Performance has gotten significantly worse

[brson]

Rustbot shows graph500-bfs, rustc, k-nucleotide, mandelbrot, and others as losing a lot of perf.

graph500-bfs went from a few seconds to thousands of seconds. Rustbot fingers 1d6fb24 as the culprit.

[eholk]

It seems like there's a good chance this is related to me moving vector append into the library. I'm making a few tweaks now that should improve performance.

[eholk]

Actually, after looking at the graphs some more, it looks like my change isn't responsible for the latest perf regression. That said, there's still a few v += [x] expressions that can be replaced with push(v, x).

[msullivan]

Tracked it down. It looks like the libcore vector code was (accidentally) not getting called until my patch landed.

So the regression is due to moving vec append to the library, and the extra copies that causes.

[eholk]

I just did a grep for += [ over the whole tree, and there are still a lot of singleton vector appends to track down. This should win back the perf we lost on rustc.

How hard would it be to let us pass self by - or +? This seems like a better long term fix than telling everyone just to use vec::push.

The issue for allowing modes on self is #2585.

[pcwalton]

I think dvec is probably the real long-term fix, no?

[eholk]

@msullivan just pointed out that vec::push_all and vec::append are doing a lot of unnecessary bounds checks that could probably be removed using unsafe code. This could gain some of our speed back.

[catamorphism]

@eholk and @msullivan : is this resolved for now?

[eholk]

The perf bots are still showing that we're about 5 seconds behind where we used to be on rustc. There's definitely more work to do, but to me the performance is tolerable again.

I can keep trying to win back those 5 seconds if people don't want to take the hit.

[eholk]

I replaced pretty much all the calls to + with various append, push, push_all, append_one calls, and now rustc is faster than it was before.

[brson]

Nice work @eholk!