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Use unchecked_sub in str indexing #123561

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Merged
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Apr 7, 2024
Merged

Use unchecked_sub in str indexing #123561

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15 changes: 9 additions & 6 deletions library/core/src/str/traits.rs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
//! Trait implementations for `str`.

use crate::cmp::Ordering;
use crate::intrinsics::unchecked_sub;
use crate::ops;
use crate::ptr;
use crate::slice::SliceIndex;
Expand Down Expand Up @@ -210,9 +211,10 @@ unsafe impl SliceIndex<str> for ops::Range<usize> {

// SAFETY: the caller guarantees that `self` is in bounds of `slice`
// which satisfies all the conditions for `add`.
let ptr = unsafe { slice.as_ptr().add(self.start) };
let len = self.end - self.start;
ptr::slice_from_raw_parts(ptr, len) as *const str
unsafe {
let new_len = unchecked_sub(self.end, self.start);
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Does this get different codegen if you just use the public stdlib unchecked_sub?

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It gets a boatload more MIR, or at least it will once #121571 lands.

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(We really need a "look, nobody cares about the parameter debug info from this method" for things like that and i32:PartialOrd and …)

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Ah okay! Cool then.

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No, but it doubles up on the precondition check.

ptr::slice_from_raw_parts(slice.as_ptr().add(self.start), new_len) as *const str
}
}
#[inline]
unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
Expand All @@ -229,9 +231,10 @@ unsafe impl SliceIndex<str> for ops::Range<usize> {
);

// SAFETY: see comments for `get_unchecked`.
let ptr = unsafe { slice.as_mut_ptr().add(self.start) };
let len = self.end - self.start;
ptr::slice_from_raw_parts_mut(ptr, len) as *mut str
unsafe {
let new_len = unchecked_sub(self.end, self.start);
ptr::slice_from_raw_parts_mut(slice.as_mut_ptr().add(self.start), new_len) as *mut str
}
}
#[inline]
fn index(self, slice: &str) -> &Self::Output {
Expand Down
28 changes: 28 additions & 0 deletions tests/codegen/slice-indexing.rs
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Expand Up @@ -32,3 +32,31 @@ pub unsafe fn get_unchecked_mut_by_range(x: &mut [i32], r: Range<usize>) -> &mut
// CHECK: sub nuw i64
x.get_unchecked_mut(r)
}

// CHECK-LABEL: @str_index_by_range(
#[no_mangle]
pub fn str_index_by_range(x: &str, r: Range<usize>) -> &str {
// CHECK: sub nuw i64
&x[r]
}

// CHECK-LABEL: @str_get_unchecked_by_range(
#[no_mangle]
pub unsafe fn str_get_unchecked_by_range(x: &str, r: Range<usize>) -> &str {
// CHECK: sub nuw i64
x.get_unchecked(r)
}

// CHECK-LABEL: @str_index_mut_by_range(
#[no_mangle]
pub fn str_index_mut_by_range(x: &mut str, r: Range<usize>) -> &mut str {
// CHECK: sub nuw i64
&mut x[r]
}

// CHECK-LABEL: @str_get_unchecked_mut_by_range(
#[no_mangle]
pub unsafe fn str_get_unchecked_mut_by_range(x: &mut str, r: Range<usize>) -> &mut str {
// CHECK: sub nuw i64
x.get_unchecked_mut(r)
}
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