gcc/sort.cc

   1 /* Platform-independent deterministic sort function.
   2    Copyright (C) 2018-2024 Free Software Foundation, Inc.
   3    Contributed by Alexander Monakov.
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify it
   8 under the terms of the GNU General Public License as published by the
   9 Free Software Foundation; either version 3, or (at your option) any
  10 later version.
  11
  12 GCC is distributed in the hope that it will be useful, but WITHOUT
  13 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  14 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  15 for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GCC; see the file COPYING3.  If not see
  19 <http://www.gnu.org/licenses/>.  */
  20
  21 /* This implements a sort function suitable for GCC use cases:
  22    - signature-compatible to C qsort, but relaxed contract:
  23      - may apply the comparator to elements in a temporary buffer
  24      - may abort on allocation failure
  25    - deterministic (but not necessarily stable)
  26    - fast, especially for common cases (0-5 elements of size 8 or 4)
  27
  28    The implementation uses sorting networks for up to 5 elements and
  29    a merge sort on top of that.  Neither stage has branches depending on
  30    comparator result, trading extra arithmetic for branch mispredictions.  */
  31
  32 #ifdef GENERATOR_FILE
  33 #include "bconfig.h"
  34 #else
  35 #include "config.h"
  36 #endif
  37
  38 #include "system.h"
  39
  40 #ifdef __GNUC__
  41 #define noinline __attribute__ ((__noinline__))
  42 #else
  43 #define noinline
  44 #endif
  45
  46 /* C-style qsort comparator function type.  */
  47 typedef int cmp_fn (const void *, const void *);
  48
  49 /* Structure holding read-mostly (read-only in netsort) context.  */
  50 struct sort_ctx
  51 {
  52   cmp_fn *cmp; // pointer to comparator
  53   char   *out; // output buffer
  54   size_t n;    // number of elements
  55   size_t size; // element size
  56   size_t nlim; // limit for using sorting networks
  57 };
  58
  59 /* Like sort_ctx, but for use with qsort_r-style comparators.  Several
  60    functions in this file are templates that work with either context type.  */
  61 struct sort_r_ctx
  62 {
  63   void          *data;
  64   sort_r_cmp_fn *cmp_;
  65   char   *out;
  66   size_t n;
  67   size_t size;
  68   size_t nlim;
  69   int cmp (const void *a, const void *b)
  70   {
  71     return cmp_ (a, b, data);
  72   }
  73 };
  74
  75 /* Helper for netsort. Permute, possibly in-place, 2 or 3 elements,
  76    placing E0 to C->OUT, E1 to C->OUT + C->SIZE, and so on.  */
  77 template<typename sort_ctx>
  78 static void
  79 reorder23 (sort_ctx *c, char *e0, char *e1, char *e2)
  80 {
  81 #define REORDER_23(TYPE, STRIDE, OFFSET)                 \
  82 do {                                                     \
  83   TYPE t0, t1;                                           \
  84   memcpy (&t0, e0 + OFFSET, sizeof (TYPE));              \
  85   memcpy (&t1, e1 + OFFSET, sizeof (TYPE));              \
  86   char *out = c->out + OFFSET;                           \
  87   if (LIKELY (c->n == 3))                                \
  88     memmove (out + 2*STRIDE, e2 + OFFSET, sizeof (TYPE));\
  89   memcpy (out, &t0, sizeof (TYPE)); out += STRIDE;       \
  90   memcpy (out, &t1, sizeof (TYPE));                      \
  91 } while (0)
  92
  93   if (LIKELY (c->size == sizeof (size_t)))
  94     REORDER_23 (size_t, sizeof (size_t), 0);
  95   else if (LIKELY (c->size == sizeof (int)))
  96     REORDER_23 (int, sizeof (int), 0);
  97   else
  98     {
  99       size_t offset = 0, step = sizeof (size_t);
 100       for (; offset + step <= c->size; offset += step)
 101         REORDER_23 (size_t, c->size, offset);
 102       for (; offset < c->size; offset++)
 103         REORDER_23 (char, c->size, offset);
 104     }
 105 }
 106
 107 /* Like reorder23, but permute 4 or 5 elements.  */
 108 template<typename sort_ctx>
 109 static void
 110 reorder45 (sort_ctx *c, char *e0, char *e1, char *e2, char *e3, char *e4)
 111 {
 112 #define REORDER_45(TYPE, STRIDE, OFFSET)                 \
 113 do {                                                     \
 114   TYPE t0, t1, t2, t3;                                   \
 115   memcpy (&t0, e0 + OFFSET, sizeof (TYPE));              \
 116   memcpy (&t1, e1 + OFFSET, sizeof (TYPE));              \
 117   memcpy (&t2, e2 + OFFSET, sizeof (TYPE));              \
 118   memcpy (&t3, e3 + OFFSET, sizeof (TYPE));              \
 119   char *out = c->out + OFFSET;                           \
 120   if (LIKELY (c->n == 5))                                \
 121     memmove (out + 4*STRIDE, e4 + OFFSET, sizeof (TYPE));\
 122   memcpy (out, &t0, sizeof (TYPE)); out += STRIDE;       \
 123   memcpy (out, &t1, sizeof (TYPE)); out += STRIDE;       \
 124   memcpy (out, &t2, sizeof (TYPE)); out += STRIDE;       \
 125   memcpy (out, &t3, sizeof (TYPE));                      \
 126 } while (0)
 127
 128   if (LIKELY (c->size == sizeof (size_t)))
 129     REORDER_45 (size_t, sizeof (size_t), 0);
 130   else if (LIKELY (c->size == sizeof (int)))
 131     REORDER_45 (int,  sizeof (int), 0);
 132   else
 133     {
 134       size_t offset = 0, step = sizeof (size_t);
 135       for (; offset + step <= c->size; offset += step)
 136         REORDER_45 (size_t, c->size, offset);
 137       for (; offset < c->size; offset++)
 138         REORDER_45 (char, c->size, offset);
 139     }
 140 }
 141
 142 /* Helper for netsort. Invoke comparator CMP on E0 and E1.
 143    Return E0^E1 if E0 compares less than E1, zero otherwise.
 144    This is noinline to avoid code growth and confine invocation
 145    to a single call site, assisting indirect branch prediction.  */
 146 template<typename sort_ctx>
 147 noinline static intptr_t
 148 cmp1 (char *e0, char *e1, sort_ctx *c)
 149 {
 150   intptr_t x = (intptr_t)e0 ^ (intptr_t)e1;
 151   return x & (c->cmp (e0, e1) >> 31);
 152 }
 153
 154 /* Apply a sorting network to 2 to 5 elements from IN, placing them into C->OUT.
 155    IN may be equal to C->OUT, in which case elements are sorted in place.  */
 156 template<typename sort_ctx>
 157 static void
 158 netsort (char *in, sort_ctx *c)
 159 {
 160 #define CMP(e0, e1)                   \
 161 do {                                  \
 162   intptr_t x = cmp1 (e1, e0, c);      \
 163   e0 = (char *)((intptr_t)e0 ^ x);    \
 164   e1 = (char *)((intptr_t)e1 ^ x);    \
 165 } while (0)
 166
 167   char *e0 = in, *e1 = e0 + c->size, *e2 = e1 + c->size;
 168   CMP (e0, e1);
 169   if (LIKELY (c->n == 3))
 170     {
 171       CMP (e1, e2);
 172       CMP (e0, e1);
 173     }
 174   if (c->n <= 3)
 175     return reorder23 (c, e0, e1, e2);
 176   char *e3 = e2 + c->size, *e4 = e3 + c->size;
 177   if (LIKELY (c->n == 5))
 178     {
 179       CMP (e3, e4);
 180       CMP (e2, e4);
 181     }
 182   CMP (e2, e3);
 183   if (LIKELY (c->n == 5))
 184     {
 185       CMP (e0, e3);
 186       CMP (e1, e4);
 187     }
 188   CMP (e0, e2);
 189   CMP (e1, e3);
 190   CMP (e1, e2);
 191   reorder45 (c, e0, e1, e2, e3, e4);
 192 }
 193
 194 /* Execute merge sort on N elements from IN, placing them into OUT,
 195    using TMP as temporary storage if IN is equal to OUT.
 196    This is a stable sort if netsort is used only for 2 or 3 elements.  */
 197 template<typename sort_ctx>
 198 static void
 199 mergesort (char *in, sort_ctx *c, size_t n, char *out, char *tmp)
 200 {
 201   if (LIKELY (n <= c->nlim))
 202     {
 203       c->out = out;
 204       c->n = n;
 205       return netsort (in, c);
 206     }
 207   size_t nl = n / 2, nr = n - nl, sz = nl * c->size;
 208   char *mid = in + sz, *r = out + sz, *l = in == out ? tmp : in;
 209   /* Sort the right half, outputting to right half of OUT.  */
 210   mergesort (mid, c, nr, r, tmp);
 211   /* Sort the left half, leaving left half of OUT free.  */
 212   mergesort (in, c, nl, l, mid);
 213   /* Merge sorted halves given by L, R to [OUT, END).  */
 214 #define MERGE_ELTSIZE(SIZE)                     \
 215 do {                                            \
 216   intptr_t mr = c->cmp (r, l) >> 31;            \
 217   intptr_t lr = (intptr_t)l ^ (intptr_t)r;      \
 218   lr = (intptr_t)l ^ (lr & mr);                 \
 219   out = (char *)memcpy (out, (char *)lr, SIZE); \
 220   out += SIZE;                                  \
 221   r += mr & SIZE;                               \
 222   if (r == out) return;                         \
 223   l += ~mr & SIZE;                              \
 224 } while (r != end)
 225
 226   if (LIKELY (c->cmp (r, l + (r - out) - c->size) < 0))
 227     {
 228       char *end = out + n * c->size;
 229       if (sizeof (size_t) == 8 && LIKELY (c->size == 8))
 230         MERGE_ELTSIZE (8);
 231       else if (LIKELY (c->size == 4))
 232         MERGE_ELTSIZE (4);
 233       else
 234         MERGE_ELTSIZE (c->size);
 235     }
 236   memcpy (out, l, r - out);
 237 }
 238
 239 #if CHECKING_P
 240   /* Don't complain about cast from void* to function pointer.  */
 241 #pragma GCC diagnostic push
 242 #pragma GCC diagnostic ignored "-Wconditionally-supported"
 243
 244 /* Adapter for using two-argument comparators in functions expecting the
 245    three-argument sort_r_cmp_fn type.  */
 246 static int
 247 cmp2to3 (const void *a, const void *b, void *c)
 248 {
 249   return ((cmp_fn *)c) (a, b);
 250 }
 251 #endif
 252
 253 /* Replacement for C qsort.  */
 254 void
 255 gcc_qsort (void *vbase, size_t n, size_t size, cmp_fn *cmp)
 256 {
 257   if (n < 2)
 258     return;
 259   size_t nlim = 5;
 260   bool stable = (ssize_t) size < 0;
 261   if (stable)
 262     nlim = 3, size = ~size;
 263   char *base = (char *)vbase;
 264   sort_ctx c = {cmp, base, n, size, nlim};
 265   long long scratch[32];
 266   size_t bufsz = (n / 2) * size;
 267   void *buf = bufsz <= sizeof scratch ? scratch : xmalloc (bufsz);
 268   mergesort (base, &c, n, base, (char *)buf);
 269   if (buf != scratch)
 270     free (buf);
 271 #if CHECKING_P
 272   qsort_chk (vbase, n, size, cmp2to3, (void*)cmp);
 273 #pragma GCC diagnostic pop
 274 #endif
 275 }
 276
 277 /* Substitute for Glibc qsort_r.  */
 278 void
 279 gcc_sort_r (void *vbase, size_t n, size_t size, sort_r_cmp_fn *cmp, void *data)
 280 {
 281   if (n < 2)
 282     return;
 283   size_t nlim = 5;
 284   bool stable = (ssize_t) size < 0;
 285   if (stable)
 286     nlim = 3, size = ~size;
 287   char *base = (char *)vbase;
 288   sort_r_ctx c = {data, cmp, base, n, size, nlim};
 289   long long scratch[32];
 290   size_t bufsz = (n / 2) * size;
 291   void *buf = bufsz <= sizeof scratch ? scratch : xmalloc (bufsz);
 292   mergesort (base, &c, n, base, (char *)buf);
 293   if (buf != scratch)
 294     free (buf);
 295 #if CHECKING_P
 296   qsort_chk (vbase, n, size, cmp, data);
 297 #endif
 298 }
 299
 300 /* Stable sort, signature-compatible to C qsort.  */
 301 void
 302 gcc_stablesort (void *vbase, size_t n, size_t size, cmp_fn *cmp)
 303 {
 304   gcc_qsort (vbase, n, ~size, cmp);
 305 }
 306
 307 /* Stable sort, signature-compatible to Glibc qsort_r.  */
 308 void
 309 gcc_stablesort_r (void *vbase, size_t n, size_t size, sort_r_cmp_fn *cmp,
 310                   void *data)
 311 {
 312   gcc_sort_r (vbase, n, ~size, cmp, data);
 313 }