Auto merge of #37270 - Mark-Simulacrum:smallvec-optimized-arenas, r=eddyb

Add ArrayVec and AccumulateVec to reduce heap allocations during interning of slices

Updates `mk_tup`, `mk_type_list`, and `mk_substs` to allow interning directly from iterators. The previous PR, #37220, changed some of the calls to pass a borrowed slice from `Vec` instead of directly passing the iterator, and these changes further optimize that to avoid the allocation entirely.

This change yields 50% less malloc calls in [some cases](https://pastebin.mozilla.org/8921686). It also yields decent, though not amazing, performance improvements:
```
futures-rs-test  4.091s vs  4.021s --> 1.017x faster (variance: 1.004x, 1.004x)
helloworld       0.219s vs  0.220s --> 0.993x faster (variance: 1.010x, 1.018x)
html5ever-2016-  3.805s vs  3.736s --> 1.018x faster (variance: 1.003x, 1.009x)
hyper.0.5.0      4.609s vs  4.571s --> 1.008x faster (variance: 1.015x, 1.017x)
inflate-0.1.0    3.864s vs  3.883s --> 0.995x faster (variance: 1.232x, 1.005x)
issue-32062-equ  0.309s vs  0.299s --> 1.033x faster (variance: 1.014x, 1.003x)
issue-32278-big  1.614s vs  1.594s --> 1.013x faster (variance: 1.007x, 1.004x)
jld-day15-parse  1.390s vs  1.326s --> 1.049x faster (variance: 1.006x, 1.009x)
piston-image-0. 10.930s vs 10.675s --> 1.024x faster (variance: 1.006x, 1.010x)
reddit-stress    2.302s vs  2.261s --> 1.019x faster (variance: 1.010x, 1.026x)
regex.0.1.30     2.250s vs  2.240s --> 1.005x faster (variance: 1.087x, 1.011x)
rust-encoding-0  1.895s vs  1.887s --> 1.005x faster (variance: 1.005x, 1.018x)
syntex-0.42.2   29.045s vs 28.663s --> 1.013x faster (variance: 1.004x, 1.006x)
syntex-0.42.2-i 13.925s vs 13.868s --> 1.004x faster (variance: 1.022x, 1.007x)
```

We implement a small-size optimized vector, intended to be used primarily for collection of presumed to be short iterators. This vector cannot be "upsized/reallocated" into a heap-allocated vector, since that would require (slow) branching logic, but during the initial collection from an iterator heap-allocation is possible.

We make the new `AccumulateVec` and `ArrayVec` generic over implementors of the `Array` trait, of which there is currently one, `[T; 8]`. In the future, this is likely to expand to other values of N.

Huge thanks to @nnethercote for collecting the performance and other statistics mentioned above.

1 files changed, 106 insertions, 0 deletions

diff --git a/src/librustc_data_structures/array_vec.rs b/src/librustc_data_structures/array_vec.rs
new file mode 100644
index 00000000000..f87426cee59
--- /dev/null
+++ b/src/librustc_data_structures/array_vec.rs
@@ -0,0 +1,106 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! A stack-allocated vector, allowing storage of N elements on the stack.
+//!
+//! Currently, only the N = 8 case is supported (due to Array only being impl-ed for [T; 8]).
+
+use std::marker::Unsize;
+use std::iter::Extend;
+use std::ptr::drop_in_place;
+use std::ops::{Deref, DerefMut};
+use std::slice;
+use std::fmt;
+
+pub unsafe trait Array {
+    type Element;
+    type PartialStorage: Default + Unsize<[ManuallyDrop<Self::Element>]>;
+    const LEN: usize;
+}
+
+unsafe impl<T> Array for [T; 8] {
+    type Element = T;
+    type PartialStorage = [ManuallyDrop<T>; 8];
+    const LEN: usize = 8;
+}
+
+pub struct ArrayVec<A: Array> {
+    count: usize,
+    values: A::PartialStorage
+}
+
+impl<A: Array> ArrayVec<A> {
+    pub fn new() -> Self {
+        ArrayVec {
+            count: 0,
+            values: Default::default(),
+        }
+    }
+}
+
+impl<A> fmt::Debug for ArrayVec<A>
+    where A: Array,
+          A::Element: fmt::Debug {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self[..].fmt(f)
+    }
+}
+
+impl<A: Array> Deref for ArrayVec<A> {
+    type Target = [A::Element];
+    fn deref(&self) -> &Self::Target {
+        unsafe {
+            slice::from_raw_parts(&self.values as *const _ as *const A::Element, self.count)
+        }
+    }
+}
+
+impl<A: Array> DerefMut for ArrayVec<A> {
+    fn deref_mut(&mut self) -> &mut [A::Element] {
+        unsafe {
+            slice::from_raw_parts_mut(&mut self.values as *mut _ as *mut A::Element, self.count)
+        }
+    }
+}
+
+impl<A: Array> Drop for ArrayVec<A> {
+    fn drop(&mut self) {
+        unsafe {
+            drop_in_place(&mut self[..])
+        }
+    }
+}
+
+impl<A: Array> Extend<A::Element> for ArrayVec<A> {
+    fn extend<I>(&mut self, iter: I) where I: IntoIterator<Item=A::Element> {
+        for el in iter {
+            unsafe {
+                let arr = &mut self.values as &mut [ManuallyDrop<_>];
+                arr[self.count].value = el;
+            }
+            self.count += 1;
+        }
+    }
+}
+
+// FIXME: This should use repr(transparent) from rust-lang/rfcs#1758.
+#[allow(unions_with_drop_fields)]
+pub union ManuallyDrop<T> {
+    value: T,
+    #[allow(dead_code)]
+    empty: (),
+}
+
+impl<T> Default for ManuallyDrop<T> {
+    fn default() -> Self {
+        ManuallyDrop { empty: () }
+    }
+}
+