9fcb1658ab
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.
103 lines
3.4 KiB
Rust
103 lines
3.4 KiB
Rust
use std::ptr;
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use smallvec::{Array, SmallVec};
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pub trait MapInPlace<T>: Sized {
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fn map_in_place<F>(&mut self, mut f: F) where F: FnMut(T) -> T {
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self.flat_map_in_place(|e| Some(f(e)))
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}
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fn flat_map_in_place<F, I>(&mut self, f: F)
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where F: FnMut(T) -> I,
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I: IntoIterator<Item=T>;
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}
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impl<T> MapInPlace<T> for Vec<T> {
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fn flat_map_in_place<F, I>(&mut self, mut f: F)
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where F: FnMut(T) -> I,
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I: IntoIterator<Item=T>
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{
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let mut read_i = 0;
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let mut write_i = 0;
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unsafe {
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let mut old_len = self.len();
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self.set_len(0); // make sure we just leak elements in case of panic
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while read_i < old_len {
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// move the read_i'th item out of the vector and map it
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// to an iterator
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let e = ptr::read(self.get_unchecked(read_i));
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let iter = f(e).into_iter();
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read_i += 1;
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for e in iter {
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if write_i < read_i {
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ptr::write(self.get_unchecked_mut(write_i), e);
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write_i += 1;
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} else {
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// If this is reached we ran out of space
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// in the middle of the vector.
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// However, the vector is in a valid state here,
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// so we just do a somewhat inefficient insert.
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self.set_len(old_len);
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self.insert(write_i, e);
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old_len = self.len();
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self.set_len(0);
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read_i += 1;
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write_i += 1;
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}
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}
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}
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// write_i tracks the number of actually written new items.
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self.set_len(write_i);
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}
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}
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}
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impl<T, A: Array<Item = T>> MapInPlace<T> for SmallVec<A> {
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fn flat_map_in_place<F, I>(&mut self, mut f: F)
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where F: FnMut(T) -> I,
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I: IntoIterator<Item=T>
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{
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let mut read_i = 0;
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let mut write_i = 0;
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unsafe {
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let mut old_len = self.len();
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self.set_len(0); // make sure we just leak elements in case of panic
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while read_i < old_len {
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// move the read_i'th item out of the vector and map it
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// to an iterator
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let e = ptr::read(self.get_unchecked(read_i));
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let iter = f(e).into_iter();
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read_i += 1;
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for e in iter {
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if write_i < read_i {
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ptr::write(self.get_unchecked_mut(write_i), e);
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write_i += 1;
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} else {
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// If this is reached we ran out of space
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// in the middle of the vector.
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// However, the vector is in a valid state here,
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// so we just do a somewhat inefficient insert.
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self.set_len(old_len);
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self.insert(write_i, e);
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old_len = self.len();
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self.set_len(0);
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read_i += 1;
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write_i += 1;
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}
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}
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}
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// write_i tracks the number of actually written new items.
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self.set_len(write_i);
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}
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}
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}
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