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131
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1 /* Platform-independent deterministic sort function.
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2 Copyright (C) 2018 Free Software Foundation, Inc.
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3 Contributed by Alexander Monakov.
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4
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5 This file is part of GCC.
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6
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7 GCC is free software; you can redistribute it and/or modify it
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8 under the terms of the GNU General Public License as published by the
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9 Free Software Foundation; either version 3, or (at your option) any
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10 later version.
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11
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12 GCC is distributed in the hope that it will be useful, but WITHOUT
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13 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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14 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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15 for more details.
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16
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17 You should have received a copy of the GNU General Public License
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18 along with GCC; see the file COPYING3. If not see
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19 <http://www.gnu.org/licenses/>. */
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20
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21 /* This implements a sort function suitable for GCC use cases:
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22 - signature-compatible to C qsort, but relaxed contract:
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23 - may apply the comparator to elements in a temporary buffer
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24 - may abort on allocation failure
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25 - deterministic (but not necessarily stable)
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26 - fast, especially for common cases (0-5 elements of size 8 or 4)
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27
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28 The implementation uses a network sort for up to 5 elements and
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29 a merge sort on top of that. Neither stage has branches depending on
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30 comparator result, trading extra arithmetic for branch mispredictions. */
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31
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32 #ifdef GENERATOR_FILE
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33 #include "bconfig.h"
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34 #else
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35 #include "config.h"
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36 #endif
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37
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38 #include "system.h"
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39
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40 #define likely(cond) __builtin_expect ((cond), 1)
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41
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42 #ifdef __GNUC__
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43 #define noinline __attribute__ ((__noinline__))
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44 #else
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45 #define noinline
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46 #endif
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47
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48 /* C-style qsort comparator function type. */
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49 typedef int cmp_fn (const void *, const void *);
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50
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51 /* Structure holding read-mostly (read-only in netsort) context. */
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52 struct sort_ctx
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53 {
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54 cmp_fn *cmp; // pointer to comparator
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55 char *out; // output buffer
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56 size_t n; // number of elements
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57 size_t size; // element size
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58 size_t nlim; // limit for network sort
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59 };
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60
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61 /* Helper for netsort. Permute, possibly in-place, 2 or 3 elements,
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62 placing E0 to C->OUT, E1 to C->OUT + C->SIZE, and so on. */
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63 static void
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64 reorder23 (sort_ctx *c, char *e0, char *e1, char *e2)
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65 {
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66 #define REORDER_23(TYPE, STRIDE, OFFSET) \
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67 do { \
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68 TYPE t0, t1; \
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69 memcpy (&t0, e0 + OFFSET, sizeof (TYPE)); \
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70 memcpy (&t1, e1 + OFFSET, sizeof (TYPE)); \
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71 char *out = c->out + OFFSET; \
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72 if (likely (c->n == 3)) \
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73 memmove (out + 2*STRIDE, e2 + OFFSET, sizeof (TYPE));\
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74 memcpy (out, &t0, sizeof (TYPE)); out += STRIDE; \
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75 memcpy (out, &t1, sizeof (TYPE)); \
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76 } while (0)
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77
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78 if (likely (c->size == sizeof (size_t)))
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79 REORDER_23 (size_t, sizeof (size_t), 0);
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80 else if (likely (c->size == sizeof (int)))
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81 REORDER_23 (int, sizeof (int), 0);
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82 else
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83 {
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84 size_t offset = 0, step = sizeof (size_t);
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85 for (; offset + step <= c->size; offset += step)
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86 REORDER_23 (size_t, c->size, offset);
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87 for (; offset < c->size; offset++)
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88 REORDER_23 (char, c->size, offset);
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89 }
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90 }
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91
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92 /* Like reorder23, but permute 4 or 5 elements. */
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93 static void
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94 reorder45 (sort_ctx *c, char *e0, char *e1, char *e2, char *e3, char *e4)
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95 {
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96 #define REORDER_45(TYPE, STRIDE, OFFSET) \
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97 do { \
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98 TYPE t0, t1, t2, t3; \
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99 memcpy (&t0, e0 + OFFSET, sizeof (TYPE)); \
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100 memcpy (&t1, e1 + OFFSET, sizeof (TYPE)); \
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101 memcpy (&t2, e2 + OFFSET, sizeof (TYPE)); \
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102 memcpy (&t3, e3 + OFFSET, sizeof (TYPE)); \
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103 char *out = c->out + OFFSET; \
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104 if (likely (c->n == 5)) \
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105 memmove (out + 4*STRIDE, e4 + OFFSET, sizeof (TYPE));\
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106 memcpy (out, &t0, sizeof (TYPE)); out += STRIDE; \
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107 memcpy (out, &t1, sizeof (TYPE)); out += STRIDE; \
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108 memcpy (out, &t2, sizeof (TYPE)); out += STRIDE; \
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109 memcpy (out, &t3, sizeof (TYPE)); \
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110 } while (0)
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111
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112 if (likely (c->size == sizeof (size_t)))
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113 REORDER_45 (size_t, sizeof (size_t), 0);
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114 else if (likely(c->size == sizeof (int)))
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115 REORDER_45 (int, sizeof (int), 0);
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116 else
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117 {
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118 size_t offset = 0, step = sizeof (size_t);
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119 for (; offset + step <= c->size; offset += step)
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120 REORDER_45 (size_t, c->size, offset);
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121 for (; offset < c->size; offset++)
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122 REORDER_45 (char, c->size, offset);
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123 }
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124 }
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125
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126 /* Helper for netsort. Invoke comparator CMP on E0 and E1.
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127 Return E0^E1 if E0 compares less than E1, zero otherwise.
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128 This is noinline to avoid code growth and confine invocation
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129 to a single call site, assisting indirect branch prediction. */
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130 noinline static intptr_t
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131 cmp1 (char *e0, char *e1, cmp_fn *cmp)
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132 {
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133 intptr_t x = (intptr_t)e0 ^ (intptr_t)e1;
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134 return x & (cmp (e0, e1) >> 31);
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135 }
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136
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137 /* Execute network sort on 2 to 5 elements from IN, placing them into C->OUT.
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138 IN may be equal to C->OUT, in which case elements are sorted in place. */
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139 static void
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140 netsort (char *in, sort_ctx *c)
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141 {
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142 #define CMP(e0, e1) \
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143 do { \
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144 intptr_t x = cmp1 (e1, e0, c->cmp); \
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145 e0 = (char *)((intptr_t)e0 ^ x); \
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146 e1 = (char *)((intptr_t)e1 ^ x); \
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147 } while (0)
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148
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149 char *e0 = in, *e1 = e0 + c->size, *e2 = e1 + c->size;
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150 CMP (e0, e1);
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151 if (likely (c->n == 3))
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152 {
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153 CMP (e1, e2);
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154 CMP (e0, e1);
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155 }
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156 if (c->n <= 3)
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157 return reorder23 (c, e0, e1, e2);
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158 char *e3 = e2 + c->size, *e4 = e3 + c->size;
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159 if (likely (c->n == 5))
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160 {
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161 CMP (e3, e4);
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162 CMP (e2, e4);
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163 }
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164 CMP (e2, e3);
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165 if (likely (c->n == 5))
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166 {
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167 CMP (e0, e3);
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168 CMP (e1, e4);
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169 }
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170 CMP (e0, e2);
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171 CMP (e1, e3);
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172 CMP (e1, e2);
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173 reorder45 (c, e0, e1, e2, e3, e4);
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174 }
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175
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176 /* Execute merge sort on N elements from IN, placing them into OUT,
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177 using TMP as temporary storage if IN is equal to OUT.
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178 This is a stable sort if netsort is used only for 2 or 3 elements. */
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179 static void
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180 mergesort (char *in, sort_ctx *c, size_t n, char *out, char *tmp)
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181 {
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182 if (likely (n <= c->nlim))
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183 {
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184 c->out = out;
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185 c->n = n;
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186 return netsort (in, c);
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187 }
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188 size_t nl = n / 2, nr = n - nl, sz = nl * c->size;
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189 char *mid = in + sz, *r = out + sz, *l = in == out ? tmp : in;
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190 /* Sort the right half, outputting to right half of OUT. */
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191 mergesort (mid, c, nr, r, tmp);
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192 /* Sort the left half, leaving left half of OUT free. */
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193 mergesort (in, c, nl, l, mid);
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194 /* Merge sorted halves given by L, R to [OUT, END). */
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195 #define MERGE_ELTSIZE(SIZE) \
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196 do { \
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197 intptr_t mr = c->cmp (r, l) >> 31; \
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198 intptr_t lr = (intptr_t)l ^ (intptr_t)r; \
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199 lr = (intptr_t)l ^ (lr & mr); \
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200 out = (char *)memcpy (out, (char *)lr, SIZE); \
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201 out += SIZE; \
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202 r += mr & SIZE; \
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203 if (r == out) return; \
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204 l += ~mr & SIZE; \
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205 } while (r != end)
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206
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207 if (likely (c->cmp(r, l + (r - out) - c->size) < 0))
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208 {
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209 char *end = out + n * c->size;
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210 if (sizeof (size_t) == 8 && likely (c->size == 8))
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211 MERGE_ELTSIZE (8);
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212 else if (likely (c->size == 4))
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213 MERGE_ELTSIZE (4);
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214 else
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215 MERGE_ELTSIZE (c->size);
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216 }
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217 memcpy (out, l, r - out);
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218 }
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219
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220 void
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221 gcc_qsort (void *vbase, size_t n, size_t size, cmp_fn *cmp)
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222 {
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223 if (n < 2)
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224 return;
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225 size_t nlim = 5;
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226 bool stable = (ssize_t) size < 0;
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227 if (stable)
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228 nlim = 3, size = ~size;
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229 char *base = (char *)vbase;
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230 sort_ctx c = {cmp, base, n, size, nlim};
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231 long long scratch[32];
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232 size_t bufsz = (n / 2) * size;
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233 void *buf = bufsz <= sizeof scratch ? scratch : xmalloc (bufsz);
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234 mergesort (base, &c, n, base, (char *)buf);
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235 if (buf != scratch)
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236 free (buf);
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237 #if CHECKING_P
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238 qsort_chk (vbase, n, size, cmp);
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239 #endif
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240 }
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241
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242 void
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243 gcc_stablesort (void *vbase, size_t n, size_t size, cmp_fn *cmp)
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244 {
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245 gcc_qsort (vbase, n, ~size, cmp);
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246 }
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