Control visibility of dylib symbols

[mprobinson]

Compiling a dylib on Linux, I expected only extern fns in the root crate to be visible. Instead every public symbol in every crate I used was visible, and no dead code was eliminated, resulting in a very bloated dylib.

I was able to fix this by creating a linker version script with only the intended public symbols in the global section and passing linker arguments:
-C link-args="-Wl,-version-script=version.script -Wl,-gc-sections"

This works but it requires extra manual effort maintaining the version script and it is not cross-platform. It would be nice if Rust supported this natively.

[alexchandel]

dynamic linking is evil anyways

[mprobinson]

I'm writing to an existing C API so I have no choice but to use it.

[alexchandel]

You're linking against a dylib, or you're producing one for consumption by other C libraries? If possible, just have all your consumers statically link against your library.

In #14344, the linker unintentionally optimizes away unused symbols. It's interesting that here, it's not removing any.

[mprobinson]

I'm producing one for consumption by C code, and it's not possible for me to have all consumers statically link.

[mprobinson]

As an example of the problem, consider this code:

#![no_std]
#![feature(lang_items, globs)]
#![crate_type = "dylib"]

extern crate core;
extern crate collections;
use core::prelude::*;
use collections::string::String;

#[no_mangle]
pub extern "C" fn get_len() -> uint {
    let mut s = String::from_str("Test");
    s.push('_'); // Do enough work to prevent LLVM optimizing it out
    s.len()
}

#[lang = "stack_exhausted"] extern fn stack_exhausted() { }
#[lang = "eh_personality"] extern fn eh_personality() { }
#[lang = "panic_fmt"] fn panic_fmt() -> ! { loop { } }

Compiling with rustc --opt-level 3 lib.rs:

$ ls -hs liblib.so
1.5M liblib.so

Compiling with rustc --opt-level 3 -C link-args="-Wl,-version-script=version.script -Wl,-gc-sections" lib.rs, and version.script containing only get_len in the global section:

$ ls -hs liblib.so 
328K liblib.so

GCC supports -fvisibility=hidden and __attribute__ ((visibility ("default"))) for this, could Rust support something similar?

[mprobinson]

The correct solution is to enable dylib LTO, so closing in favor of #18863.