Auto merge of #120050 - scottmcm:vec-resize-memset, r=<try>

`Vec::resize` for bytes should be a single `memset`

Really I just started by trying to see if specializing `iter::repeat_n` would help the perf issue that kept me from removing `Vec::extend_with` last time I tried, but I noticed in the process that a resize for bytes doesn't set all the new space with a single `memset`: <https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=35175ec844b46fcd95e2d0aad526859e>

So using `repeat_n` to implement it -- like `VecDeque` uses, with the specialization for `next` to avoid a branch -- means that the optimizer notices the resize can set all the values with a single memset.

library/alloc/src/vec/mod.rs

@@ -2448,7 +2448,7 @@ impl<T: Clone, A: Allocator> Vec<T, A> {
         let len = self.len();
 
         if new_len > len {
-            self.extend_with(new_len - len, value)
+            self.extend_trusted(core::iter::repeat_n(value, new_len - len));
         } else {
             self.truncate(new_len);
         }
@@ -2562,38 +2562,6 @@ impl<T, A: Allocator, const N: usize> Vec<[T; N], A> {
     }
 }
 
-impl<T: Clone, A: Allocator> Vec<T, A> {
-    #[cfg(not(no_global_oom_handling))]
-    /// Extend the vector by `n` clones of value.
-    fn extend_with(&mut self, n: usize, value: T) {
-        self.reserve(n);
-
-        unsafe {
-            let mut ptr = self.as_mut_ptr().add(self.len());
-            // Use SetLenOnDrop to work around bug where compiler
-            // might not realize the store through `ptr` through self.set_len()
-            // don't alias.
-            let mut local_len = SetLenOnDrop::new(&mut self.len);
-
-            // Write all elements except the last one
-            for _ in 1..n {
-                ptr::write(ptr, value.clone());
-                ptr = ptr.add(1);
-                // Increment the length in every step in case clone() panics
-                local_len.increment_len(1);
-            }
-
-            if n > 0 {
-                // We can write the last element directly without cloning needlessly
-                ptr::write(ptr, value);
-                local_len.increment_len(1);
-            }
-
-            // len set by scope guard
-        }
-    }
-}
-
 impl<T: PartialEq, A: Allocator> Vec<T, A> {
     /// Removes consecutive repeated elements in the vector according to the
     /// [`PartialEq`] trait implementation.

library/alloc/src/vec/spec_from_elem.rs

@@ -13,7 +13,7 @@ pub(super) trait SpecFromElem: Sized {
 impl<T: Clone> SpecFromElem for T {
     default fn from_elem<A: Allocator>(elem: Self, n: usize, alloc: A) -> Vec<Self, A> {
         let mut v = Vec::with_capacity_in(n, alloc);
-        v.extend_with(n, elem);
+        v.extend_trusted(core::iter::repeat_n(elem, n));
         v
     }
 }
@@ -25,7 +25,7 @@ impl<T: Clone + IsZero> SpecFromElem for T {
             return Vec { buf: RawVec::with_capacity_zeroed_in(n, alloc), len: n };
         }
         let mut v = Vec::with_capacity_in(n, alloc);
-        v.extend_with(n, elem);
+        v.extend_trusted(core::iter::repeat_n(elem, n));
         v
     }
 }

library/core/src/iter/sources/repeat_n.rs

@@ -110,6 +110,34 @@ impl<A> Drop for RepeatN<A> {
     }
 }
 
+trait SpecRepeatN<A> {
+    unsafe fn spec_next_unchecked(&mut self) -> A;
+}
+
+impl<A: Clone> SpecRepeatN<A> for RepeatN<A> {
+    default unsafe fn spec_next_unchecked(&mut self) -> A {
+        self.count -= 1;
+
+        if self.count == 0 {
+            // SAFETY: we just lowered the count to zero so it won't be dropped
+            // later, and thus it's okay to take it here.
+            unsafe { ManuallyDrop::take(&mut self.element) }
+        } else {
+            A::clone(&self.element)
+        }
+    }
+}
+
+impl<A: Copy> SpecRepeatN<A> for RepeatN<A> {
+    unsafe fn spec_next_unchecked(&mut self) -> A {
+        self.count -= 1;
+
+        // For `Copy` types, we can always just read the item directly,
+        // so skip having a branch that would need to be optimized out.
+        *self.element
+    }
+}
+
 #[unstable(feature = "iter_repeat_n", issue = "104434")]
 impl<A: Clone> Iterator for RepeatN<A> {
     type Item = A;
@@ -120,15 +148,8 @@ impl<A: Clone> Iterator for RepeatN<A> {
             return None;
         }
 
-        self.count -= 1;
-        Some(if self.count == 0 {
-            // SAFETY: the check above ensured that the count used to be non-zero,
-            // so element hasn't been dropped yet, and we just lowered the count to
-            // zero so it won't be dropped later, and thus it's okay to take it here.
-            unsafe { ManuallyDrop::take(&mut self.element) }
-        } else {
-            A::clone(&self.element)
-        })
+        // SAFETY: Just confirmed above that the iterator is non-empty
+        unsafe { Some(self.spec_next_unchecked()) }
     }
 
     #[inline]

tests/codegen/vec-resize-memset.rs

@@ -0,0 +1,13 @@
+// compile-flags: -O
+// no-system-llvm
+// only-64bit
+// ignore-debug (the extra assertions get in the way)
+
+#![crate_type = "lib"]
+
+pub fn resize_with_bytes_is_one_memset(x: &mut Vec<u8>) {
+    let new_len = x.len() + 456789;
+    x.resize(new_len, 123);
+}
+
+// CHECK: call void @llvm.memset.p0.i64({{.+}}, i8 123, i64 456789, i1 false)