foo {
    #[linkage="private"]
    static mut FOO: usize = 0;

    fn inc_non_generic() {
        unsafe { FOO += 1; }
    }

    fn inc_generic<T>(stuff: &Vec<T>) {
        unsafe { FOO += stuff.len(); }
    }
}

You'd think that these access the same FOO but the two functions would actually each access a private copy. That's very much not what one would expect. So anything that accesses something with private or internal linkage would have to be forced into the same CGU. It's doable but it would be a bit of work.

[mjbshaw]

Yeah, as much as I would love to be able to inline a static (so it gets duplicated across codegen units), I too think that would be too surprising. I think Rust would need some kind of annotation that allowed developers to opt-in to that kind of behavior on a per-static basis.

For this particular issue at hand, I think a minimal solution would be acceptable. That is, a private static nested inside of a generic function should always go in the same codegen unit as the monomorphisations of that generic function. This minimal solution would suffice for my personal needs.

Extending that further, a slightly better solution (but I'm guessing more work) would be to make sure that private statics outside of the function (but within the same module) get put into the same codegen units as the functions that use them.

The "nuclear" solution is to make sure that all code (even separate modules) that uses private statics gets put into the same codegen unit. That sounds like the most work to me and I don't think it's currently worth it.

The implication of neither duplicating private statics nor merging code that uses them into a single codegen unit is that these functions cannot be inlined, or else linking will fail. The user must explicitly annotate them with #[inline(never)]. I think that's acceptable for the time being.

[michaelwoerister]

One thing we could experiment with -- and which would make sure that things always work as long as they are in the same module -- is to make incremental compilation not split generic and non-generic code into different CGUs. That might make compilation slower (because the cache is hit less often during incr. comp.) but it might also make compilation faster (because less per-CGU work has to be duplicated, such as instantiating inline functions). That would be a simple change to make and we could test it on perf.rust-lang.org before merging.

[michaelwoerister]

One thing we could experiment with -- and which would make sure that things always work as long as they are in the same module -- is to make incremental compilation not split generic and non-generic code into different CGUs.

I tried this out in #53963 but unfortunately the performance hit was too big.

[pnkfelix]

discussed in today's meeting; wg-incr-comp thinks this might be fixed...

[wesleywiser]

This does not seem to be fixed. cargo build now reports a slightly different error:

LLVM ERROR: Undefined temporary symbol .L_this_is_a_private_symbol