Identical code folding

[Kobzol]

Since Rust can't do polymorphization properly yet, using generics generates a lot of duplicated functions because of monomorphization. These functions take space in the binary, even though they have completely the same instructions.

Some linkers (gold, lld) can deduplicate these identical functions using Identical Code Folding, and thus reduce binary size (and potentially also improve usage of the i-cache).

You can specify this linker option using a linker flag, for example like this:

$ RUSTFLAGS="-Clink-args=-fuse-ld=lld -Clink-args=-Wl,--icf=all" cargo build

I measured the binary size change for the following program:

fn foo() {
    let mut a: Vec<u8> = vec![1, 2, 3];
    a.pop();

    let mut b: Vec<u32> = vec![1, 2, 3];
    b.pop();

    let mut c: Vec<i32> = vec![1, 2, 3];
    c.pop();
}

fn main() {
    foo();
}

Here are binary sizes are after running strip on them:

Linker	Mode	Binary size (B)	ICF (Identical Code Folding)
gold	debug	342696	No
gold	debug	330408	Yes
gold	release	322216	No
gold	release	318120	Yes
lld	debug	330968	No
lld	debug	321840	Yes
lld	release	310616	No
lld	release	306848	Yes

[johnthagen]

It would be interesting to see how this interacts with -Z build-std=std,panic_abort as then the linker might also be able to fold identical code in std as well? 🤔

[Kobzol]

I tried it, but it didn't change the binary size. I suppose that's expected, since the program links statically to libstd, the code of libstd is already inside the binary when ICF runs, so it's applied to it even without build-std. It's done on the linker/binary level, so it won't do much for libstd unless we link to it as a dynamic library I guess :)