Optimize Rc<T>::default

library/alloc/src/rc.rs

@@ -2312,7 +2312,16 @@ impl<T: Default> Default for Rc<T> {
  /// ```
  #[inline]
  fn default() -> Rc<T> {
- Rc::new(Default::default())
+ unsafe {
+ Self::from_inner(
+ Box::leak(Box::write(Box::new_uninit(), RcInner {
+ strong: Cell::new(1),
+ weak: Cell::new(1),
+ value: T::default(),
+ }))
+ .into(),
+ )
+ }
  }
 }
 

library/alloc/src/sync.rs

@@ -3447,13 +3447,16 @@ impl<T: Default> Default for Arc<T> {
  /// assert_eq!(*x, 0);
  /// ```
  fn default() -> Arc<T> {
- let x = Box::into_raw(Box::write(Box::new_uninit(), ArcInner {
- strong: atomic::AtomicUsize::new(1),
- weak: atomic::AtomicUsize::new(1),
- data: T::default(),
- }));
- // SAFETY: `Box::into_raw` consumes the `Box` and never returns null
- unsafe { Self::from_inner(NonNull::new_unchecked(x)) }
+ unsafe {
+ Self::from_inner(
+ Box::leak(Box::write(Box::new_uninit(), ArcInner {
+ strong: atomic::AtomicUsize::new(1),
+ weak: atomic::AtomicUsize::new(1),
+ data: T::default(),
+ }))
+ .into(),
+ )
+ }
  }
 }
 

tests/codegen/placement-new.rs

@@ -1,9 +1,11 @@
 //@ compile-flags: -O
+//@ compile-flags: -Zmerge-functions=disabled
 #![crate_type = "lib"]
 
 // Test to check that types with "complex" destructors, but trivial `Default` impls
 // are constructed directly into the allocation in `Box::default` and `Arc::default`.
 
+use std::rc::Rc;
 use std::sync::Arc;
 
 // CHECK-LABEL: @box_default_inplace
@@ -16,6 +18,16 @@ pub fn box_default_inplace() -> Box<(String, String)> {
  Box::default()
 }
 
+// CHECK-LABEL: @rc_default_inplace
+#[no_mangle]
+pub fn rc_default_inplace() -> Rc<(String, String)> {
+ // CHECK-NOT: alloca
+ // CHECK: [[RC:%.*]] = {{.*}}call {{.*}}__rust_alloc(
+ // CHECK-NOT: call void @llvm.memcpy
+ // CHECK: ret ptr [[RC]]
+ Rc::default()
+}
+
 // CHECK-LABEL: @arc_default_inplace
 #[no_mangle]
 pub fn arc_default_inplace() -> Arc<(String, String)> {