From f539134e2098bcc02d5c6228034974c67196adab Mon Sep 17 00:00:00 2001 From: Stepan Koltsov Date: Fri, 29 Mar 2024 03:24:02 +0000 Subject: [PATCH] Do not allocate for ZST ThinBox attempt 2 (using const_allocate) There's PR https://github.com/rust-lang/rust/pull/123184 which avoids allocation for ZST ThinBox. That PR has an issue with unsoundness with misuse of `MaybeUninit` (see comments in that PR). This PR is much simpler implementation which does not have this problem, but it uses `const_allocate` feature. --- library/alloc/src/boxed/thin.rs | 96 ++++++++++++++++++++++++++++----- library/alloc/src/lib.rs | 2 + 2 files changed, 85 insertions(+), 13 deletions(-) diff --git a/library/alloc/src/boxed/thin.rs b/library/alloc/src/boxed/thin.rs index 0421a12b3a952..8b145b67bf186 100644 --- a/library/alloc/src/boxed/thin.rs +++ b/library/alloc/src/boxed/thin.rs @@ -4,10 +4,14 @@ use crate::alloc::{self, Layout, LayoutError}; use core::error::Error; use core::fmt::{self, Debug, Display, Formatter}; +#[cfg(not(no_global_oom_handling))] +use core::intrinsics::const_allocate; use core::marker::PhantomData; #[cfg(not(no_global_oom_handling))] use core::marker::Unsize; -use core::mem::{self, SizedTypeProperties}; +use core::mem; +#[cfg(not(no_global_oom_handling))] +use core::mem::SizedTypeProperties; use core::ops::{Deref, DerefMut}; use core::ptr::Pointee; use core::ptr::{self, NonNull}; @@ -109,9 +113,14 @@ impl ThinBox { where T: Unsize, { - let meta = ptr::metadata(&value as &Dyn); - let ptr = WithOpaqueHeader::new(meta, value); - ThinBox { ptr, _marker: PhantomData } + if mem::size_of::() == 0 { + let ptr = WithOpaqueHeader::new_unsize_zst::(value); + ThinBox { ptr, _marker: PhantomData } + } else { + let meta = ptr::metadata(&value as &Dyn); + let ptr = WithOpaqueHeader::new(meta, value); + ThinBox { ptr, _marker: PhantomData } + } } } @@ -200,6 +209,16 @@ impl WithOpaqueHeader { Self(ptr.0) } + #[cfg(not(no_global_oom_handling))] + fn new_unsize_zst(value: T) -> Self + where + Dyn: ?Sized, + T: Unsize, + { + let ptr = WithHeader::<::Metadata>::new_unsize_zst::(value); + Self(ptr.0) + } + fn try_new(header: H, value: T) -> Result { WithHeader::try_new(header, value).map(|ptr| Self(ptr.0)) } @@ -288,6 +307,58 @@ impl WithHeader { } } + // `Dyn` is `?Sized` type like `[u32]`, and `T` is ZST type like `[u32; 0]`. + #[cfg(not(no_global_oom_handling))] + fn new_unsize_zst(value: T) -> WithHeader + where + Dyn: Pointee + ?Sized, + T: Unsize, + { + assert!(mem::size_of::() == 0); + + const fn max(a: usize, b: usize) -> usize { + if a > b { a } else { b } + } + + // Compute a pointer to the right metadata. This will point to the beginning + // of the header, past the padding, so the assigned type makes sense. + // It also ensures that the address at the end of the header is sufficiently + // aligned for T. + let alloc: &::Metadata = const { + // FIXME: just call `WithHeader::alloc_layout` with size reset to 0. + // Currently that's blocked on `Layout::extend` not being `const fn`. + + let alloc_align = + max(mem::align_of::(), mem::align_of::<::Metadata>()); + + let alloc_size = + max(mem::align_of::(), mem::size_of::<::Metadata>()); + + unsafe { + // SAFETY: align is power of two because it is the maximum of two alignments. + let alloc: *mut u8 = const_allocate(alloc_size, alloc_align); + + let metadata_offset = + alloc_size.checked_sub(mem::size_of::<::Metadata>()).unwrap(); + // SAFETY: adding offset within the allocation. + let metadata_ptr: *mut ::Metadata = + alloc.add(metadata_offset).cast(); + // SAFETY: `*metadata_ptr` is within the allocation. + metadata_ptr.write(ptr::metadata::(ptr::dangling::() as *const Dyn)); + + // SAFETY: we have just written the metadata. + &*(metadata_ptr) + } + }; + + // SAFETY: `alloc` points to `::Metadata`, so addition stays in-bounds. + let value_ptr = + unsafe { (alloc as *const ::Metadata).add(1) }.cast::().cast_mut(); + debug_assert!(value_ptr.is_aligned()); + mem::forget(value); + WithHeader(NonNull::new(value_ptr.cast()).unwrap(), PhantomData) + } + // Safety: // - Assumes that either `value` can be dereferenced, or is the // `NonNull::dangling()` we use when both `T` and `H` are ZSTs. @@ -300,20 +371,19 @@ impl WithHeader { impl Drop for DropGuard { fn drop(&mut self) { + // All ZST are allocated statically. + if self.value_layout.size() == 0 { + return; + } + unsafe { // SAFETY: Layout must have been computable if we're in drop let (layout, value_offset) = WithHeader::::alloc_layout(self.value_layout).unwrap_unchecked(); - // Note: Don't deallocate if the layout size is zero, because the pointer - // didn't come from the allocator. - if layout.size() != 0 { - alloc::dealloc(self.ptr.as_ptr().sub(value_offset), layout); - } else { - debug_assert!( - value_offset == 0 && H::IS_ZST && self.value_layout.size() == 0 - ); - } + // Since we only allocate for non-ZSTs, the layout size cannot be zero. + debug_assert!(layout.size() != 0); + alloc::dealloc(self.ptr.as_ptr().sub(value_offset), layout); } } } diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs index cafd59cb0d954..34146918288fa 100644 --- a/library/alloc/src/lib.rs +++ b/library/alloc/src/lib.rs @@ -114,8 +114,10 @@ #![feature(const_box)] #![feature(const_cow_is_borrowed)] #![feature(const_eval_select)] +#![feature(const_heap)] #![feature(const_maybe_uninit_as_mut_ptr)] #![feature(const_maybe_uninit_write)] +#![feature(const_option)] #![feature(const_pin)] #![feature(const_refs_to_cell)] #![feature(const_size_of_val)]