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

[Michael Jones via llvm-commits]

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@@ -0,0 +1,92 @@
+//===-- 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 "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;
+  }
+
+  const size_t len_div_8 = len / 8;
+
+  const size_t a = 0;             // [0, floor(n/8))
+  const size_t b = len_div_8 * 4; // [4*floor(n/8), 5*floor(n/8))
+  const size_t c = len_div_8 * 7; // [7*floor(n/8), 8*floor(n/8))
+
+  if (len < PSEUDO_MEDIAN_REC_THRESHOLD) {
+    return median3(array, a, b, c, is_less);
+  } else {
+    return median3_rec(array, a, b, c, len_div_8, is_less);
+  }
+}
+
+// Calculates an approximate median of 3 elements from sections a, b, c, or
+// recursively from an approximation of each, if they're large enough. By
+// dividing the size of each section by 8 when recursing we have logarithmic
+// recursion depth and overall sample from f(n) = 3*f(n/8) -> f(n) =
+// O(n^(log(3)/log(8))) ~= O(n^0.528) elements.
+template <typename A, typename F>
+size_t median3_rec(const A &array, size_t a, size_t b, size_t c, size_t n,
+                   const F &is_less) {
+  if (n * 8 >= PSEUDO_MEDIAN_REC_THRESHOLD) {
+    const size_t n8 = n / 8;
+    a = median3_rec(array, a, a + (n8 * 4), a + (n8 * 7), n8, is_less);
+    b = median3_rec(array, b, b + (n8 * 4), b + (n8 * 7), n8, is_less);
+    c = median3_rec(array, c, c + (n8 * 4), c + (n8 * 7), n8, is_less);
+  }
+  return median3(array, a, b, c, is_less);
+}
+
+/// Calculates the median of 3 elements.
+template <typename A, typename F>
+size_t median3(const A &array, size_t a, size_t b, size_t c, const F &is_less) {
+  const void *a_ptr = array.get(a);
+  const void *b_ptr = array.get(b);
+  const void *c_ptr = array.get(c);
+
+  const bool x = is_less(a_ptr, b_ptr);
+  const bool y = is_less(a_ptr, c_ptr);
+  if (x == y) {
+    // If x=y=0 then b, c <= a. In this case we want to return max(b, c).
+    // If x=y=1 then a < b, c. In this case we want to return min(b, c).
+    // By toggling the outcome of b < c using XOR x we get this behavior.
+    const bool z = is_less(b_ptr, c_ptr);
+    if (z ^ x) {
+      return c;
+    } else {
+      return b;
+    }
----------------
michaelrj-google wrote:

Yeah, the LLVM style guide recommends skipping them in places where it's obvious: https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements

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