allocate before calling T::default in <Arc<T>>::default()

Same rationale as in the previous commit.
rust-lang · Oct 9, 2024 · f72c897 · f72c897
1 parent 2f4ef71
commit f72c897
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Showing 2 changed files with 9 additions and 3 deletions.
diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
@@ -3447,7 +3447,13 @@ impl<T: Default> Default for Arc<T> {
     /// assert_eq!(*x, 0);
     /// ```
     fn default() -> Arc<T> {
-        Arc::new(Default::default())
+        let x = Box::into_non_null(Box::write(Box::new_uninit(), ArcInner {
+            strong: atomic::AtomicUsize::new(1),
+            weak: atomic::AtomicUsize::new(1),
+            data: T::default(),
+        }));
+        // SAFETY: `Box::into_non_null` consumes the `Box`
+        unsafe { Self::from_inner(x) }
     }
 }
 

diff --git a/tests/codegen/placement-new.rs b/tests/codegen/placement-new.rs
@@ -19,9 +19,9 @@ pub fn box_default_inplace() -> Box<(String, String)> {
 // CHECK-LABEL: @arc_default_inplace
 #[no_mangle]
 pub fn arc_default_inplace() -> Arc<(String, String)> {
-    // CHECK: [[ALLOCA:%.*]] = alloca
+    // CHECK-NOT: alloca
     // CHECK: [[ARC:%.*]] = {{.*}}call {{.*}}__rust_alloc(
-    // CHECK: call void @llvm.memcpy.p0.p0.i64(
+    // CHECK-NOT: call void @llvm.memcpy.p0.p0.i64(
     // CHECK: ret ptr [[ARC]]
     Arc::default()
 }