Adding --emit=asm speeds up generated code because of codegen units

[jrmuizel]

With the following rust code:

pub fn main() {
    print_triples();
    println!("hello");
}


fn print_triples() {
    let mut i = 0 as i32;
    for z in 1.. {
        for x in 1..=z {
            for y in x..=z {
                if x*x + y*y == z*z {
                    i = i + 1;
                    if i == 1000 {
                        return;
                    }
                }
            }
        }
    }
}

I get:

/tmp/pythagoras$ rustc --emit=link -O simple.rs
/tmp/pythagoras$ time ./simple
hello

real	0m0.290s
user	0m0.287s
sys	0m0.002s
/tmp/pythagoras$ rustc --emit=asm,link -O simple.rs
/tmp/pythagoras$ time ./simple
hello

real	0m0.005s
user	0m0.002s
sys	0m0.002s
/tmp/pythagoras$ rustc --version
rustc 1.32.0-nightly (400c2bc5e 2018-11-27)
/tmp/pythagoras$

[jonas-schievink]

AFAIK this forces rustc to use a single codegen unit. This generally makes code run faster because every function is available for inlining, although I don't see what might cause such a drastic difference.

You can try to reproduce by using -Ccodegen-units=1.

[jrmuizel]

Indeed -Ccodegen-units=1 fixes the problem. It's pretty surprising/dangerous that --emit=asm changes the generated code. Why is a single codegen unit forced with --emit=asm?

[jonas-schievink]

This was done in #30208. Multiple codegen units would result in multiple compilation outputs, which is generally not expected when using --emit (which should only output a single file).

EDIT: Also see #30063, which is now obsolete since the build system changed, but the discussion there is still relevant.

[nikic]

@jrmuizel I think the answer is that nobody has implemented the necessary handling for that. If there are multiple codegen units and we want to produce a single artifact, we'd have to merge the LLVM modules prior to emitting IR/BC/asm (unless LTO, either thin or fat, already takes care of that).

I agree that the current behavior is not great, as these are often used for debugging performance issues and changing the number of codegen-units can impact optimization a lot.

[lambda]

One thing to note is that the difference in speed listed here is a bit artificial; this program doesn't actually do anything in the print_triples loop, so it looks like with one codegen unit, it's eliminated entirely, while with the default settings its actually running the loop even though it has no effect.