This reduces by 12% the number of allocations done for a "clean
incremental" of `webrender_api`, which reduces the instruction count by
about 0.5%.
It also reduces instruction counts by up to 1.4% across a range of
rustc-perf benchmark runs.
Overhaul `syntax::fold::Folder`.
This PR changes `syntax::fold::Folder` from a functional style
(where most methods take a `T` and produce a new `T`) to a more
imperative style (where most methods take and modify a `&mut T`), and
renames it `syntax::mut_visit::MutVisitor`.
This makes the code faster and more concise.
This commit changes `syntax::fold::Folder` from a functional style
(where most methods take a `T` and produce a new `T`) to a more
imperative style (where most methods take and modify a `&mut T`), and
renames it `syntax::mut_visit::MutVisitor`.
The first benefit is speed. The functional style does not require any
reallocations, due to the use of `P::map` and
`MoveMap::move_{,flat_}map`. However, every field in the AST must be
overwritten; even those fields that are unchanged are overwritten with
the same value. This causes a lot of unnecessary memory writes. The
imperative style reduces instruction counts by 1--3% across a wide range
of workloads, particularly incremental workloads.
The second benefit is conciseness; the imperative style is usually more
concise. E.g. compare the old functional style:
```
fn fold_abc(&mut self, abc: ABC) {
ABC {
a: fold_a(abc.a),
b: fold_b(abc.b),
c: abc.c,
}
}
```
with the imperative style:
```
fn visit_abc(&mut self, ABC { a, b, c: _ }: &mut ABC) {
visit_a(a);
visit_b(b);
}
```
(The reductions get larger in more complex examples.)
Overall, the patch removes over 200 lines of code -- even though the new
code has more comments -- and a lot of the remaining lines have fewer
characters.
Some notes:
- The old style used methods called `fold_*`. The new style mostly uses
methods called `visit_*`, but there are a few methods that map a `T`
to something other than a `T`, which are called `flat_map_*` (`T` maps
to multiple `T`s) or `filter_map_*` (`T` maps to 0 or 1 `T`s).
- `move_map.rs`/`MoveMap`/`move_map`/`move_flat_map` are renamed
`map_in_place.rs`/`MapInPlace`/`map_in_place`/`flat_map_in_place` to
reflect their slightly changed signatures.
- Although this commit renames the `fold` module as `mut_visit`, it
keeps it in the `fold.rs` file, so as not to confuse git. The next
commit will rename the file.
This requires a pre-pass over the input streams. But that is cheap
compared to the quadratic blowup associated with reallocating the
accumulating vector on-the-fly.
`TokenStream::Stream` can represent a token stream containing any number
of token trees. `TokenStream::Tree` is the special case representing a
single token tree. The latter doesn't occur all that often dynamically,
so this commit removes it, which simplifies the code quite a bit.
This change has mixed performance effects.
- The size of `TokenStream` drops from 32 bytes to 8 bytes, and there
is one less case for all the match statements.
- The conversion of a `TokenTree` to a `TokenStream` now requires two
allocations, for the creation of a single element Lrc<Vec<_>>. (But a
subsequent commit in this PR will reduce the main source of such
conversions.)
`TokenStream` is currently recursive in *two* ways:
- the `TokenTree` variant contains a `ThinTokenStream`, which can
contain a `TokenStream`;
- the `TokenStream` variant contains a `Vec<TokenStream>`.
The latter is not necessary and causes significant complexity. This
commit replaces it with the simpler `Vec<(TokenTree, IsJoint)>`.
This reduces complexity significantly. In particular, `StreamCursor` is
eliminated, and `Cursor` becomes much simpler, consisting now of just a
`TokenStream` and an index.
The commit also removes the `Extend` impl for `TokenStream`, because it
is only used in tests. (The commit also removes those tests.)
Overall, the commit reduces the number of lines of code by almost 200.
Remove `TokenStream::JointTree`.
This is done by adding a new `IsJoint` field to `TokenStream::Tree`,
which simplifies a lot of `match` statements. And likewise for
`CursorKind`.
The commit also adds a new method `TokenTree:stream()` which can replace
a choice between `.into()` and `.joint()`.
This is done by adding a new `IsJoint` field to `TokenStream::Tree`,
which simplifies a lot of `match` statements. And likewise for
`CursorKind`.
The commit also adds a new method `TokenTree:stream()` which can replace
a choice between `.into()` and `.joint()`.
