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Auto merge of #111646 - Voultapher:restore-branchless-code-gen-for-merge, r=cuviper

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Use code with reliable branchless code-gen for slice::sort merge

The recent LLVM 16 update changes code-gen to be not branchless anymore, in the slice::sort implementation merge function. This improves performance by 30% for random patterns, restoring the performance to the state with LLVM 15.

Fixes #111559
This commit is contained in:
bors 2023-05-20 21:52:10 +00:00
commit fe76e14955
1 changed files with 12 additions and 26 deletions
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38
library/core/src/slice/sort.rs
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@ -1085,12 +1085,12 @@ where
// SAFETY: left and right must be valid and part of v same for out.
unsafe {
let to_copy = if is_less(&*right, &**left) {
get_and_increment(&mut right)
} else {
get_and_increment(left)
};
ptr::copy_nonoverlapping(to_copy, get_and_increment(out), 1);
let is_l = is_less(&*right, &**left);
let to_copy = if is_l { right } else { *left };
ptr::copy_nonoverlapping(to_copy, *out, 1);
*out = out.add(1);
right = right.add(is_l as usize);
*left = left.add(!is_l as usize);
}
}
} else {
@ -1113,32 +1113,18 @@ where
// SAFETY: left and right must be valid and part of v same for out.
unsafe {
let to_copy = if is_less(&*right.sub(1), &*left.sub(1)) {
decrement_and_get(left)
} else {
decrement_and_get(right)
};
ptr::copy_nonoverlapping(to_copy, decrement_and_get(&mut out), 1);
let is_l = is_less(&*right.sub(1), &*left.sub(1));
*left = left.sub(is_l as usize);
*right = right.sub(!is_l as usize);
let to_copy = if is_l { *left } else { *right };
out = out.sub(1);
ptr::copy_nonoverlapping(to_copy, out, 1);
}
}
}
// Finally, `hole` gets dropped. If the shorter run was not fully consumed, whatever remains of
// it will now be copied into the hole in `v`.
unsafe fn get_and_increment<T>(ptr: &mut *mut T) -> *mut T {
let old = *ptr;
// SAFETY: ptr.add(1) must still be a valid pointer and part of `v`.
*ptr = unsafe { ptr.add(1) };
old
}
unsafe fn decrement_and_get<T>(ptr: &mut *mut T) -> *mut T {
// SAFETY: ptr.sub(1) must still be a valid pointer and part of `v`.
*ptr = unsafe { ptr.sub(1) };
*ptr
}
// When dropped, copies the range `start..end` into `dest..`.
struct MergeHole<T> {
start: *mut T,