[libc] Improve qsort (#120450)

Author: Voultapher

libc/src/stdlib/heap_sort.h

@@ -18,11 +18,12 @@ namespace internal {
 // A simple in-place heapsort implementation.
 // Follow the implementation in https://en.wikipedia.org/wiki/Heapsort.
 
-LIBC_INLINE void heap_sort(const Array &array) {
-  size_t end = array.size();
+template <typename A, typename F>
+LIBC_INLINE void heap_sort(const A &array, const F &is_less) {
+  size_t end = array.len();
   size_t start = end / 2;
 
-  auto left_child = [](size_t i) -> size_t { return 2 * i + 1; };
+  const auto left_child = [](size_t i) -> size_t { return 2 * i + 1; };
 
   while (end > 1) {
     if (start > 0) {
@@ -40,12 +41,11 @@ LIBC_INLINE void heap_sort(const Array &array) {
     while (left_child(root) < end) {
       size_t child = left_child(root);
       // If there are two children, set child to the greater.
-      if (child + 1 < end &&
-          array.elem_compare(child, array.get(child + 1)) < 0)
+      if ((child + 1 < end) && is_less(array.get(child), array.get(child + 1)))
         ++child;
 
       // If the root is less than the greater child
-      if (array.elem_compare(root, array.get(child)) >= 0)
+      if (!is_less(array.get(root), array.get(child)))
         break;
 
       // Swap the root with the greater child and continue sifting down.

libc/src/stdlib/qsort.cpp

@@ -18,14 +18,12 @@ namespace LIBC_NAMESPACE_DECL {
 LLVM_LIBC_FUNCTION(void, qsort,
                    (void *array, size_t array_size, size_t elem_size,
                     int (*compare)(const void *, const void *))) {
-  if (array == nullptr || array_size == 0 || elem_size == 0)
-    return;
-  internal::Comparator c(compare);
 
-  auto arr = internal::Array(reinterpret_cast<uint8_t *>(array), array_size,
-                             elem_size, c);
+  const auto is_less = [compare](const void *a, const void *b) -> bool {
+    return compare(a, b) < 0;
+  };
 
-  internal::sort(arr);
+  internal::unstable_sort(array, array_size, elem_size, is_less);
 }
 
 } // namespace LIBC_NAMESPACE_DECL

libc/src/stdlib/qsort_data.h

@@ -17,91 +17,122 @@
 namespace LIBC_NAMESPACE_DECL {
 namespace internal {
 
-using Compare = int(const void *, const void *);
-using CompareWithState = int(const void *, const void *, void *);
-
-enum class CompType { COMPARE, COMPARE_WITH_STATE };
-
-struct Comparator {
-  union {
-    Compare *comp_func;
-    CompareWithState *comp_func_r;
-  };
-  const CompType comp_type;
-
-  void *arg;
-
-  Comparator(Compare *func)
-      : comp_func(func), comp_type(CompType::COMPARE), arg(nullptr) {}
-
-  Comparator(CompareWithState *func, void *arg_val)
-      : comp_func_r(func), comp_type(CompType::COMPARE_WITH_STATE),
-        arg(arg_val) {}
-
-#if defined(__clang__)
-  // Recent upstream changes to -fsanitize=function find more instances of
-  // function type mismatches. One case is with the comparator passed to this
-  // class. Libraries will tend to pass comparators that take pointers to
-  // varying types while this comparator expects to accept const void pointers.
-  // Ideally those tools would pass a function that strictly accepts const
-  // void*s to avoid UB, or would use qsort_r to pass their own comparator.
-  [[clang::no_sanitize("function")]]
-#endif
-  int comp_vals(const void *a, const void *b) const {
-    if (comp_type == CompType::COMPARE) {
-      return comp_func(a, b);
-    } else {
-      return comp_func_r(a, b, arg);
+class ArrayGenericSize {
+  cpp::byte *array_base;
+  size_t array_len;
+  size_t elem_size;
+
+  LIBC_INLINE cpp::byte *get_internal(size_t i) const {
+    return array_base + (i * elem_size);
+  }
+
+public:
+  LIBC_INLINE ArrayGenericSize(void *a, size_t s, size_t e)
+      : array_base(reinterpret_cast<cpp::byte *>(a)), array_len(s),
+        elem_size(e) {}
+
+  static constexpr bool has_fixed_size() { return false; }
+
+  LIBC_INLINE void *get(size_t i) const { return get_internal(i); }
+
+  LIBC_INLINE void swap(size_t i, size_t j) const {
+    // It's possible to use 8 byte blocks with `uint64_t`, but that
+    // generates more machine code as the remainder loop gets
+    // unrolled, plus 4 byte operations are more likely to be
+    // efficient on a wider variety of hardware. On x86 LLVM tends
+    // to unroll the block loop again into 2 16 byte swaps per
+    // iteration which is another reason that 4 byte blocks yields
+    // good performance even for big types.
+    using block_t = uint32_t;
+    constexpr size_t BLOCK_SIZE = sizeof(block_t);
+
+    alignas(block_t) cpp::byte tmp_block[BLOCK_SIZE];
+
+    cpp::byte *elem_i = get_internal(i);
+    cpp::byte *elem_j = get_internal(j);
+
+    const size_t elem_size_rem = elem_size % BLOCK_SIZE;
+    const cpp::byte *elem_i_block_end = elem_i + (elem_size - elem_size_rem);
+
+    while (elem_i != elem_i_block_end) {
+      __builtin_memcpy(tmp_block, elem_i, BLOCK_SIZE);
+      __builtin_memcpy(elem_i, elem_j, BLOCK_SIZE);
+      __builtin_memcpy(elem_j, tmp_block, BLOCK_SIZE);
+
+      elem_i += BLOCK_SIZE;
+      elem_j += BLOCK_SIZE;
+    }
+
+    for (size_t n = 0; n < elem_size_rem; ++n) {
+      cpp::byte tmp = elem_i[n];
+      elem_i[n] = elem_j[n];
+      elem_j[n] = tmp;
     }
   }
+
+  LIBC_INLINE size_t len() const { return array_len; }
+
+  // Make an Array starting at index |i| and length |s|.
+  LIBC_INLINE ArrayGenericSize make_array(size_t i, size_t s) const {
+    return ArrayGenericSize(get_internal(i), s, elem_size);
+  }
+
+  // Reset this Array to point at a different interval of the same
+  // items starting at index |i|.
+  LIBC_INLINE void reset_bounds(size_t i, size_t s) {
+    array_base = get_internal(i);
+    array_len = s;
+  }
 };
 
-class Array {
-  uint8_t *array;
-  size_t array_size;
-  size_t elem_size;
-  Comparator compare;
+// Having a specialized Array type for sorting that knows at
+// compile-time what the size of the element is, allows for much more
+// efficient swapping and for cheaper offset calculations.
+template <size_t ELEM_SIZE> class ArrayFixedSize {
+  cpp::byte *array_base;
+  size_t array_len;
 
-public:
-  Array(uint8_t *a, size_t s, size_t e, Comparator c)
-      : array(a), array_size(s), elem_size(e), compare(c) {}
-
-  uint8_t *get(size_t i) const { return array + i * elem_size; }
-
-  void swap(size_t i, size_t j) const {
-    uint8_t *elem_i = get(i);
-    uint8_t *elem_j = get(j);
-    for (size_t b = 0; b < elem_size; ++b) {
-      uint8_t temp = elem_i[b];
-      elem_i[b] = elem_j[b];
-      elem_j[b] = temp;
-    }
+  LIBC_INLINE cpp::byte *get_internal(size_t i) const {
+    return array_base + (i * ELEM_SIZE);
   }
 
-  int elem_compare(size_t i, const uint8_t *other) const {
-    // An element must compare equal to itself so we don't need to consult the
-    // user provided comparator.
-    if (get(i) == other)
-      return 0;
-    return compare.comp_vals(get(i), other);
+public:
+  LIBC_INLINE ArrayFixedSize(void *a, size_t s)
+      : array_base(reinterpret_cast<cpp::byte *>(a)), array_len(s) {}
+
+  // Beware this function is used a heuristic for cheap to swap types, so
+  // instantiating `ArrayFixedSize` with `ELEM_SIZE > 100` is probably a bad
+  // idea perf wise.
+  static constexpr bool has_fixed_size() { return true; }
+
+  LIBC_INLINE void *get(size_t i) const { return get_internal(i); }
+
+  LIBC_INLINE void swap(size_t i, size_t j) const {
+    alignas(32) cpp::byte tmp[ELEM_SIZE];
+
+    cpp::byte *elem_i = get_internal(i);
+    cpp::byte *elem_j = get_internal(j);
+
+    __builtin_memcpy(tmp, elem_i, ELEM_SIZE);
+    __builtin_memmove(elem_i, elem_j, ELEM_SIZE);
+    __builtin_memcpy(elem_j, tmp, ELEM_SIZE);
   }
 
-  size_t size() const { return array_size; }
+  LIBC_INLINE size_t len() const { return array_len; }
 
-  // Make an Array starting at index |i| and size |s|.
-  LIBC_INLINE Array make_array(size_t i, size_t s) const {
-    return Array(get(i), s, elem_size, compare);
+  // Make an Array starting at index |i| and length |s|.
+  LIBC_INLINE ArrayFixedSize<ELEM_SIZE> make_array(size_t i, size_t s) const {
+    return ArrayFixedSize<ELEM_SIZE>(get_internal(i), s);
   }
 
-  // Reset this Array to point at a different interval of the same items.
-  LIBC_INLINE void reset_bounds(uint8_t *a, size_t s) {
-    array = a;
-    array_size = s;
+  // Reset this Array to point at a different interval of the same
+  // items starting at index |i|.
+  LIBC_INLINE void reset_bounds(size_t i, size_t s) {
+    array_base = get_internal(i);
+    array_len = s;
   }
 };
 
-using SortingRoutine = void(const Array &);
-
 } // namespace internal
 } // namespace LIBC_NAMESPACE_DECL
 

libc/src/stdlib/qsort_pivot.h

@@ -0,0 +1,85 @@
+//===-- 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;
+  }
+
+  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);
+    return z ^ x ? c : b;
+  } else {
+    // Either c <= a < b or b <= a < c, thus a is our median.
+    return a;
+  }
+}
+
+} // namespace internal
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_STDLIB_QSORT_PIVOT_H

libc/src/stdlib/qsort_r.cpp

@@ -19,13 +19,12 @@ LLVM_LIBC_FUNCTION(void, qsort_r,
                    (void *array, size_t array_size, size_t elem_size,
                     int (*compare)(const void *, const void *, void *),
                     void *arg)) {
-  if (array == nullptr || array_size == 0 || elem_size == 0)
-    return;
-  internal::Comparator c(compare, arg);
-  auto arr = internal::Array(reinterpret_cast<uint8_t *>(array), array_size,
-                             elem_size, c);
 
-  internal::sort(arr);
+  const auto is_less = [compare, arg](const void *a, const void *b) -> bool {
+    return compare(a, b, arg) < 0;
+  };
+
+  internal::unstable_sort(array, array_size, elem_size, is_less);
 }
 
 } // namespace LIBC_NAMESPACE_DECL