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Extra allocations are a significant cost of NLL, and the most common
ones come from within `Canonicalizer`. In particular, `canonical_var()`
contains this code:
indices
.entry(kind)
.or_insert_with(|| {
let cvar1 = variables.push(info);
let cvar2 = var_values.push(kind);
assert_eq!(cvar1, cvar2);
cvar1
})
.clone()
`variables` and `var_values` are `Vec`s. `indices` is a `HashMap` used
to track what elements have been inserted into `var_values`. If `kind`
hasn't been seen before, `indices`, `variables` and `var_values` all get
a new element. (The number of elements in each container is always the
same.) This results in lots of allocations.
In practice, most of the time these containers only end up holding a few
elements. This PR changes them to avoid heap allocations in the common
case, by changing the `Vec`s to `SmallVec`s and only using `indices`
once enough elements are present. (When the number of elements is small,
a direct linear search of `var_values` is as good or better than a
hashmap lookup.)
The changes to `variables` are straightforward and contained within
`Canonicalizer`. The changes to `indices` are more complex but also
contained within `Canonicalizer`. The changes to `var_values` are more
intrusive because they require defining a new type
`SmallCanonicalVarValues` -- which is to `CanonicalVarValues` as
`SmallVec` is to `Vec -- and passing stack-allocated values of that type
in from outside.
All this speeds up a number of NLL "check" builds, the best by 2%.
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convert NLL ops to caches
This is a extension of <https://github.com/rust-lang/rust/pull/51460>. It uses a lot more caching than we used to do. This caching is not yet as efficient as it could be, but I'm curious to see the current perf results.
This is the high-level idea: in the MIR type checker, use [canonicalized queries](https://rust-lang-nursery.github.io/rustc-guide/traits/canonical-queries.html) for all the major operations. This is helpful because the MIR type check is operating in a context where all types are fully known (mostly, anyway) but regions are completely renumbered. This means we often wind up with duplicate queries like `Foo<'1, '2> :Bar` and `Foo<'3, '4>: Bar`. Canonicalized queries let us re-use the results. By the final commit in this PR, we can essentially just "read off" the resulting region relations and add them to the NLL type check.
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This lets us simplify a few type aliases.
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Notably, excluding ReSkolemized
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It is backed by the new `normalize_projection_ty` query, which uses
canonicalization.
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