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Optimize layout calculations in HashMap

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This now produces the same assembly code as the previous implementation.
This commit is contained in:
Amanieu d'Antras 2018-06-04 09:22:07 +01:00
parent 6232478d26
commit b69724f37c
1 changed files with 16 additions and 3 deletions
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19
src/libstd/collections/hash/table.rs
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@ -15,6 +15,7 @@ use mem::{size_of, needs_drop};
use mem;
use ops::{Deref, DerefMut};
use ptr::{self, Unique, NonNull};
use hint;
use self::BucketState::*;
@ -655,7 +656,17 @@ impl<K, V, M> GapThenFull<K, V, M>
fn calculate_layout<K, V>(capacity: usize) -> Result<(Layout, usize), LayoutErr> {
let hashes = Layout::array::<HashUint>(capacity)?;
let pairs = Layout::array::<(K, V)>(capacity)?;
hashes.extend(pairs)
hashes.extend(pairs).map(|(layout, _)| {
// LLVM seems to have trouble properly const-propagating pairs.align(),
// possibly due to the use of NonZeroUsize. This little hack allows it
// to generate optimal code.
//
// See https://github.com/rust-lang/rust/issues/51346 for more details.
(
layout,
hashes.size() + hashes.padding_needed_for(mem::align_of::<(K, V)>()),
)
})
}
pub(crate) enum Fallibility {
@ -711,7 +722,8 @@ impl<K, V> RawTable<K, V> {
}
fn raw_bucket_at(&self, index: usize) -> RawBucket<K, V> {
let (_, pairs_offset) = calculate_layout::<K, V>(self.capacity()).unwrap();
let (_, pairs_offset) = calculate_layout::<K, V>(self.capacity())
.unwrap_or_else(|_| unsafe { hint::unreachable_unchecked() });
let buffer = self.hashes.ptr() as *mut u8;
unsafe {
RawBucket {
@ -1109,7 +1121,8 @@ unsafe impl<#[may_dangle] K, #[may_dangle] V> Drop for RawTable<K, V> {
}
}
let (layout, _) = calculate_layout::<K, V>(self.capacity()).unwrap();
let (layout, _) = calculate_layout::<K, V>(self.capacity())
.unwrap_or_else(|_| unsafe { hint::unreachable_unchecked() });
unsafe {
Global.dealloc(NonNull::new_unchecked(self.hashes.ptr()).as_opaque(), layout);
// Remember how everything was allocated out of one buffer