&[u8].to_owned() does not compile to a memcpy

[alexcrichton]

I imagine that something is happening along the way which is preventing this from becoming a memcpy.

~ $ cat foo.rs                                
use std::vec;

fn main() {
    let a = vec::from_fn(50 * 1024 * 1024, |_| 0u8);

    for _ in range(0, 30) {
        a.to_owned();
    }
}
~ $ cat bar.rs                                      
use std::vec;

fn main() {
    let a = vec::from_fn(50 * 1024 * 1024, |_| 0u8);

    for _ in range(0, 30) {
        let mut b = vec::with_capacity::<u8>(50 * 1024 * 1024);
        unsafe {
            vec::raw::set_len(&mut b, 50 * 1024 * 1024);
        }
        vec::bytes::copy_memory(b, a, a.len());
    }
}
~ $ rustc -O foo.rs                             
~ $ rustc -O bar.rs                                 
~ $ time ./foo                 
./foo  3.41s user 0.20s system 99% cpu 3.624 total
~ $ time ./bar                                        
./bar  0.38s user 0.20s system 97% cpu 0.593 total

The memcpy is a 6x speedup compared to to_owned()

[Aatch]

Looks like the write to the length in push when copying the elements over is making LLVM wary and preventing some optimisation opportunities.

Better alias analysis would help here.

[alexcrichton]

Closed by #13539, thanks @Aatch!

[alexcrichton]

(as in, the Vec case was fixed, and the ~[u8] case will become irrelevant in the near future)