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Refactor binary_search_by to use conditional moves

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Refactor the if/else checking on cmp::Ordering variants to a
"branchless" reassignment of left and right. This change results
in fewer branches and instructions.
This commit is contained in:
okaneco 2023-11-08 14:44:13 -05:00
parent 341efb1017
commit d585eecb05
1 changed files with 8 additions and 9 deletions
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17
library/core/src/slice/mod.rs
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@ -6,7 +6,7 @@
#![stable(feature = "rust1", since = "1.0.0")]
use crate::cmp::Ordering::{self, Greater, Less};
use crate::cmp::Ordering::{self, Equal, Greater, Less};
use crate::fmt;
use crate::intrinsics::{assert_unsafe_precondition, exact_div};
use crate::marker::Copy;
@ -2844,14 +2844,13 @@ impl<T> [T] {
// we have `left + size/2 < self.len()`, and this is in-bounds.
let cmp = f(unsafe { self.get_unchecked(mid) });
// The reason why we use if/else control flow rather than match
// is because match reorders comparison operations, which is perf sensitive.
// This is x86 asm for u8: https://rust.godbolt.org/z/8Y8Pra.
if cmp == Less {
left = mid + 1;
} else if cmp == Greater {
right = mid;
} else {
// This control flow produces conditional moves, which results in
// fewer branches and instructions than if/else or matching on
// cmp::Ordering.
// This is x86 asm for u8: https://rust.godbolt.org/z/698eYffTx.
left = if cmp == Less { mid + 1 } else { left };
right = if cmp == Greater { mid } else { right };
if cmp == Equal {
// SAFETY: same as the `get_unchecked` above
unsafe { crate::intrinsics::assume(mid < self.len()) };
return Ok(mid);