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//===-- Implementation header for qsort utilities ---------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_LIBC_SRC_STDLIB_QUICK_SORT_H
#define LLVM_LIBC_SRC_STDLIB_QUICK_SORT_H
#include "hdr/stdint_proxy.h"
#include "src/__support/CPP/bit.h"
#include "src/__support/CPP/cstddef.h"
#include "src/__support/macros/config.h"
#include "src/stdlib/qsort_pivot.h"
namespace LIBC_NAMESPACE_DECL {
namespace internal {
// Branchless Lomuto partition based on the implementation by Lukas
// Bergdoll and Orson Peters
// https://github.com/Voultapher/sort-research-rs/blob/main/writeup/lomcyc_partition/text.md.
// Simplified to avoid having to stack allocate.
template <typename A, typename F>
LIBC_INLINE size_t partition_lomuto_branchless(const A &array,
const void *pivot,
const F &is_less) {
const size_t array_len = array.len();
size_t left = 0;
size_t right = 0;
while (right < array_len) {
const bool right_is_lt = is_less(array.get(right), pivot);
array.swap(left, right);
left += static_cast<size_t>(right_is_lt);
right += 1;
}
return left;
}
// Optimized for large types that are expensive to move. Not optimized
// for integers. It's possible to use a cyclic permutation here for
// large types as done in ipnsort but the advantages of this are limited
// as `is_less` is a small wrapper around a call to a function pointer
// and won't incur much binary-size overhead. The other reason to use
// cyclic permutation is to have more efficient swapping, but we don't
// know the element size so this isn't applicable here either.
template <typename A, typename F>
LIBC_INLINE size_t partition_hoare_branchy(const A &array, const void *pivot,
const F &is_less) {
const size_t array_len = array.len();
size_t left = 0;
size_t right = array_len;
while (true) {
while (left < right && is_less(array.get(left), pivot))
++left;
while (true) {
--right;
if (left >= right || is_less(array.get(right), pivot)) {
break;
}
}
if (left >= right)
break;
array.swap(left, right);
++left;
}
return left;
}
template <typename A, typename F>
LIBC_INLINE size_t partition(const A &array, size_t pivot_index,
const F &is_less) {
// Place the pivot at the beginning of the array.
if (pivot_index != 0) {
array.swap(0, pivot_index);
}
const A array_without_pivot = array.make_array(1, array.len() - 1);
const void *pivot = array.get(0);
size_t num_lt;
if constexpr (A::has_fixed_size()) {
// Branchless Lomuto avoid branch misprediction penalties, but
// it also swaps more often which is only faster if the swap is a fast
// constant operation.
num_lt = partition_lomuto_branchless(array_without_pivot, pivot, is_less);
} else {
num_lt = partition_hoare_branchy(array_without_pivot, pivot, is_less);
}
// Place the pivot between the two partitions.
array.swap(0, num_lt);
return num_lt;
}
template <typename A, typename F>
LIBC_INLINE void quick_sort_impl(A &array, const void *ancestor_pivot,
size_t limit, const F &is_less) {
while (true) {
const size_t array_len = array.len();
if (array_len <= 1)
return;
// If too many bad pivot choices were made, simply fall back to
// heapsort in order to guarantee `O(N x log(N))` worst-case.
if (limit == 0) {
heap_sort(array, is_less);
return;
}
limit -= 1;
const size_t pivot_index = choose_pivot(array, is_less);
// If the chosen pivot is equal to the predecessor, then it's the smallest
// element in the slice. Partition the slice into elements equal to and
// elements greater than the pivot. This case is usually hit when the slice
// contains many duplicate elements.
if (ancestor_pivot) {
if (!is_less(ancestor_pivot, array.get(pivot_index))) {
const size_t num_lt =
partition(array, pivot_index,
[is_less](const void *a, const void *b) -> bool {
return !is_less(b, a);
});
// Continue sorting elements greater than the pivot. We know that
// `num_lt` cont
array.reset_bounds(num_lt + 1, array.len() - (num_lt + 1));
ancestor_pivot = nullptr;
continue;
}
}
size_t split_index = partition(array, pivot_index, is_less);
if (array_len == 2)
// The partition operation sorts the two element array.
return;
// Split the array into `left`, `pivot`, and `right`.
A left = array.make_array(0, split_index);
const void *pivot = array.get(split_index);
const size_t right_start = split_index + 1;
A right = array.make_array(right_start, array.len() - right_start);
// Recurse into the left side. We have a fixed recursion limit,
// testing shows no real benefit for recursing into the shorter
// side.
quick_sort_impl(left, ancestor_pivot, limit, is_less);
// Continue with the right side.
array = right;
ancestor_pivot = pivot;
}
}
constexpr size_t ilog2(size_t n) {
return static_cast<size_t>(cpp::bit_width(n)) - 1;
}
template <typename A, typename F>
LIBC_INLINE void quick_sort(A &array, const F &is_less) {
const void *ancestor_pivot = nullptr;
// Limit the number of imbalanced partitions to `2 * floor(log2(len))`.
// The binary OR by one is used to eliminate the zero-check in the logarithm.
const size_t limit = 2 * ilog2((array.len() | 1));
quick_sort_impl(array, ancestor_pivot, limit, is_less);
}
} // namespace internal
} // namespace LIBC_NAMESPACE_DECL
#endif // LLVM_LIBC_SRC_STDLIB_QUICK_SORT_H
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