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Drop unsafe code from next_u64 #963

merged 1 commit into from
Apr 6, 2020


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@Shnatsel Shnatsel commented Apr 4, 2020

A better version of #961. This produces identical assembly as the version with the unsafe code, since LLVM is smart enough to optimize this into a single load. I've just disassembled the entire binary with objdump and compared outputs so that I don't end up looking at the wrong function like last time.

Code used for testing:

use rand;
use rand::Rng;
use rand_hc::Hc128Rng;
use rand::SeedableRng;

fn main() {
    let mut rng: Hc128Rng = Hc128Rng::from_entropy();
    let value: u64 = rng.gen();
    println!("{}", value);

I have also confirmed that this particular path is exercised by my test code by inserting panic!() in the function and observing that my test code does indeed panic.

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@dhardy dhardy left a comment

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Thanks @Shnatsel. LGTM and I don't recall why we did this in the first place. It may be that an earlier compiler wasn't so good at optimising?

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Shnatsel commented Apr 5, 2020

git blame led me to a huge commit moving code around, so I couldn't find the original reasoning. However, I've tested Rust 1.32 (current MSRV) and it does optimize this correctly.

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dhardy commented Apr 6, 2020

Lets go with this then. Thanks again for the fixes @Shnatsel!

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2 participants