[libc-commits] [libc] [llvm] [libc] Improve qsort (PR #120450)

[Schrodinger ZHU Yifan via libc-commits]

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@@ -0,0 +1,89 @@
+//===-- 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_QSORT_PIVOT_H
+#define LLVM_LIBC_SRC_STDLIB_QSORT_PIVOT_H
+
+#include <stdint.h>
+
+namespace LIBC_NAMESPACE_DECL {
+namespace internal {
+
+// Recursively select a pseudomedian if above this threshold.
+constexpr size_t PSEUDO_MEDIAN_REC_THRESHOLD = 64;
+
+// Selects a pivot from `array`. Algorithm taken from glidesort by Orson Peters.
+//
+// This chooses a pivot by sampling an adaptive amount of points, approximating
+// the quality of a median of sqrt(n) elements.
+template <typename A, typename F>
+size_t choose_pivot(const A &array, const F &is_less) {
+  const size_t len = array.len();
+
+  if (len < 8) {
+    return 0;
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SchrodingerZhu wrote:

This brace can also be removed.

https://github.com/llvm/llvm-project/pull/120450