--- /dev/null
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * A fast, small, non-recursive O(n log n) sort for the Linux kernel
+ *
+ * This performs n*log2(n) + 0.37*n + o(n) comparisons on average,
+ * and 1.5*n*log2(n) + O(n) in the (very contrived) worst case.
+ *
+ * Glibc qsort() manages n*log2(n) - 1.26*n for random inputs (1.63*n
+ * better) at the expense of stack usage and much larger code to avoid
+ * quicksort's O(n^2) worst case.
+ */
+
+#include <stdio.h>
+#include <stdbool.h>
+#include <linux/types.h>
+
+#include "sort.h"
+#include "linux_types.h"
+
+/**
+ * is_aligned - is this pointer & size okay for word-wide copying?
+ * @base: pointer to data
+ * @size: size of each element
+ * @align: required alignment (typically 4 or 8)
+ *
+ * Returns true if elements can be copied using word loads and stores.
+ * The size must be a multiple of the alignment.
+ *
+ * For some reason, gcc doesn't know to optimize "if (a & mask || b & mask)"
+ * to "if ((a | b) & mask)", so we do that by hand.
+ */
+__const __always_inline
+static bool is_aligned(const void *base, size_t size, unsigned char align)
+{
+ unsigned char lsbits = (unsigned char)size;
+
+ (void)base;
+ return (lsbits & (align - 1)) == 0;
+}
+
+/**
+ * swap_words_32 - swap two elements in 32-bit chunks
+ * @a: pointer to the first element to swap
+ * @b: pointer to the second element to swap
+ * @n: element size (must be a multiple of 4)
+ *
+ * Exchange the two objects in memory. This exploits base+index addressing,
+ * which basically all CPUs have, to minimize loop overhead computations.
+ *
+ * For some reason, on x86 gcc 7.3.0 adds a redundant test of n at the
+ * bottom of the loop, even though the zero flag is still valid from the
+ * subtract (since the intervening mov instructions don't alter the flags).
+ * Gcc 8.1.0 doesn't have that problem.
+ */
+static void swap_words_32(void *a, void *b, size_t n)
+{
+ do {
+ u32 t = *(u32 *)(a + (n -= 4));
+ *(u32 *)(a + n) = *(u32 *)(b + n);
+ *(u32 *)(b + n) = t;
+ } while (n);
+}
+
+/**
+ * swap_words_64 - swap two elements in 64-bit chunks
+ * @a: pointer to the first element to swap
+ * @b: pointer to the second element to swap
+ * @n: element size (must be a multiple of 8)
+ *
+ * Exchange the two objects in memory. This exploits base+index
+ * addressing, which basically all CPUs have, to minimize loop overhead
+ * computations.
+ *
+ * We'd like to use 64-bit loads if possible. If they're not, emulating
+ * one requires base+index+4 addressing which x86 has but most other
+ * processors do not.
+ */
+static void swap_words_64(void *a, void *b, size_t n)
+{
+ do {
+ u64 t = *(u64 *)(a + (n -= 8));
+ *(u64 *)(a + n) = *(u64 *)(b + n);
+ *(u64 *)(b + n) = t;
+ } while (n);
+}
+
+/**
+ * swap_bytes - swap two elements a byte at a time
+ * @a: pointer to the first element to swap
+ * @b: pointer to the second element to swap
+ * @n: element size
+ *
+ * This is the fallback if alignment doesn't allow using larger chunks.
+ */
+static void swap_bytes(void *a, void *b, size_t n)
+{
+ do {
+ char t = ((char *)a)[--n];
+ ((char *)a)[n] = ((char *)b)[n];
+ ((char *)b)[n] = t;
+ } while (n);
+}
+
+/*
+ * The values are arbitrary as long as they can't be confused with
+ * a pointer, but small integers make for the smallest compare
+ * instructions.
+ */
+#define SWAP_WORDS_64 (swap_r_func_t)0
+#define SWAP_WORDS_32 (swap_r_func_t)1
+#define SWAP_BYTES (swap_r_func_t)2
+#define SWAP_WRAPPER (swap_r_func_t)3
+
+struct wrapper {
+ cmp_func_t cmp;
+ swap_func_t swap;
+};
+
+/*
+ * The function pointer is last to make tail calls most efficient if the
+ * compiler decides not to inline this function.
+ */
+static void do_swap(void *a, void *b, size_t size, swap_r_func_t swap_func, const void *priv)
+{
+ if (swap_func == SWAP_WRAPPER) {
+ ((const struct wrapper *)priv)->swap(a, b, (int)size);
+ return;
+ }
+
+ if (swap_func == SWAP_WORDS_64)
+ swap_words_64(a, b, size);
+ else if (swap_func == SWAP_WORDS_32)
+ swap_words_32(a, b, size);
+ else if (swap_func == SWAP_BYTES)
+ swap_bytes(a, b, size);
+ else
+ swap_func(a, b, (int)size, priv);
+}
+
+#define _CMP_WRAPPER ((cmp_r_func_t)0L)
+
+static int do_cmp(const void *a, const void *b, cmp_r_func_t cmp, const void *priv)
+{
+ if (cmp == _CMP_WRAPPER)
+ return ((const struct wrapper *)priv)->cmp(a, b);
+ return cmp(a, b, priv);
+}
+
+/**
+ * parent - given the offset of the child, find the offset of the parent.
+ * @i: the offset of the heap element whose parent is sought. Non-zero.
+ * @lsbit: a precomputed 1-bit mask, equal to "size & -size"
+ * @size: size of each element
+ *
+ * In terms of array indexes, the parent of element j = @i/@size is simply
+ * (j-1)/2. But when working in byte offsets, we can't use implicit
+ * truncation of integer divides.
+ *
+ * Fortunately, we only need one bit of the quotient, not the full divide.
+ * @size has a least significant bit. That bit will be clear if @i is
+ * an even multiple of @size, and set if it's an odd multiple.
+ *
+ * Logically, we're doing "if (i & lsbit) i -= size;", but since the
+ * branch is unpredictable, it's done with a bit of clever branch-free
+ * code instead.
+ */
+__const __always_inline
+static size_t parent(size_t i, unsigned int lsbit, size_t size)
+{
+ i -= size;
+ i -= size & -(i & lsbit);
+ return i / 2;
+}
+
+/**
+ * sort_r - sort an array of elements
+ * @base: pointer to data to sort
+ * @num: number of elements
+ * @size: size of each element
+ * @cmp_func: pointer to comparison function
+ * @swap_func: pointer to swap function or NULL
+ * @priv: third argument passed to comparison function
+ *
+ * This function does a heapsort on the given array. You may provide
+ * a swap_func function if you need to do something more than a memory
+ * copy (e.g. fix up pointers or auxiliary data), but the built-in swap
+ * avoids a slow retpoline and so is significantly faster.
+ *
+ * Sorting time is O(n log n) both on average and worst-case. While
+ * quicksort is slightly faster on average, it suffers from exploitable
+ * O(n*n) worst-case behavior and extra memory requirements that make
+ * it less suitable for kernel use.
+ */
+void sort_r(void *base, size_t num, size_t size,
+ cmp_r_func_t cmp_func,
+ swap_r_func_t swap_func,
+ const void *priv)
+{
+ /* pre-scale counters for performance */
+ size_t n = num * size, a = (num/2) * size;
+ const unsigned int lsbit = size & -size; /* Used to find parent */
+
+ if (!a) /* num < 2 || size == 0 */
+ return;
+
+ /* called from 'sort' without swap function, let's pick the default */
+ if (swap_func == SWAP_WRAPPER && !((struct wrapper *)priv)->swap)
+ swap_func = NULL;
+
+ if (!swap_func) {
+ if (is_aligned(base, size, 8))
+ swap_func = SWAP_WORDS_64;
+ else if (is_aligned(base, size, 4))
+ swap_func = SWAP_WORDS_32;
+ else
+ swap_func = SWAP_BYTES;
+ }
+
+ /*
+ * Loop invariants:
+ * 1. elements [a,n) satisfy the heap property (compare greater than
+ * all of their children),
+ * 2. elements [n,num*size) are sorted, and
+ * 3. a <= b <= c <= d <= n (whenever they are valid).
+ */
+ for (;;) {
+ size_t b, c, d;
+
+ if (a) /* Building heap: sift down --a */
+ a -= size;
+ else if (n -= size) /* Sorting: Extract root to --n */
+ do_swap(base, base + n, size, swap_func, priv);
+ else /* Sort complete */
+ break;
+
+ /*
+ * Sift element at "a" down into heap. This is the
+ * "bottom-up" variant, which significantly reduces
+ * calls to cmp_func(): we find the sift-down path all
+ * the way to the leaves (one compare per level), then
+ * backtrack to find where to insert the target element.
+ *
+ * Because elements tend to sift down close to the leaves,
+ * this uses fewer compares than doing two per level
+ * on the way down. (A bit more than half as many on
+ * average, 3/4 worst-case.)
+ */
+ for (b = a; c = 2*b + size, (d = c + size) < n;)
+ b = do_cmp(base + c, base + d, cmp_func, priv) >= 0 ? c : d;
+ if (d == n) /* Special case last leaf with no sibling */
+ b = c;
+
+ /* Now backtrack from "b" to the correct location for "a" */
+ while (b != a && do_cmp(base + a, base + b, cmp_func, priv) >= 0)
+ b = parent(b, lsbit, size);
+ c = b; /* Where "a" belongs */
+ while (b != a) { /* Shift it into place */
+ b = parent(b, lsbit, size);
+ do_swap(base + b, base + c, size, swap_func, priv);
+ }
+ }
+}
+
+void sort(void *base, size_t num, size_t size,
+ cmp_func_t cmp_func,
+ swap_func_t swap_func)
+{
+ struct wrapper w = {
+ .cmp = cmp_func,
+ .swap = swap_func,
+ };
+
+ return sort_r(base, num, size, _CMP_WRAPPER, SWAP_WRAPPER, &w);
+}
projects / mtd-utils.git / commitdiff