1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * A fast, small, non-recursive O(n log n) sort for the Linux kernel
4 *
5 * This performs n*log2(n) + 0.37*n + o(n) comparisons on average,
6 * and 1.5*n*log2(n) + O(n) in the (very contrived) worst case.
7 *
8 * Glibc qsort() manages n*log2(n) - 1.26*n for random inputs (1.63*n
9 * better) at the expense of stack usage and much larger code to avoid
10 * quicksort's O(n^2) worst case.
11 */
12
13 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
14
15 #include <linux/types.h>
16 #include <linux/export.h>
17 #include <linux/sort.h>
18
19 /**
20 * is_aligned - is this pointer & size okay for word-wide copying?
21 * @base: pointer to data
22 * @size: size of each element
23 * @align: required alignment (typically 4 or 8)
24 *
25 * Returns true if elements can be copied using word loads and stores.
26 * The size must be a multiple of the alignment, and the base address must
27 * be if we do not have CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS.
28 *
29 * For some reason, gcc doesn't know to optimize "if (a & mask || b & mask)"
30 * to "if ((a | b) & mask)", so we do that by hand.
31 */
32 __attribute_const__ __always_inline
33 static bool is_aligned(const void *base, size_t size, unsigned char align)
34 {
35 unsigned char lsbits = (unsigned char)size;
36
37 (void)base;
38 #ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
39 lsbits |= (unsigned char)(uintptr_t)base;
40 #endif
41 return (lsbits & (align - 1)) == 0;
42 }
43
44 /**
45 * swap_words_32 - swap two elements in 32-bit chunks
46 * @a: pointer to the first element to swap
47 * @b: pointer to the second element to swap
48 * @n: element size (must be a multiple of 4)
49 *
50 * Exchange the two objects in memory. This exploits base+index addressing,
51 * which basically all CPUs have, to minimize loop overhead computations.
52 *
53 * For some reason, on x86 gcc 7.3.0 adds a redundant test of n at the
54 * bottom of the loop, even though the zero flag is stil valid from the
55 * subtract (since the intervening mov instructions don't alter the flags).
56 * Gcc 8.1.0 doesn't have that problem.
57 */
58 static void swap_words_32(void *a, void *b, size_t n)
59 {
60 do {
61 u32 t = *(u32 *)(a + (n -= 4));
62 *(u32 *)(a + n) = *(u32 *)(b + n);
63 *(u32 *)(b + n) = t;
64 } while (n);
65 }
66
67 /**
68 * swap_words_64 - swap two elements in 64-bit chunks
69 * @a: pointer to the first element to swap
70 * @b: pointer to the second element to swap
71 * @n: element size (must be a multiple of 8)
72 *
73 * Exchange the two objects in memory. This exploits base+index
74 * addressing, which basically all CPUs have, to minimize loop overhead
75 * computations.
76 *
77 * We'd like to use 64-bit loads if possible. If they're not, emulating
78 * one requires base+index+4 addressing which x86 has but most other
79 * processors do not. If CONFIG_64BIT, we definitely have 64-bit loads,
80 * but it's possible to have 64-bit loads without 64-bit pointers (e.g.
81 * x32 ABI). Are there any cases the kernel needs to worry about?
82 */
83 static void swap_words_64(void *a, void *b, size_t n)
84 {
85 do {
86 #ifdef CONFIG_64BIT
87 u64 t = *(u64 *)(a + (n -= 8));
88 *(u64 *)(a + n) = *(u64 *)(b + n);
89 *(u64 *)(b + n) = t;
90 #else
91 /* Use two 32-bit transfers to avoid base+index+4 addressing */
92 u32 t = *(u32 *)(a + (n -= 4));
93 *(u32 *)(a + n) = *(u32 *)(b + n);
94 *(u32 *)(b + n) = t;
95
96 t = *(u32 *)(a + (n -= 4));
97 *(u32 *)(a + n) = *(u32 *)(b + n);
98 *(u32 *)(b + n) = t;
99 #endif
100 } while (n);
101 }
102
103 /**
104 * swap_bytes - swap two elements a byte at a time
105 * @a: pointer to the first element to swap
106 * @b: pointer to the second element to swap
107 * @n: element size
108 *
109 * This is the fallback if alignment doesn't allow using larger chunks.
110 */
111 static void swap_bytes(void *a, void *b, size_t n)
112 {
113 do {
114 char t = ((char *)a)[--n];
115 ((char *)a)[n] = ((char *)b)[n];
116 ((char *)b)[n] = t;
117 } while (n);
118 }
119
120 typedef void (*swap_func_t)(void *a, void *b, int size);
121
122 /*
123 * The values are arbitrary as long as they can't be confused with
124 * a pointer, but small integers make for the smallest compare
125 * instructions.
126 */
127 #define SWAP_WORDS_64 (swap_func_t)0
128 #define SWAP_WORDS_32 (swap_func_t)1
129 #define SWAP_BYTES (swap_func_t)2
130
131 /*
132 * The function pointer is last to make tail calls most efficient if the
133 * compiler decides not to inline this function.
134 */
135 static void do_swap(void *a, void *b, size_t size, swap_func_t swap_func)
136 {
137 if (swap_func == SWAP_WORDS_64)
138 swap_words_64(a, b, size);
139 else if (swap_func == SWAP_WORDS_32)
140 swap_words_32(a, b, size);
141 else if (swap_func == SWAP_BYTES)
142 swap_bytes(a, b, size);
143 else
144 swap_func(a, b, (int)size);
145 }
146
147 typedef int (*cmp_func_t)(const void *, const void *);
148 typedef int (*cmp_r_func_t)(const void *, const void *, const void *);
149 #define _CMP_WRAPPER ((cmp_r_func_t)0L)
150
151 static int do_cmp(const void *a, const void *b,
152 cmp_r_func_t cmp, const void *priv)
153 {
154 if (cmp == _CMP_WRAPPER)
155 return ((cmp_func_t)(priv))(a, b);
156 return cmp(a, b, priv);
157 }
158
159 /**
160 * parent - given the offset of the child, find the offset of the parent.
161 * @i: the offset of the heap element whose parent is sought. Non-zero.
162 * @lsbit: a precomputed 1-bit mask, equal to "size & -size"
163 * @size: size of each element
164 *
165 * In terms of array indexes, the parent of element j = @i/@size is simply
166 * (j-1)/2. But when working in byte offsets, we can't use implicit
167 * truncation of integer divides.
168 *
169 * Fortunately, we only need one bit of the quotient, not the full divide.
170 * @size has a least significant bit. That bit will be clear if @i is
171 * an even multiple of @size, and set if it's an odd multiple.
172 *
173 * Logically, we're doing "if (i & lsbit) i -= size;", but since the
174 * branch is unpredictable, it's done with a bit of clever branch-free
175 * code instead.
176 */
177 __attribute_const__ __always_inline
178 static size_t parent(size_t i, unsigned int lsbit, size_t size)
179 {
180 i -= size;
181 i -= size & -(i & lsbit);
182 return i / 2;
183 }
184
185 /**
186 * sort_r - sort an array of elements
187 * @base: pointer to data to sort
188 * @num: number of elements
189 * @size: size of each element
190 * @cmp_func: pointer to comparison function
191 * @swap_func: pointer to swap function or NULL
192 * @priv: third argument passed to comparison function
193 *
194 * This function does a heapsort on the given array. You may provide
195 * a swap_func function if you need to do something more than a memory
196 * copy (e.g. fix up pointers or auxiliary data), but the built-in swap
197 * avoids a slow retpoline and so is significantly faster.
198 *
199 * Sorting time is O(n log n) both on average and worst-case. While
200 * quicksort is slightly faster on average, it suffers from exploitable
201 * O(n*n) worst-case behavior and extra memory requirements that make
202 * it less suitable for kernel use.
203 */
204 void sort_r(void *base, size_t num, size_t size,
205 int (*cmp_func)(const void *, const void *, const void *),
206 void (*swap_func)(void *, void *, int size),
207 const void *priv)
208 {
209 /* pre-scale counters for performance */
210 size_t n = num * size, a = (num/2) * size;
211 const unsigned int lsbit = size & -size; /* Used to find parent */
212
213 if (!a) /* num < 2 || size == 0 */
214 return;
215
216 if (!swap_func) {
217 if (is_aligned(base, size, 8))
218 swap_func = SWAP_WORDS_64;
219 else if (is_aligned(base, size, 4))
220 swap_func = SWAP_WORDS_32;
221 else
222 swap_func = SWAP_BYTES;
223 }
224
225 /*
226 * Loop invariants:
227 * 1. elements [a,n) satisfy the heap property (compare greater than
228 * all of their children),
229 * 2. elements [n,num*size) are sorted, and
230 * 3. a <= b <= c <= d <= n (whenever they are valid).
231 */
232 for (;;) {
233 size_t b, c, d;
234
235 if (a) /* Building heap: sift down --a */
236 a -= size;
237 else if (n -= size) /* Sorting: Extract root to --n */
238 do_swap(base, base + n, size, swap_func);
239 else /* Sort complete */
240 break;
241
242 /*
243 * Sift element at "a" down into heap. This is the
244 * "bottom-up" variant, which significantly reduces
245 * calls to cmp_func(): we find the sift-down path all
246 * the way to the leaves (one compare per level), then
247 * backtrack to find where to insert the target element.
248 *
249 * Because elements tend to sift down close to the leaves,
250 * this uses fewer compares than doing two per level
251 * on the way down. (A bit more than half as many on
252 * average, 3/4 worst-case.)
253 */
254 for (b = a; c = 2*b + size, (d = c + size) < n;)
255 b = do_cmp(base + c, base + d, cmp_func, priv) >= 0 ? c : d;
256 if (d == n) /* Special case last leaf with no sibling */
257 b = c;
258
259 /* Now backtrack from "b" to the correct location for "a" */
260 while (b != a && do_cmp(base + a, base + b, cmp_func, priv) >= 0)
261 b = parent(b, lsbit, size);
262 c = b; /* Where "a" belongs */
263 while (b != a) { /* Shift it into place */
264 b = parent(b, lsbit, size);
265 do_swap(base + b, base + c, size, swap_func);
266 }
267 }
268 }
269 EXPORT_SYMBOL(sort_r);
270
271 void sort(void *base, size_t num, size_t size,
272 int (*cmp_func)(const void *, const void *),
273 void (*swap_func)(void *, void *, int size))
274 {
275 return sort_r(base, num, size, _CMP_WRAPPER, swap_func, cmp_func);
276 }
277 EXPORT_SYMBOL(sort);