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 * Quicksort 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 #include <linux/types.h> 14 #include <linux/export.h> 15 #include <linux/sort.h> 16 17 /** 18 * is_aligned - is this pointer & size okay for word-wide copying? 19 * @base: pointer to data 20 * @size: size of each element 21 * @align: required alignment (typically 4 or 8) 22 * 23 * Returns true if elements can be copied using word loads and stores. 24 * The size must be a multiple of the alignment, and the base address must 25 * be if we do not have CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS. 26 * 27 * For some reason, gcc doesn't know to optimize "if (a & mask || b & mask)" 28 * to "if ((a | b) & mask)", so we do that by hand. 29 */ 30 __attribute_const__ __always_inline 31 static bool is_aligned(const void *base, size_t size, unsigned char align) 32 { 33 unsigned char lsbits = (unsigned char)size; 34 35 (void)base; 36 #ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS 37 lsbits |= (unsigned char)(uintptr_t)base; 38 #endif 39 return (lsbits & (align - 1)) == 0; 40 } 41 42 /** 43 * swap_words_32 - swap two elements in 32-bit chunks 44 * @a: pointer to the first element to swap 45 * @b: pointer to the second element to swap 46 * @n: element size (must be a multiple of 4) 47 * 48 * Exchange the two objects in memory. This exploits base+index addressing, 49 * which basically all CPUs have, to minimize loop overhead computations. 50 * 51 * For some reason, on x86 gcc 7.3.0 adds a redundant test of n at the 52 * bottom of the loop, even though the zero flag is still valid from the 53 * subtract (since the intervening mov instructions don't alter the flags). 54 * Gcc 8.1.0 doesn't have that problem. 55 */ 56 static void swap_words_32(void *a, void *b, size_t n) 57 { 58 do { 59 u32 t = *(u32 *)(a + (n -= 4)); 60 *(u32 *)(a + n) = *(u32 *)(b + n); 61 *(u32 *)(b + n) = t; 62 } while (n); 63 } 64 65 /** 66 * swap_words_64 - swap two elements in 64-bit chunks 67 * @a: pointer to the first element to swap 68 * @b: pointer to the second element to swap 69 * @n: element size (must be a multiple of 8) 70 * 71 * Exchange the two objects in memory. This exploits base+index 72 * addressing, which basically all CPUs have, to minimize loop overhead 73 * computations. 74 * 75 * We'd like to use 64-bit loads if possible. If they're not, emulating 76 * one requires base+index+4 addressing which x86 has but most other 77 * processors do not. If CONFIG_64BIT, we definitely have 64-bit loads, 78 * but it's possible to have 64-bit loads without 64-bit pointers (e.g. 79 * x32 ABI). Are there any cases the kernel needs to worry about? 80 */ 81 static void swap_words_64(void *a, void *b, size_t n) 82 { 83 do { 84 #ifdef CONFIG_64BIT 85 u64 t = *(u64 *)(a + (n -= 8)); 86 *(u64 *)(a + n) = *(u64 *)(b + n); 87 *(u64 *)(b + n) = t; 88 #else 89 /* Use two 32-bit transfers to avoid base+index+4 addressing */ 90 u32 t = *(u32 *)(a + (n -= 4)); 91 *(u32 *)(a + n) = *(u32 *)(b + n); 92 *(u32 *)(b + n) = t; 93 94 t = *(u32 *)(a + (n -= 4)); 95 *(u32 *)(a + n) = *(u32 *)(b + n); 96 *(u32 *)(b + n) = t; 97 #endif 98 } while (n); 99 } 100 101 /** 102 * swap_bytes - swap two elements a byte at a time 103 * @a: pointer to the first element to swap 104 * @b: pointer to the second element to swap 105 * @n: element size 106 * 107 * This is the fallback if alignment doesn't allow using larger chunks. 108 */ 109 static void swap_bytes(void *a, void *b, size_t n) 110 { 111 do { 112 char t = ((char *)a)[--n]; 113 ((char *)a)[n] = ((char *)b)[n]; 114 ((char *)b)[n] = t; 115 } while (n); 116 } 117 118 /* 119 * The values are arbitrary as long as they can't be confused with 120 * a pointer, but small integers make for the smallest compare 121 * instructions. 122 */ 123 #define SWAP_WORDS_64 (swap_r_func_t)0 124 #define SWAP_WORDS_32 (swap_r_func_t)1 125 #define SWAP_BYTES (swap_r_func_t)2 126 #define SWAP_WRAPPER (swap_r_func_t)3 127 128 struct wrapper { 129 cmp_func_t cmp; 130 swap_func_t swap; 131 }; 132 133 /* 134 * The function pointer is last to make tail calls most efficient if the 135 * compiler decides not to inline this function. 136 */ 137 static void do_swap(void *a, void *b, size_t size, swap_r_func_t swap_func, const void *priv) 138 { 139 if (swap_func == SWAP_WRAPPER) { 140 ((const struct wrapper *)priv)->swap(a, b, (int)size); 141 return; 142 } 143 144 if (swap_func == SWAP_WORDS_64) 145 swap_words_64(a, b, size); 146 else if (swap_func == SWAP_WORDS_32) 147 swap_words_32(a, b, size); 148 else if (swap_func == SWAP_BYTES) 149 swap_bytes(a, b, size); 150 else 151 swap_func(a, b, (int)size, priv); 152 } 153 154 #define _CMP_WRAPPER ((cmp_r_func_t)0L) 155 156 static int do_cmp(const void *a, const void *b, cmp_r_func_t cmp, const void *priv) 157 { 158 if (cmp == _CMP_WRAPPER) 159 return ((const struct wrapper *)priv)->cmp(a, b); 160 return cmp(a, b, priv); 161 } 162 163 /** 164 * parent - given the offset of the child, find the offset of the parent. 165 * @i: the offset of the heap element whose parent is sought. Non-zero. 166 * @lsbit: a precomputed 1-bit mask, equal to "size & -size" 167 * @size: size of each element 168 * 169 * In terms of array indexes, the parent of element j = @i/@size is simply 170 * (j-1)/2. But when working in byte offsets, we can't use implicit 171 * truncation of integer divides. 172 * 173 * Fortunately, we only need one bit of the quotient, not the full divide. 174 * @size has a least significant bit. That bit will be clear if @i is 175 * an even multiple of @size, and set if it's an odd multiple. 176 * 177 * Logically, we're doing "if (i & lsbit) i -= size;", but since the 178 * branch is unpredictable, it's done with a bit of clever branch-free 179 * code instead. 180 */ 181 __attribute_const__ __always_inline 182 static size_t parent(size_t i, unsigned int lsbit, size_t size) 183 { 184 i -= size; 185 i -= size & -(i & lsbit); 186 return i / 2; 187 } 188 189 /** 190 * sort_r - sort an array of elements 191 * @base: pointer to data to sort 192 * @num: number of elements 193 * @size: size of each element 194 * @cmp_func: pointer to comparison function 195 * @swap_func: pointer to swap function or NULL 196 * @priv: third argument passed to comparison function 197 * 198 * This function does a heapsort on the given array. You may provide 199 * a swap_func function if you need to do something more than a memory 200 * copy (e.g. fix up pointers or auxiliary data), but the built-in swap 201 * avoids a slow retpoline and so is significantly faster. 202 * 203 * Sorting time is O(n log n) both on average and worst-case. While 204 * quicksort is slightly faster on average, it suffers from exploitable 205 * O(n*n) worst-case behavior and extra memory requirements that make 206 * it less suitable for kernel use. 207 */ 208 void sort_r(void *base, size_t num, size_t size, 209 cmp_r_func_t cmp_func, 210 swap_r_func_t swap_func, 211 const void *priv) 212 { 213 /* pre-scale counters for performance */ 214 size_t n = num * size, a = (num/2) * size; 215 const unsigned int lsbit = size & -size; /* Used to find parent */ 216 size_t shift = 0; 217 218 if (!a) /* num < 2 || size == 0 */ 219 return; 220 221 /* called from 'sort' without swap function, let's pick the default */ 222 if (swap_func == SWAP_WRAPPER && !((struct wrapper *)priv)->swap) 223 swap_func = NULL; 224 225 if (!swap_func) { 226 if (is_aligned(base, size, 8)) 227 swap_func = SWAP_WORDS_64; 228 else if (is_aligned(base, size, 4)) 229 swap_func = SWAP_WORDS_32; 230 else 231 swap_func = SWAP_BYTES; 232 } 233 234 /* 235 * Loop invariants: 236 * 1. elements [a,n) satisfy the heap property (compare greater than 237 * all of their children), 238 * 2. elements [n,num*size) are sorted, and 239 * 3. a <= b <= c <= d <= n (whenever they are valid). 240 */ 241 for (;;) { 242 size_t b, c, d; 243 244 if (a) /* Building heap: sift down a */ 245 a -= size << shift; 246 else if (n > 3 * size) { /* Sorting: Extract two largest elements */ 247 n -= size; 248 do_swap(base, base + n, size, swap_func, priv); 249 shift = do_cmp(base + size, base + 2 * size, cmp_func, priv) <= 0; 250 a = size << shift; 251 n -= size; 252 do_swap(base + a, base + n, size, swap_func, priv); 253 } else { /* Sort complete */ 254 break; 255 } 256 257 /* 258 * Sift element at "a" down into heap. This is the 259 * "bottom-up" variant, which significantly reduces 260 * calls to cmp_func(): we find the sift-down path all 261 * the way to the leaves (one compare per level), then 262 * backtrack to find where to insert the target element. 263 * 264 * Because elements tend to sift down close to the leaves, 265 * this uses fewer compares than doing two per level 266 * on the way down. (A bit more than half as many on 267 * average, 3/4 worst-case.) 268 */ 269 for (b = a; c = 2*b + size, (d = c + size) < n;) 270 b = do_cmp(base + c, base + d, cmp_func, priv) > 0 ? c : d; 271 if (d == n) /* Special case last leaf with no sibling */ 272 b = c; 273 274 /* Now backtrack from "b" to the correct location for "a" */ 275 while (b != a && do_cmp(base + a, base + b, cmp_func, priv) >= 0) 276 b = parent(b, lsbit, size); 277 c = b; /* Where "a" belongs */ 278 while (b != a) { /* Shift it into place */ 279 b = parent(b, lsbit, size); 280 do_swap(base + b, base + c, size, swap_func, priv); 281 } 282 } 283 284 n -= size; 285 do_swap(base, base + n, size, swap_func, priv); 286 if (n == size * 2 && do_cmp(base, base + size, cmp_func, priv) > 0) 287 do_swap(base, base + size, size, swap_func, priv); 288 } 289 EXPORT_SYMBOL(sort_r); 290 291 void sort(void *base, size_t num, size_t size, 292 cmp_func_t cmp_func, 293 swap_func_t swap_func) 294 { 295 struct wrapper w = { 296 .cmp = cmp_func, 297 .swap = swap_func, 298 }; 299 300 return sort_r(base, num, size, _CMP_WRAPPER, SWAP_WRAPPER, &w); 301 } 302 EXPORT_SYMBOL(sort); 303