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Get rid of bounds check in slice::chunks_exact() and related functions during construction

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LLVM can't figure out in

    let rem = self.len() % chunk_size;
    let len = self.len() - rem;
    let (fst, snd) = self.split_at(len);

and

    let rem = self.len() % chunk_size;
    let (fst, snd) = self.split_at(rem);

that the index passed to split_at() is smaller than the slice length and
adds a bounds check plus panic for it.

Apart from removing the overhead of the bounds check this also allows
LLVM to optimize code around the ChunksExact iterator better.
This commit is contained in:
Sebastian Dröge 2020-08-26 09:59:04 +03:00
parent 30dc32b10e
commit d08996ac54
1 changed files with 10 additions and 6 deletions
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16
library/core/src/slice/mod.rs
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@ -865,8 +865,9 @@ pub fn chunks_mut(&mut self, chunk_size: usize) -> ChunksMut<'_, T> {
pub fn chunks_exact(&self, chunk_size: usize) -> ChunksExact<'_, T> {
assert_ne!(chunk_size, 0);
let rem = self.len() % chunk_size;
let len = self.len() - rem;
let (fst, snd) = self.split_at(len);
let fst_len = self.len() - rem;
// SAFETY: 0 <= fst_len <= self.len() by construction above
let (fst, snd) = unsafe { self.split_at_unchecked(fst_len) };
ChunksExact { v: fst, rem: snd, chunk_size }
}
@ -910,8 +911,9 @@ pub fn chunks_exact(&self, chunk_size: usize) -> ChunksExact<'_, T> {
pub fn chunks_exact_mut(&mut self, chunk_size: usize) -> ChunksExactMut<'_, T> {
assert_ne!(chunk_size, 0);
let rem = self.len() % chunk_size;
let len = self.len() - rem;
let (fst, snd) = self.split_at_mut(len);
let fst_len = self.len() - rem;
// SAFETY: 0 <= fst_len <= self.len() by construction above
let (fst, snd) = unsafe { self.split_at_mut_unchecked(fst_len) };
ChunksExactMut { v: fst, rem: snd, chunk_size }
}
@ -1063,7 +1065,8 @@ pub fn rchunks_mut(&mut self, chunk_size: usize) -> RChunksMut<'_, T> {
pub fn rchunks_exact(&self, chunk_size: usize) -> RChunksExact<'_, T> {
assert!(chunk_size != 0);
let rem = self.len() % chunk_size;
let (fst, snd) = self.split_at(rem);
// SAFETY: 0 <= rem <= self.len() by construction above
let (fst, snd) = unsafe { self.split_at_unchecked(rem) };
RChunksExact { v: snd, rem: fst, chunk_size }
}
@ -1108,7 +1111,8 @@ pub fn rchunks_exact(&self, chunk_size: usize) -> RChunksExact<'_, T> {
pub fn rchunks_exact_mut(&mut self, chunk_size: usize) -> RChunksExactMut<'_, T> {
assert!(chunk_size != 0);
let rem = self.len() % chunk_size;
let (fst, snd) = self.split_at_mut(rem);
// SAFETY: 0 <= rem <= self.len() by construction above
let (fst, snd) = unsafe { self.split_at_mut_unchecked(rem) };
RChunksExactMut { v: snd, rem: fst, chunk_size }
}