Slower binaries since some days

[leonardo-m]

This could have the same causes of Issue 42808 (i.e. regression due to #42313), or perhaps not.

This small test program solves the Euler problem number 214:

#![feature(step_by)]

fn main() {
    fn e214() -> u64 {
        const N_MAX: usize = 40_000_000;
        const SIZE: usize = N_MAX / 2;
        const LEN: u8 = 25;

        let mut chain_length = vec![2u8; SIZE];
        let mut tot: u64 = 0;

        for i in 1 .. SIZE {
            // if not prime, then try next one
            if chain_length[i] > 2 { continue; }

            // p is prime
            let p = 2 * i + 1;

            // calculate q*2^k = p-1
            let mut q = i;
            let mut k = 1;
            while (q & 1) == 0 {
                q /= 2;
                k += 1;
            }

            // calculate cl = chain_length(p)
            let cl = chain_length[q / 2] + k;

            // add p to tot if the length matches.
            if cl == LEN { tot += p as u64; }

            // loop over powers of p.
            let mut pp = p as u64;
            while pp < N_MAX as u64 {
                // Loop over multiples of pp.
                for j in ((pp / 2) .. SIZE as u64).step_by(pp) {
                    chain_length[j as usize] += cl - 2;
                }
                pp *= p as u64;
            }
        }
        tot
    }
    assert_eq!(e214(), 1_677_366_278_943);
}

On my PC using nightly-2017-06-18-x86_64-pc-windows-gnu it runs in about 0.39 seconds, while using a more recent Nightly, like the current rustc 1.19.0-nightly (78d8416 2017-06-17), it runs in about 0.87 seconds.

[leonardo-m]

I compile with:
rustc -C opt-level=3 -C target-cpu=native -C panic=abort

[leonardo-m]

Old asm compared to the new asm:
https://gist.github.com/anonymous/c4708600946f04b87c8b9586b15e3017

(Moved to a gist after suggestion by @pnkfelix).

[pnkfelix]

I also observe a time slowdown on when compiling executing this benchmark on a Linux system (using the same flags -C opt-level=3 -C target-cpu=native -C panic=abort as documented in comments above), comparing e00c040 to 1143eb2

e00c040	1143eb2
real 0m0.265s	real 0m0.690s

(I note this largely to add the data point that this is not just a windows issue.)

[pnkfelix]

@leonardo-m it might be good to move those asm dumps to a gist; they're pretty long to scroll through on the thread...

[pnkfelix]

(Seems obvious from the asm dumps that one piece of low hanging fruit would be to add some inlining directives to Layout::from_size_align, I think...)

[leonardo-m]

I have few other similar small programs that got slower in this timeframe (but their slowdown is smaller. On average I've seen a 10%+ increase of run-time).

I also observe a time slowdown on when compiling this benchmark

Yes, compilation time of my programs is also increased (perhaps because the compiler as well is slower).

[pnkfelix]

Yes, compilation time of my programs is also increased (perhaps because the compiler as well is slower).

Sorry, I meant "executing this benchmark when compiled with new Allocator support." (The part I was trying to emphasize was that I duplicated the problem on a non-Windows platform)

[Mark-Simulacrum]

Yeah, this was picked up by perf.rlo as well. MIR end regions and then allocators slowed down compilation time in all crates, I believe.

[pnkfelix]

(yeah a little embarrassing to have authored both of those PRs and have to deal with the fallout now...)

[pnkfelix]

A collection of judiciously placed #[inline] attributes seems to completely fix the speed regression here.

[leonardo-m]

A collection of judiciously placed #[inline] attributes seems to completely fix the speed regression here.

Very nice. I'll test your changes when they are in a Nightly. And I'll report if there are other performance regressions.

[pnkfelix]

However, the #[inline] directives make the binary size regression for my microbenchmark on #42808 worse

[pnkfelix]

Also, it seems like @alexcrichton has independently added many of the same #[inline] directives in PR #42727, as he mentioned here

[pnkfelix]

(yes, I just built a rustc atop #42727, and it looks like it will definitely fix the execution time regression described in this issue.)

[leonardo-m]

Replacing the step_by feature with iterator_step_by, and using .step_by(pp as usize), now the best-of-three run-time is 0.47 seconds.

[pnkfelix]

Hmm indeed, seems like the running times regressed further between (734c83642 2017-07-03) and (720c596ec 2017-07-08) (or maybe I'm not adapting the benchmark correctly...)

[arielb1]

1.20 is now beta

[alexcrichton]

Looking into this, some things I've learned:

• Technically I don't think this is anything other than a nightly regresion due to the nightly feature usage. Despite this though if you use RUSTC_BOOTSTRAP=1 to test 1.19.0, 1.20.0, and 1.21.0, you can see that performance regressed in 1.20.0 and was fixed in 1.21.0.
• Historically this program used Range::step_by.
• The Range::step_by function was removed in #43012 (not in 1.20.0)
• The new method this program is using is Iterator::step_by, introduced in #41439 (not in 1.20.0)
• The StepBy iterator implementation calls the nth method to do iteration.
• The nth implementation for ops::Range calls the add_usize method
• The add_usize method calls TryFrom
• On 1.20.0 the number 2 function in the profile is a TryFrom-related method
• This method, which should be inlined, was inlined in #43248

tl;dr this is fixed, no need for a backport, closing.

[leonardo-m]

Thank you, this specific program is indeed fixed (run-time is now 0.40 s), but other performance regressions are not yet fixed, so I'll open other bug reports.

[alexcrichton]

Yes that'd be much appreciated, thanks @leonardo-m!