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DroplessArena: Allocate objects from the end of memory chunk 

Allocating from the end of memory chunk simplifies the alignment code
and reduces the number of checked arithmetic operations.
This commit is contained in:
Tomasz Miąsko 2020-09-21 00:00:00 +00:00
parent 70148d7b31
commit c7e887c64a
1 changed files with 16 additions and 21 deletions
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37
compiler/rustc_arena/src/lib.rs
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@ -299,11 +299,13 @@ unsafe impl<#[may_dangle] T> Drop for TypedArena<T> {
unsafe impl<T: Send> Send for TypedArena<T> {}
pub struct DroplessArena {
/// A pointer to the next object to be allocated.
ptr: Cell<*mut u8>,
/// A pointer to the start of the free space.
start: Cell<*mut u8>,
/// A pointer to the end of the allocated area. When this pointer is
/// reached, a new chunk is allocated.
/// A pointer to the end of free space.
///
/// The allocation proceeds from the end of the chunk towards the start.
/// When this pointer crosses the start pointer, a new chunk is allocated.
end: Cell<*mut u8>,
/// A vector of arena chunks.
@ -316,7 +318,7 @@ impl Default for DroplessArena {
#[inline]
fn default() -> DroplessArena {
DroplessArena {
ptr: Cell::new(ptr::null_mut()),
start: Cell::new(ptr::null_mut()),
end: Cell::new(ptr::null_mut()),
chunks: Default::default(),
}
@ -348,7 +350,7 @@ impl DroplessArena {
new_cap = cmp::max(additional, new_cap);
let mut chunk = TypedArenaChunk::<u8>::new(new_cap);
self.ptr.set(chunk.start());
self.start.set(chunk.start());
self.end.set(chunk.end());
chunks.push(chunk);
}
@ -359,24 +361,17 @@ impl DroplessArena {
/// request.
#[inline]
fn alloc_raw_without_grow(&self, layout: Layout) -> Option<*mut u8> {
let ptr = self.ptr.get() as usize;
let start = self.start.get() as usize;
let end = self.end.get() as usize;
let align = layout.align();
let bytes = layout.size();
// The allocation request fits into the current chunk iff:
//
// let aligned = align_to(ptr, align);
// ptr <= aligned && aligned + bytes <= end
//
// Except that we work with fixed width integers and need to be careful
// about potential overflow in the calcuation. If the overflow does
// happen, then we definitely don't have enough free and need to grow
// the arena.
let aligned = ptr.checked_add(align - 1)? & !(align - 1);
let new_ptr = aligned.checked_add(bytes)?;
if new_ptr <= end {
self.ptr.set(new_ptr as *mut u8);
Some(aligned as *mut u8)
let new_end = end.checked_sub(bytes)? & !(align - 1);
if start <= new_end {
let new_end = new_end as *mut u8;
self.end.set(new_end);
Some(new_end)
} else {
None
}