Auto merge of rust-lang#87777 - the8472:fix-mir-max-rss, r=oli-obk,joshtriplett

Use zeroed allocations in the mir interpreter instead eagerly touching the memory

rust-lang#86255 introduced a 30% regression in [page faults](https://perf.rust-lang.org/compare.html?start=64ae15ddd3f3cca7036ab2b2f3a6b130b62af4da&end=39e20f1ae5f13451eb35247808d6a2527cb7d060&stat=faults
) and a 3% regression in [max-rss](https://perf.rust-lang.org/index.html?start=2021-07-01&end=&absolute=false&stat=max-rss) in the ctfe-stress benchmarks.
That's most likely happened because it separated allocation from initialization of the vec which defeats the zero-optimization.

Currently there's no allocation API that is fallible, zeroing and returns a slice, so this PR introduces one and then uses that to solve the problem. In principle `vec.resize(len, 0)` could be optimized to use `alloc::grow_zeroed` where appropriate but that would require new specializations and new plumbing in `RawVec`.

Author: bors

alloc/src/boxed.rs

@@ -157,7 +157,6 @@ use crate::alloc::{handle_alloc_error, WriteCloneIntoRaw};
 use crate::alloc::{AllocError, Allocator, Global, Layout};
 #[cfg(not(no_global_oom_handling))]
 use crate::borrow::Cow;
-#[cfg(not(no_global_oom_handling))]
 use crate::raw_vec::RawVec;
 #[cfg(not(no_global_oom_handling))]
 use crate::str::from_boxed_utf8_unchecked;
@@ -589,6 +588,71 @@ impl<T> Box<[T]> {
     pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
         unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
     }
+
+    /// Constructs a new boxed slice with uninitialized contents. Returns an error if
+    /// the allocation fails
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api, new_uninit)]
+    ///
+    /// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
+    /// let values = unsafe {
+    ///     // Deferred initialization:
+    ///     values[0].as_mut_ptr().write(1);
+    ///     values[1].as_mut_ptr().write(2);
+    ///     values[2].as_mut_ptr().write(3);
+    ///     values.assume_init()
+    /// };
+    ///
+    /// assert_eq!(*values, [1, 2, 3]);
+    /// # Ok::<(), std::alloc::AllocError>(())
+    /// ```
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[inline]
+    pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
+        unsafe {
+            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
+                Ok(l) => l,
+                Err(_) => return Err(AllocError),
+            };
+            let ptr = Global.allocate(layout)?;
+            Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
+        }
+    }
+
+    /// Constructs a new boxed slice with uninitialized contents, with the memory
+    /// being filled with `0` bytes. Returns an error if the allocation fails
+    ///
+    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
+    /// of this method.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api, new_uninit)]
+    ///
+    /// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
+    /// let values = unsafe { values.assume_init() };
+    ///
+    /// assert_eq!(*values, [0, 0, 0]);
+    /// # Ok::<(), std::alloc::AllocError>(())
+    /// ```
+    ///
+    /// [zeroed]: mem::MaybeUninit::zeroed
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[inline]
+    pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
+        unsafe {
+            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
+                Ok(l) => l,
+                Err(_) => return Err(AllocError),
+            };
+            let ptr = Global.allocate_zeroed(layout)?;
+            Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
+        }
+    }
 }
 
 impl<T, A: Allocator> Box<[T], A> {