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This commit is contained in:
John Kåre Alsaker 2023-08-14 21:29:19 +02:00
parent d5d72168fa
commit 6f86591b87
1 changed files with 16 additions and 7 deletions
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23
compiler/rustc_arena/src/lib.rs
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@ -366,16 +366,18 @@ unsafe impl<T: Send> Send for TypedArena<T> {}
#[inline(always)]
fn align_down(val: usize, align: usize) -> usize {
assert!(align.is_power_of_two());
debug_assert!(align.is_power_of_two());
val & !(align - 1)
}
#[inline(always)]
fn align(val: usize, align: usize) -> usize {
assert!(align.is_power_of_two());
fn align_up(val: usize, align: usize) -> usize {
debug_assert!(align.is_power_of_two());
(val + align - 1) & !(align - 1)
}
// Pointer alignment is common in compiler types, so keep `DroplessArena` aligned to them
// to optimize away alignment code.
const DROPLESS_ALIGNMENT: usize = mem::align_of::<usize>();
/// An arena that can hold objects of multiple different types that impl `Copy`
@ -390,6 +392,8 @@ pub struct DroplessArena {
/// start. (This is slightly simpler and faster than allocating upwards,
/// see <https://fitzgeraldnick.com/2019/11/01/always-bump-downwards.html>.)
/// When this pointer crosses the start pointer, a new chunk is allocated.
///
/// This is kept aligned to DROPLESS_ALIGNMENT.
end: Cell<*mut u8>,
/// A vector of arena chunks.
@ -433,13 +437,16 @@ impl DroplessArena {
// Also ensure that this chunk can fit `additional`.
new_cap = cmp::max(additional, new_cap);
let mut chunk = ArenaChunk::new(align(new_cap, PAGE));
let mut chunk = ArenaChunk::new(align_up(new_cap, PAGE));
self.start.set(chunk.start());
// Align the end to DROPLESS_ALIGNMENT
let end = align_down(chunk.end().addr(), DROPLESS_ALIGNMENT);
// Make sure we don't go past `start`
let end = cmp::max(chunk.start().addr(), end);
// Make sure we don't go past `start`. This should not happen since the allocation
// should be at least DROPLESS_ALIGNMENT - 1 bytes.
debug_assert!(chunk.start().addr() <= end);
self.end.set(chunk.end().with_addr(end));
chunks.push(chunk);
@ -469,7 +476,7 @@ impl DroplessArena {
let end = old_end.addr();
// Align allocated bytes so that `self.end` stays aligned to DROPLESS_ALIGNMENT
let bytes = align(layout.size(), DROPLESS_ALIGNMENT);
let bytes = align_up(layout.size(), DROPLESS_ALIGNMENT);
// Tell LLVM that `end` is aligned to DROPLESS_ALIGNMENT
unsafe { intrinsics::assume(end == align_down(end, DROPLESS_ALIGNMENT)) };
@ -477,6 +484,8 @@ impl DroplessArena {
let new_end = align_down(end.checked_sub(bytes)?, layout.align());
if start <= new_end {
let new_end = old_end.with_addr(new_end);
// `new_end` is aligned to DROPLESS_ALIGNMENT as `align_down` preserves alignment
// as both `end` and `bytes` are already aligned to DROPLESS_ALIGNMENT.
self.end.set(new_end);
Some(new_end)
} else {