rust: Import LLD for linking wasm objects by alexcrichton · Pull Request #48125 · rust-lang/rust (original) (raw)

It's also important to note that this PR actually regresses the apparent hello size of "hello world" in wasm. Fear not, though, this is intended and not as bad as it seems! Today this program:

#![crate_type = "cdylib"]

#[no_mangle]
pub extern fn foo(a: u32) -> u32 { a + 1
}

generates a 17k hello world with rustc +nightly foo.rs -O --target wasm32-unknown-unknown. Now note that 17k is also way too big in this case! The 17k, though, is due to a lack of a linker (aka lld) to remove all the extra compiler-rt intrinsics by default. When you run wasm-gc this hello world today reduces to 137 bytes as you'd expect.

With this PR, however, the size of the first compilation is 633k (!) and with wasm-gc it only reduces to 209k (!). Not to fear, though, this is expected! This "bug" is due to the fact that the release_60 branch of LLD does not yet implement --gc-sections for wasm, only for other targets. As a result, we're not invoking LLD with --gc-sections yet. The way to recover this size difference is to actually do what wasm is already doing today, compile with LTO! Compiling with rustc +nightly foo.rs -C lto -O --target wasm32-unknown-unknown. This yield a 149 byte executable without running wasm-gc. The gc step can slim it down by a few dozen bytes, but there's also some bugs for us to fix in rustc.

To recap, here's the size of the program above in various stages:

As always for the smallest binaries it's recommended to compile with -O -C lto and run wasm-gc, and while this PR will drastically change the initial state it'll get better when LLD is upgraded again!