nll experiment: compute SCCs instead of iterative region solving
This is an attempt to speed up region solving by replacing the current iterative dataflow with a SCC computation. The idea is to detect cycles (SCCs) amongst region constraints and then compute just one value per cycle. The graph with all cycles removed is of course a DAG, so we can then solve constraints "bottom up" once the liveness values are known.
I kinda ran out of time this morning so the last commit is a bit sloppy but I wanted to get this posted, let travis run on it, and maybe do a perf run, before I clean it up.
It was introduced in #50240 to avoid an allocation when creating a new
BTreeMap, which gave some speed-ups. But then #50352 made that the
default behaviour for BTreeMap, so LazyBTreeMap is no longer necessary.
Implement LazyBTreeMap and use it in a few places.
This is a thin wrapper around BTreeMap that avoids allocating upon creation.
I would prefer to change BTreeMap directly to make it lazy (like I did with HashSet in #36734) and I initially attempted that by making BTreeMap::root an Option<>. But then I also had to change Iter and Range to handle trees with no root, and those types have stability markers on them and I wasn't sure if that was acceptable. Also, BTreeMap has a lot of complex code and changing it all was challenging, and I didn't have high confidence about my general approach.
So I prototyped this wrapper instead and used it in the hottest locations to get some measurements about the effect. The measurements are pretty good!
- Doing a debug build of serde, it reduces the total number of heap allocations from 17,728,709 to 13,359,384, a 25% reduction. The number of bytes allocated drops from 7,474,672,966 to 5,482,308,388, a 27% reduction.
- It gives speedups of up to 3.6% on some rustc-perf benchmark jobs. crates.io, futures, and serde benefit most.
```
futures-check
avg: -1.9% min: -3.6% max: -0.5%
serde-check
avg: -2.1% min: -3.5% max: -0.7%
crates.io-check
avg: -1.7% min: -3.5% max: -0.3%
serde
avg: -2.0% min: -3.0% max: -0.9%
serde-opt
avg: -1.8% min: -2.9% max: -0.3%
futures
avg: -1.5% min: -2.8% max: -0.4%
tokio-webpush-simple-check
avg: -1.1% min: -2.2% max: -0.1%
futures-opt
avg: -1.2% min: -2.1% max: -0.4%
piston-image-check
avg: -0.8% min: -1.1% max: -0.3%
crates.io
avg: -0.6% min: -1.0% max: -0.3%
```
@Gankro, how do you think I should proceed here? Is leaving this as a wrapper reasonable? Or should I try to make BTreeMap itself lazy? If so, can I change the representation of Iter and Range?
Thanks!
Stabilize more APIs for the 1.20.0 release
In addition to the few stabilizations that have already landed, this cleans up the remaining APIs that are in `final-comment-period` right now to be stable by the 1.20.0 release
