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Commit 4e431fa

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Auto merge of #123561 - saethlin:str-unchecked-sub-index, r=scottmcm
Use unchecked_sub in str indexing #108763 applied this logic to indexing for slices, but of course `str` has its own separate impl. Found this by skimming over the codegen for https://github.com/oxidecomputer/hubris/; their dist builds enable overflow checks so the lack of `unchecked_sub` was producing an impossible-to-hit overflow check and also inhibiting some inlining. r? scottmcm
2 parents fc1a4c5 + 712aab7 commit 4e431fa

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2 files changed

+37
-6
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2 files changed

+37
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lines changed

‎library/core/src/str/traits.rs‎

Lines changed: 9 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -1,6 +1,7 @@
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//! Trait implementations for `str`.
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use crate::cmp::Ordering;
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use crate::intrinsics::unchecked_sub;
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use crate::ops;
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use crate::ptr;
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use crate::slice::SliceIndex;
@@ -210,9 +211,10 @@ unsafe impl SliceIndex<str> for ops::Range<usize> {
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// SAFETY: the caller guarantees that `self` is in bounds of `slice`
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// which satisfies all the conditions for `add`.
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let ptr = unsafe { slice.as_ptr().add(self.start) };
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let len = self.end - self.start;
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ptr::slice_from_raw_parts(ptr, len) as *const str
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unsafe {
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let new_len = unchecked_sub(self.end, self.start);
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ptr::slice_from_raw_parts(slice.as_ptr().add(self.start), new_len) as *const str
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}
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}
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#[inline]
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unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
@@ -229,9 +231,10 @@ unsafe impl SliceIndex<str> for ops::Range<usize> {
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);
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// SAFETY: see comments for `get_unchecked`.
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let ptr = unsafe { slice.as_mut_ptr().add(self.start) };
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let len = self.end - self.start;
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ptr::slice_from_raw_parts_mut(ptr, len) as *mut str
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unsafe {
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let new_len = unchecked_sub(self.end, self.start);
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ptr::slice_from_raw_parts_mut(slice.as_mut_ptr().add(self.start), new_len) as *mut str
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}
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}
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#[inline]
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fn index(self, slice: &str) -> &Self::Output {

‎tests/codegen/slice-indexing.rs‎

Lines changed: 28 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -32,3 +32,31 @@ pub unsafe fn get_unchecked_mut_by_range(x: &mut [i32], r: Range<usize>) -> &mut
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// CHECK: sub nuw i64
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x.get_unchecked_mut(r)
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}
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// CHECK-LABEL: @str_index_by_range(
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#[no_mangle]
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pub fn str_index_by_range(x: &str, r: Range<usize>) -> &str {
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// CHECK: sub nuw i64
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&x[r]
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}
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// CHECK-LABEL: @str_get_unchecked_by_range(
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#[no_mangle]
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pub unsafe fn str_get_unchecked_by_range(x: &str, r: Range<usize>) -> &str {
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// CHECK: sub nuw i64
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x.get_unchecked(r)
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}
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// CHECK-LABEL: @str_index_mut_by_range(
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#[no_mangle]
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pub fn str_index_mut_by_range(x: &mut str, r: Range<usize>) -> &mut str {
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// CHECK: sub nuw i64
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&mut x[r]
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}
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// CHECK-LABEL: @str_get_unchecked_mut_by_range(
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#[no_mangle]
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pub unsafe fn str_get_unchecked_mut_by_range(x: &mut str, r: Range<usize>) -> &mut str {
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// CHECK: sub nuw i64
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x.get_unchecked_mut(r)
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}

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