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Auto merge of rust-lang#75687 - TimDiekmann:realloc-align, r=Amanieu
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Allow reallocation to different alignment in `AllocRef`

The allocator-wg [has decided](rust-lang/wg-allocators#5 (comment)) to support reallocating to a different alignment in `AllocRef`. For more details please see the linked issue.

r? @Amanieu

closes rust-lang/wg-allocators#5
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@bors
bors committed Aug 26, 2020
2 parents 2fe9a33 + 46b547c commit ffd59bf
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Showing 5 changed files with 185 additions and 162 deletions.
89 changes: 55 additions & 34 deletions library/alloc/src/alloc.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,7 +3,7 @@
#![stable(feature = "alloc_module", since = "1.28.0")]

use core::intrinsics::{self, min_align_of_val, size_of_val};
use core::ptr::{NonNull, Unique};
use core::ptr::{self, NonNull, Unique};

#[stable(feature = "alloc_module", since = "1.28.0")]
#[doc(inline)]
Expand Down Expand Up @@ -162,36 +162,45 @@ impl Global {
unsafe fn grow_impl(
&mut self,
ptr: NonNull<u8>,
layout: Layout,
new_size: usize,
old_layout: Layout,
new_layout: Layout,
zeroed: bool,
) -> Result<NonNull<[u8]>, AllocErr> {
debug_assert!(
new_size >= layout.size(),
"`new_size` must be greater than or equal to `layout.size()`"
new_layout.size() >= old_layout.size(),
"`new_layout.size()` must be greater than or equal to `old_layout.size()`"
);

match layout.size() {
// SAFETY: the caller must ensure that the `new_size` does not overflow.
// `layout.align()` comes from a `Layout` and is thus guaranteed to be valid for a Layout.
0 => unsafe {
let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
self.alloc_impl(new_layout, zeroed)
},
match old_layout.size() {
0 => self.alloc_impl(new_layout, zeroed),

// SAFETY: `new_size` is non-zero as `old_size` is greater than or equal to `new_size`
// as required by safety conditions. Other conditions must be upheld by the caller
old_size => unsafe {
// `realloc` probably checks for `new_size >= size` or something similar.
intrinsics::assume(new_size >= layout.size());
old_size if old_layout.align() == new_layout.align() => unsafe {
let new_size = new_layout.size();

// `realloc` probably checks for `new_size >= old_layout.size()` or something similar.
intrinsics::assume(new_size >= old_layout.size());

let raw_ptr = realloc(ptr.as_ptr(), layout, new_size);
let raw_ptr = realloc(ptr.as_ptr(), old_layout, new_size);
let ptr = NonNull::new(raw_ptr).ok_or(AllocErr)?;
if zeroed {
raw_ptr.add(old_size).write_bytes(0, new_size - old_size);
}
Ok(NonNull::slice_from_raw_parts(ptr, new_size))
},

// SAFETY: because `new_layout.size()` must be greater than or equal to `old_size`,
// both the old and new memory allocation are valid for reads and writes for `old_size`
// bytes. Also, because the old allocation wasn't yet deallocated, it cannot overlap
// `new_ptr`. Thus, the call to `copy_nonoverlapping` is safe. The safety contract
// for `dealloc` must be upheld by the caller.
old_size => unsafe {
let new_ptr = self.alloc_impl(new_layout, zeroed)?;
ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), old_size);
self.dealloc(ptr, old_layout);
Ok(new_ptr)
},
}
}
}
Expand Down Expand Up @@ -221,52 +230,64 @@ unsafe impl AllocRef for Global {
unsafe fn grow(
&mut self,
ptr: NonNull<u8>,
layout: Layout,
new_size: usize,
old_layout: Layout,
new_layout: Layout,
) -> Result<NonNull<[u8]>, AllocErr> {
// SAFETY: all conditions must be upheld by the caller
unsafe { self.grow_impl(ptr, layout, new_size, false) }
unsafe { self.grow_impl(ptr, old_layout, new_layout, false) }
}

#[inline]
unsafe fn grow_zeroed(
&mut self,
ptr: NonNull<u8>,
layout: Layout,
new_size: usize,
old_layout: Layout,
new_layout: Layout,
) -> Result<NonNull<[u8]>, AllocErr> {
// SAFETY: all conditions must be upheld by the caller
unsafe { self.grow_impl(ptr, layout, new_size, true) }
unsafe { self.grow_impl(ptr, old_layout, new_layout, true) }
}

#[inline]
unsafe fn shrink(
&mut self,
ptr: NonNull<u8>,
layout: Layout,
new_size: usize,
old_layout: Layout,
new_layout: Layout,
) -> Result<NonNull<[u8]>, AllocErr> {
debug_assert!(
new_size <= layout.size(),
"`new_size` must be smaller than or equal to `layout.size()`"
new_layout.size() <= old_layout.size(),
"`new_layout.size()` must be smaller than or equal to `old_layout.size()`"
);

match new_size {
match new_layout.size() {
// SAFETY: conditions must be upheld by the caller
0 => unsafe {
self.dealloc(ptr, layout);
Ok(NonNull::slice_from_raw_parts(layout.dangling(), 0))
self.dealloc(ptr, old_layout);
Ok(NonNull::slice_from_raw_parts(new_layout.dangling(), 0))
},

// SAFETY: `new_size` is non-zero. Other conditions must be upheld by the caller
new_size => unsafe {
// `realloc` probably checks for `new_size <= size` or something similar.
intrinsics::assume(new_size <= layout.size());
new_size if old_layout.align() == new_layout.align() => unsafe {
// `realloc` probably checks for `new_size <= old_layout.size()` or something similar.
intrinsics::assume(new_size <= old_layout.size());

let raw_ptr = realloc(ptr.as_ptr(), layout, new_size);
let raw_ptr = realloc(ptr.as_ptr(), old_layout, new_size);
let ptr = NonNull::new(raw_ptr).ok_or(AllocErr)?;
Ok(NonNull::slice_from_raw_parts(ptr, new_size))
},

// SAFETY: because `new_size` must be smaller than or equal to `old_layout.size()`,
// both the old and new memory allocation are valid for reads and writes for `new_size`
// bytes. Also, because the old allocation wasn't yet deallocated, it cannot overlap
// `new_ptr`. Thus, the call to `copy_nonoverlapping` is safe. The safety contract
// for `dealloc` must be upheld by the caller.
new_size => unsafe {
let new_ptr = self.alloc(new_layout)?;
ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), new_size);
self.dealloc(ptr, old_layout);
Ok(new_ptr)
},
}
}
}
Expand All @@ -279,7 +300,7 @@ unsafe impl AllocRef for Global {
unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
let layout = unsafe { Layout::from_size_align_unchecked(size, align) };
match Global.alloc(layout) {
Ok(ptr) => ptr.as_non_null_ptr().as_ptr(),
Ok(ptr) => ptr.as_mut_ptr(),
Err(_) => handle_alloc_error(layout),
}
}
Expand Down
17 changes: 11 additions & 6 deletions library/alloc/src/raw_vec.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@

use core::alloc::LayoutErr;
use core::cmp;
use core::intrinsics;
use core::mem::{self, ManuallyDrop, MaybeUninit};
use core::ops::Drop;
use core::ptr::{NonNull, Unique};
Expand Down Expand Up @@ -465,8 +466,9 @@ impl<T, A: AllocRef> RawVec<T, A> {
let new_size = amount * mem::size_of::<T>();

let ptr = unsafe {
self.alloc.shrink(ptr, layout, new_size).map_err(|_| TryReserveError::AllocError {
layout: Layout::from_size_align_unchecked(new_size, layout.align()),
let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
self.alloc.shrink(ptr, layout, new_layout).map_err(|_| TryReserveError::AllocError {
layout: new_layout,
non_exhaustive: (),
})?
};
Expand Down Expand Up @@ -494,13 +496,16 @@ where

let memory = if let Some((ptr, old_layout)) = current_memory {
debug_assert_eq!(old_layout.align(), new_layout.align());
unsafe { alloc.grow(ptr, old_layout, new_layout.size()) }
unsafe {
// The allocator checks for alignment equality
intrinsics::assume(old_layout.align() == new_layout.align());
alloc.grow(ptr, old_layout, new_layout)
}
} else {
alloc.alloc(new_layout)
}
.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?;
};

Ok(memory)
memory.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })
}

unsafe impl<#[may_dangle] T, A: AllocRef> Drop for RawVec<T, A> {
Expand Down
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