Skip to content

Navigation Menu

Sign in
Appearance settings

Search code, repositories, users, issues, pull requests...

Provide feedback

We read every piece of feedback, and take your input very seriously.

Saved searches

Use saved searches to filter your results more quickly
Sign up
Appearance settings

Commit 1f67a7a

Browse files
committed
Auto merge of rust-lang#131460 - jwong101:default-placement-new, r=ibraheemdev
Optimize `Box::default` and `Arc::default` to construct more types in place Both the `Arc` and `Box` `Default` impls currently call `T::default()` before allocating, and then moving the resulting `T` into the allocation. Most `Default` impls are trivial, which should in theory allow LLVM to construct `T: Default` directly in the `Box` allocation when calling `<Box<T>>::default()`. However, the allocation may fail, which necessitates calling `T`'s destructor if it has one. If the destructor is non-trivial, then LLVM has a hard time proving that it's sound to elide, which makes it construct `T` on the stack first, and then copy it into the allocation. Change both of these impls to allocate first, and then call `T::default` into the uninitialized allocation, so that LLVM doesn't have to prove that it's sound to elide the destructor/initial stack copy. For example, given the following Rust code: ```rust #[derive(Default, Clone)] struct Foo { x: Vec<u8>, z: String, y: Vec<u8>, } #[no_mangle] pub fn src() -> Box<Foo> { Box::default() } ``` <details open> <summary>Before this PR:</summary> ```llvm `@__rust_no_alloc_shim_is_unstable` = external global i8 ; drop_in_place() generated in case the allocation fails ; core::ptr::drop_in_place<playground::Foo> ; Function Attrs: nounwind nonlazybind uwtable define internal fastcc void `@"_ZN4core3ptr36drop_in_place$LT$playground..Foo$GT17ドルhff376aece491233bE"(ptr` noalias nocapture noundef readonly align 8 dereferenceable(72) %_1) unnamed_addr #0 personality ptr `@rust_eh_personality` { start: %_1.val = load i64, ptr %_1, align 8 %0 = icmp eq i64 %_1.val, 0 br i1 %0, label %bb6, label %"_ZN63_$LT$alloc..alloc..Global$u20$as$u20$core..alloc..Allocator$GT10ドルdeallocate17heaa87468709346b1E.exit.i.i.i4.i" "_ZN63_$LT$alloc..alloc..Global$u20$as$u20$core..alloc..Allocator$GT10ドルdeallocate17heaa87468709346b1E.exit.i.i.i4.i": ; preds = %start %1 = getelementptr inbounds i8, ptr %_1, i64 8 %_1.val6 = load ptr, ptr %1, align 8, !nonnull !3, !noundef !3 tail call void `@__rust_dealloc(ptr` noundef nonnull %_1.val6, i64 noundef %_1.val, i64 noundef 1) rust-lang#8 br label %bb6 bb6: ; preds = %"_ZN63_$LT$alloc..alloc..Global$u20$as$u20$core..alloc..Allocator$GT10ドルdeallocate17heaa87468709346b1E.exit.i.i.i4.i", %start %2 = getelementptr inbounds i8, ptr %_1, i64 24 %.val9 = load i64, ptr %2, align 8 %3 = icmp eq i64 %.val9, 0 br i1 %3, label %bb5, label %"_ZN63_$LT$alloc..alloc..Global$u20$as$u20$core..alloc..Allocator$GT10ドルdeallocate17heaa87468709346b1E.exit.i.i.i4.i.i11" "_ZN63_$LT$alloc..alloc..Global$u20$as$u20$core..alloc..Allocator$GT10ドルdeallocate17heaa87468709346b1E.exit.i.i.i4.i.i11": ; preds = %bb6 %4 = getelementptr inbounds i8, ptr %_1, i64 32 %.val10 = load ptr, ptr %4, align 8, !nonnull !3, !noundef !3 tail call void `@__rust_dealloc(ptr` noundef nonnull %.val10, i64 noundef %.val9, i64 noundef 1) rust-lang#8 br label %bb5 bb5: ; preds = %"_ZN63_$LT$alloc..alloc..Global$u20$as$u20$core..alloc..Allocator$GT10ドルdeallocate17heaa87468709346b1E.exit.i.i.i4.i.i11", %bb6 %5 = getelementptr inbounds i8, ptr %_1, i64 48 %.val4 = load i64, ptr %5, align 8 %6 = icmp eq i64 %.val4, 0 br i1 %6, label %"_ZN4core3ptr46drop_in_place$LT$alloc..vec..Vec$LT$u8$GT$$GT17ドルhb5ca95423e113cf7E.exit16", label %"_ZN63_$LT$alloc..alloc..Global$u20$as$u20$core..alloc..Allocator$GT10ドルdeallocate17heaa87468709346b1E.exit.i.i.i4.i15" "_ZN63_$LT$alloc..alloc..Global$u20$as$u20$core..alloc..Allocator$GT10ドルdeallocate17heaa87468709346b1E.exit.i.i.i4.i15": ; preds = %bb5 %7 = getelementptr inbounds i8, ptr %_1, i64 56 %.val5 = load ptr, ptr %7, align 8, !nonnull !3, !noundef !3 tail call void `@__rust_dealloc(ptr` noundef nonnull %.val5, i64 noundef %.val4, i64 noundef 1) rust-lang#8 br label %"_ZN4core3ptr46drop_in_place$LT$alloc..vec..Vec$LT$u8$GT$$GT17ドルhb5ca95423e113cf7E.exit16" "_ZN4core3ptr46drop_in_place$LT$alloc..vec..Vec$LT$u8$GT$$GT17ドルhb5ca95423e113cf7E.exit16": ; preds = %bb5, %"_ZN63_$LT$alloc..alloc..Global$u20$as$u20$core..alloc..Allocator$GT10ドルdeallocate17heaa87468709346b1E.exit.i.i.i4.i15" ret void } ; Function Attrs: nonlazybind uwtable define noalias noundef nonnull align 8 ptr `@src()` unnamed_addr #1 personality ptr `@rust_eh_personality` { start: ; alloca to place `Foo` in. %_1 = alloca [72 x i8], align 8 call void `@llvm.lifetime.start.p0(i64` 72, ptr nonnull %_1) store i64 0, ptr %_1, align 8 %_2.sroa.4.0._1.sroa_idx = getelementptr inbounds i8, ptr %_1, i64 8 store ptr inttoptr (i64 1 to ptr), ptr %_2.sroa.4.0._1.sroa_idx, align 8 %_2.sroa.5.0._1.sroa_idx = getelementptr inbounds i8, ptr %_1, i64 16 %_3.sroa.4.0..sroa_idx = getelementptr inbounds i8, ptr %_1, i64 32 call void `@llvm.memset.p0.i64(ptr` noundef nonnull align 8 dereferenceable(16) %_2.sroa.5.0._1.sroa_idx, i8 0, i64 16, i1 false) store ptr inttoptr (i64 1 to ptr), ptr %_3.sroa.4.0..sroa_idx, align 8 %_3.sroa.5.0..sroa_idx = getelementptr inbounds i8, ptr %_1, i64 40 %_4.sroa.4.0..sroa_idx = getelementptr inbounds i8, ptr %_1, i64 56 call void `@llvm.memset.p0.i64(ptr` noundef nonnull align 8 dereferenceable(16) %_3.sroa.5.0..sroa_idx, i8 0, i64 16, i1 false) store ptr inttoptr (i64 1 to ptr), ptr %_4.sroa.4.0..sroa_idx, align 8 %_4.sroa.5.0..sroa_idx = getelementptr inbounds i8, ptr %_1, i64 64 store i64 0, ptr %_4.sroa.5.0..sroa_idx, align 8 %0 = load volatile i8, ptr `@__rust_no_alloc_shim_is_unstable,` align 1, !noalias !4 %_0.i.i.i = tail call noalias noundef align 8 dereferenceable_or_null(72) ptr `@__rust_alloc(i64` noundef 72, i64 noundef 8) rust-lang#8, !noalias !4 %1 = icmp eq ptr %_0.i.i.i, null br i1 %1, label %bb2.i, label %"_ZN5alloc5boxed12Box$LT$T$GT3ドルnew17h0864de14f863a27aE.exit" bb2.i: ; preds = %start ; invoke alloc::alloc::handle_alloc_error invoke void `@_ZN5alloc5alloc18handle_alloc_error17h98142d0d8d74161bE(i64` noundef 8, i64 noundef 72) rust-lang#9 to label %.noexc unwind label %cleanup.i .noexc: ; preds = %bb2.i unreachable cleanup.i: ; preds = %bb2.i %2 = landingpad { ptr, i32 } cleanup ; call core::ptr::drop_in_place<playground::Foo> call fastcc void `@"_ZN4core3ptr36drop_in_place$LT$playground..Foo$GT17ドルhff376aece491233bE"(ptr` noalias noundef nonnull align 8 dereferenceable(72) %_1) rust-lang#10 resume { ptr, i32 } %2 "_ZN5alloc5boxed12Box$LT$T$GT3ドルnew17h0864de14f863a27aE.exit": ; preds = %start ; Copy from stack to heap if allocation is successful call void `@llvm.memcpy.p0.p0.i64(ptr` noundef nonnull align 8 dereferenceable(72) %_0.i.i.i, ptr noundef nonnull align 8 dereferenceable(72) %_1, i64 72, i1 false) call void `@llvm.lifetime.end.p0(i64` 72, ptr nonnull %_1) ret ptr %_0.i.i.i } ``` </details> <details> <summary>After this PR</summary> ```llvm ; Notice how there's no `drop_in_place()` generated as well define noalias noundef nonnull align 8 ptr `@src()` unnamed_addr #0 personality ptr `@rust_eh_personality` { start: ; no stack allocation %0 = load volatile i8, ptr `@__rust_no_alloc_shim_is_unstable,` align 1 %_0.i.i.i.i.i = tail call noalias noundef align 8 dereferenceable_or_null(72) ptr `@__rust_alloc(i64` noundef 72, i64 noundef 8) rust-lang#5 %1 = icmp eq ptr %_0.i.i.i.i.i, null br i1 %1, label %bb3.i, label %"_ZN5alloc5boxed16Box$LT$T$C$A$GT13ドルnew_uninit_in17h80d6355ef4b73ea3E.exit" bb3.i: ; preds = %start ; call alloc::alloc::handle_alloc_error tail call void `@_ZN5alloc5alloc18handle_alloc_error17h98142d0d8d74161bE(i64` noundef 8, i64 noundef 72) rust-lang#6 unreachable "_ZN5alloc5boxed16Box$LT$T$C$A$GT13ドルnew_uninit_in17h80d6355ef4b73ea3E.exit": ; preds = %start ; construct `Foo` directly into the allocation if successful store i64 0, ptr %_0.i.i.i.i.i, align 8 %_8.sroa.4.0._1.sroa_idx = getelementptr inbounds i8, ptr %_0.i.i.i.i.i, i64 8 store ptr inttoptr (i64 1 to ptr), ptr %_8.sroa.4.0._1.sroa_idx, align 8 %_8.sroa.5.0._1.sroa_idx = getelementptr inbounds i8, ptr %_0.i.i.i.i.i, i64 16 %_8.sroa.7.0._1.sroa_idx = getelementptr inbounds i8, ptr %_0.i.i.i.i.i, i64 32 tail call void `@llvm.memset.p0.i64(ptr` noundef nonnull align 8 dereferenceable(16) %_8.sroa.5.0._1.sroa_idx, i8 0, i64 16, i1 false) store ptr inttoptr (i64 1 to ptr), ptr %_8.sroa.7.0._1.sroa_idx, align 8 %_8.sroa.8.0._1.sroa_idx = getelementptr inbounds i8, ptr %_0.i.i.i.i.i, i64 40 %_8.sroa.10.0._1.sroa_idx = getelementptr inbounds i8, ptr %_0.i.i.i.i.i, i64 56 tail call void `@llvm.memset.p0.i64(ptr` noundef nonnull align 8 dereferenceable(16) %_8.sroa.8.0._1.sroa_idx, i8 0, i64 16, i1 false) store ptr inttoptr (i64 1 to ptr), ptr %_8.sroa.10.0._1.sroa_idx, align 8 %_8.sroa.11.0._1.sroa_idx = getelementptr inbounds i8, ptr %_0.i.i.i.i.i, i64 64 store i64 0, ptr %_8.sroa.11.0._1.sroa_idx, align 8 ret ptr %_0.i.i.i.i.i } ``` </details>
2 parents 9618da7 + 5e474f7 commit 1f67a7a

File tree

4 files changed

+36
-2
lines changed

4 files changed

+36
-2
lines changed

‎library/alloc/src/boxed.rs‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1688,7 +1688,7 @@ impl<T: Default> Default for Box<T> {
16881688
/// Creates a `Box<T>`, with the `Default` value for T.
16891689
#[inline]
16901690
fn default() -> Self {
1691-
Box::new(T::default())
1691+
Box::write(Box::new_uninit(),T::default())
16921692
}
16931693
}
16941694

‎library/alloc/src/lib.rs‎

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -104,6 +104,7 @@
104104
#![feature(async_closure)]
105105
#![feature(async_fn_traits)]
106106
#![feature(async_iterator)]
107+
#![feature(box_uninit_write)]
107108
#![feature(clone_to_uninit)]
108109
#![feature(coerce_unsized)]
109110
#![feature(const_align_of_val)]

‎library/alloc/src/sync.rs‎

Lines changed: 7 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -3447,7 +3447,13 @@ impl<T: Default> Default for Arc<T> {
34473447
/// assert_eq!(*x, 0);
34483448
/// ```
34493449
fn default() -> Arc<T> {
3450-
Arc::new(Default::default())
3450+
let x = Box::into_raw(Box::write(Box::new_uninit(), ArcInner {
3451+
strong: atomic::AtomicUsize::new(1),
3452+
weak: atomic::AtomicUsize::new(1),
3453+
data: T::default(),
3454+
}));
3455+
// SAFETY: `Box::into_raw` consumes the `Box` and never returns null
3456+
unsafe { Self::from_inner(NonNull::new_unchecked(x)) }
34513457
}
34523458
}
34533459

‎tests/codegen/placement-new.rs‎

Lines changed: 27 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,27 @@
1+
//@ compile-flags: -O
2+
#![crate_type = "lib"]
3+
4+
// Test to check that types with "complex" destructors, but trivial `Default` impls
5+
// are constructed directly into the allocation in `Box::default` and `Arc::default`.
6+
7+
use std::sync::Arc;
8+
9+
// CHECK-LABEL: @box_default_inplace
10+
#[no_mangle]
11+
pub fn box_default_inplace() -> Box<(String, String)> {
12+
// CHECK-NOT: alloca
13+
// CHECK: [[BOX:%.*]] = {{.*}}call {{.*}}__rust_alloc(
14+
// CHECK-NOT: call void @llvm.memcpy
15+
// CHECK: ret ptr [[BOX]]
16+
Box::default()
17+
}
18+
19+
// CHECK-LABEL: @arc_default_inplace
20+
#[no_mangle]
21+
pub fn arc_default_inplace() -> Arc<(String, String)> {
22+
// CHECK-NOT: alloca
23+
// CHECK: [[ARC:%.*]] = {{.*}}call {{.*}}__rust_alloc(
24+
// CHECK-NOT: call void @llvm.memcpy
25+
// CHECK: ret ptr [[ARC]]
26+
Arc::default()
27+
}

0 commit comments

Comments
(0)

AltStyle によって変換されたページ (->オリジナル) /