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Avoid bounds checking at slice::binary_search #30917

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Merged
merged 1 commit into from
Jan 22, 2016
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Avoid bounds checking at slice::binary_search #30917

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23 changes: 12 additions & 11 deletions src/libcore/slice.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -295,22 +295,23 @@ impl<T> SliceExt for [T] {
fn binary_search_by<F>(&self, mut f: F) -> Result<usize, usize> where
F: FnMut(&T) -> Ordering
{
let mut base : usize = 0;
let mut lim : usize = self.len();
let mut base = 0usize;
let mut s = self;

while lim != 0 {
let ix = base + (lim >> 1);
match f(&self[ix]) {
Equal => return Ok(ix),
loop {
let (head, tail) = s.split_at(s.len() >> 1);
if tail.is_empty() {
return Err(base)
}
match f(&tail[0]) {
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Can we use split_first here instead? It will do the job of .is_empty(), getting the first element, and slicing out the &tail[1..], all in one.

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The compiler adds another branch, I'm not sure why.

assembly diff https://www.diffchecker.com/5p8siwcp

        if let Some((pivot, rest)) = tail.split_first() {
            match f(pivot) {
                Equal => return Ok(base + head.len()),
                Less => {
                    base += head.len() + 1;
                    s = rest;
                }
                Greater => s = head,
            }
        } else {
            return Err(base)
        }

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hm ok

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@ranma42 do you know what's the case here?

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I do not know why the code should not be the same.
Looking at the IR, the LLVM basic block that generates that jump seems to be

"_ZN5slice12_$u5b$T$u5d$8split_at21h15129678983694567549E.exit": ; preds = %loop_body
  %5 = getelementptr inbounds i32, i32* %s.sroa.0.0.ph, i32 %3
  %6 = icmp eq i32 %s.sroa.7.0, %3
  %switchtmp = icmp eq i32* %5, null
  %or.cond = or i1 %6, %switchtmp
  br i1 %or.cond, label %match_case, label %match_case4

Less => {
base = ix + 1;
lim -= 1;
base += head.len() + 1;
s = &tail[1..];
}
Greater => ()
Greater => s = head,
Equal => return Ok(base + head.len()),
}
lim >>= 1;
}
Err(base)
}

#[inline]
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6 changes: 1 addition & 5 deletions src/libcoretest/slice.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -11,24 +11,20 @@
use core::result::Result::{Ok, Err};

#[test]
fn binary_search_not_found() {
fn test_binary_search() {
let b = [1, 2, 4, 6, 8, 9];
assert!(b.binary_search_by(|v| v.cmp(&6)) == Ok(3));
let b = [1, 2, 4, 6, 8, 9];
assert!(b.binary_search_by(|v| v.cmp(&5)) == Err(3));
let b = [1, 2, 4, 6, 7, 8, 9];
assert!(b.binary_search_by(|v| v.cmp(&6)) == Ok(3));
let b = [1, 2, 4, 6, 7, 8, 9];
assert!(b.binary_search_by(|v| v.cmp(&5)) == Err(3));
let b = [1, 2, 4, 6, 8, 9];
assert!(b.binary_search_by(|v| v.cmp(&8)) == Ok(4));
let b = [1, 2, 4, 6, 8, 9];
assert!(b.binary_search_by(|v| v.cmp(&7)) == Err(4));
let b = [1, 2, 4, 6, 7, 8, 9];
assert!(b.binary_search_by(|v| v.cmp(&8)) == Ok(5));
let b = [1, 2, 4, 5, 6, 8, 9];
assert!(b.binary_search_by(|v| v.cmp(&7)) == Err(5));
let b = [1, 2, 4, 5, 6, 8, 9];
assert!(b.binary_search_by(|v| v.cmp(&0)) == Err(0));
let b = [1, 2, 4, 5, 6, 8];
assert!(b.binary_search_by(|v| v.cmp(&9)) == Err(6));
Expand Down