gcc/sort.cc

   1 /* Platform-independent deterministic sort function.
   2    Copyright (C) 2018-2021 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 a network sort 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 #define likely(cond) __builtin_expect ((cond), 1)
  41
  42 #ifdef __GNUC__
  43 #define noinline __attribute__ ((__noinline__))
  44 #else
  45 #define noinline
  46 #endif
  47
  48 /* C-style qsort comparator function type.  */
  49 typedef int cmp_fn (const void *, const void *);
  50
  51 /* Structure holding read-mostly (read-only in netsort) context.  */
  52 struct sort_ctx
  53 {
  54   cmp_fn *cmp; // pointer to comparator
  55   char   *out; // output buffer
  56   size_t n;    // number of elements
  57   size_t size; // element size
  58   size_t nlim; // limit for network sort
  59 };
  60
  61 /* Like sort_ctx, but for use with qsort_r-style comparators.  Several
  62    functions in this file are templates that work with either context type.  */
  63 struct sort_r_ctx
  64 {
  65   void          *data;
  66   sort_r_cmp_fn *cmp_;
  67   char   *out;
  68   size_t n;
  69   size_t size;
  70   size_t nlim;
  71   int cmp (const void *a, const void *b)
  72   {
  73     return cmp_ (a, b, data);
  74   }
  75 };
  76
  77 /* Helper for netsort. Permute, possibly in-place, 2 or 3 elements,
  78    placing E0 to C->OUT, E1 to C->OUT + C->SIZE, and so on.  */
  79 template<typename sort_ctx>
  80 static void
  81 reorder23 (sort_ctx *c, char *e0, char *e1, char *e2)
  82 {
  83 #define REORDER_23(TYPE, STRIDE, OFFSET)                 \
  84 do {                                                     \
  85   TYPE t0, t1;                                           \
  86   memcpy (&t0, e0 + OFFSET, sizeof (TYPE));              \
  87   memcpy (&t1, e1 + OFFSET, sizeof (TYPE));              \
  88   char *out = c->out + OFFSET;                           \
  89   if (likely (c->n == 3))                                \
  90     memmove (out + 2*STRIDE, e2 + OFFSET, sizeof (TYPE));\
  91   memcpy (out, &t0, sizeof (TYPE)); out += STRIDE;       \
  92   memcpy (out, &t1, sizeof (TYPE));                      \
  93 } while (0)
  94
  95   if (likely (c->size == sizeof (size_t)))
  96     REORDER_23 (size_t, sizeof (size_t), 0);
  97   else if (likely (c->size == sizeof (int)))
  98     REORDER_23 (int, sizeof (int), 0);
  99   else
 100     {
 101       size_t offset = 0, step = sizeof (size_t);
 102       for (; offset + step <= c->size; offset += step)
 103         REORDER_23 (size_t, c->size, offset);
 104       for (; offset < c->size; offset++)
 105         REORDER_23 (char, c->size, offset);
 106     }
 107 }
 108
 109 /* Like reorder23, but permute 4 or 5 elements.  */
 110 template<typename sort_ctx>
 111 static void
 112 reorder45 (sort_ctx *c, char *e0, char *e1, char *e2, char *e3, char *e4)
 113 {
 114 #define REORDER_45(TYPE, STRIDE, OFFSET)                 \
 115 do {                                                     \
 116   TYPE t0, t1, t2, t3;                                   \
 117   memcpy (&t0, e0 + OFFSET, sizeof (TYPE));              \
 118   memcpy (&t1, e1 + OFFSET, sizeof (TYPE));              \
 119   memcpy (&t2, e2 + OFFSET, sizeof (TYPE));              \
 120   memcpy (&t3, e3 + OFFSET, sizeof (TYPE));              \
 121   char *out = c->out + OFFSET;                           \
 122   if (likely (c->n == 5))                                \
 123     memmove (out + 4*STRIDE, e4 + OFFSET, sizeof (TYPE));\
 124   memcpy (out, &t0, sizeof (TYPE)); out += STRIDE;       \
 125   memcpy (out, &t1, sizeof (TYPE)); out += STRIDE;       \
 126   memcpy (out, &t2, sizeof (TYPE)); out += STRIDE;       \
 127   memcpy (out, &t3, sizeof (TYPE));                      \
 128 } while (0)
 129
 130   if (likely (c->size == sizeof (size_t)))
 131     REORDER_45 (size_t, sizeof (size_t), 0);
 132   else if (likely(c->size == sizeof (int)))
 133     REORDER_45 (int,  sizeof (int), 0);
 134   else
 135     {
 136       size_t offset = 0, step = sizeof (size_t);
 137       for (; offset + step <= c->size; offset += step)
 138         REORDER_45 (size_t, c->size, offset);
 139       for (; offset < c->size; offset++)
 140         REORDER_45 (char, c->size, offset);
 141     }
 142 }
 143
 144 /* Helper for netsort. Invoke comparator CMP on E0 and E1.
 145    Return E0^E1 if E0 compares less than E1, zero otherwise.
 146    This is noinline to avoid code growth and confine invocation
 147    to a single call site, assisting indirect branch prediction.  */
 148 template<typename sort_ctx>
 149 noinline static intptr_t
 150 cmp1 (char *e0, char *e1, sort_ctx *c)
 151 {
 152   intptr_t x = (intptr_t)e0 ^ (intptr_t)e1;
 153   return x & (c->cmp (e0, e1) >> 31);
 154 }
 155
 156 /* Execute network sort on 2 to 5 elements from IN, placing them into C->OUT.
 157    IN may be equal to C->OUT, in which case elements are sorted in place.  */
 158 template<typename sort_ctx>
 159 static void
 160 netsort (char *in, sort_ctx *c)
 161 {
 162 #define CMP(e0, e1)                   \
 163 do {                                  \
 164   intptr_t x = cmp1 (e1, e0, c);      \
 165   e0 = (char *)((intptr_t)e0 ^ x);    \
 166   e1 = (char *)((intptr_t)e1 ^ x);    \
 167 } while (0)
 168
 169   char *e0 = in, *e1 = e0 + c->size, *e2 = e1 + c->size;
 170   CMP (e0, e1);
 171   if (likely (c->n == 3))
 172     {
 173       CMP (e1, e2);
 174       CMP (e0, e1);
 175     }
 176   if (c->n <= 3)
 177     return reorder23 (c, e0, e1, e2);
 178   char *e3 = e2 + c->size, *e4 = e3 + c->size;
 179   if (likely (c->n == 5))
 180     {
 181       CMP (e3, e4);
 182       CMP (e2, e4);
 183     }
 184   CMP (e2, e3);
 185   if (likely (c->n == 5))
 186     {
 187       CMP (e0, e3);
 188       CMP (e1, e4);
 189     }
 190   CMP (e0, e2);
 191   CMP (e1, e3);
 192   CMP (e1, e2);
 193   reorder45 (c, e0, e1, e2, e3, e4);
 194 }
 195
 196 /* Execute merge sort on N elements from IN, placing them into OUT,
 197    using TMP as temporary storage if IN is equal to OUT.
 198    This is a stable sort if netsort is used only for 2 or 3 elements.  */
 199 template<typename sort_ctx>
 200 static void
 201 mergesort (char *in, sort_ctx *c, size_t n, char *out, char *tmp)
 202 {
 203   if (likely (n <= c->nlim))
 204     {
 205       c->out = out;
 206       c->n = n;
 207       return netsort (in, c);
 208     }
 209   size_t nl = n / 2, nr = n - nl, sz = nl * c->size;
 210   char *mid = in + sz, *r = out + sz, *l = in == out ? tmp : in;
 211   /* Sort the right half, outputting to right half of OUT.  */
 212   mergesort (mid, c, nr, r, tmp);
 213   /* Sort the left half, leaving left half of OUT free.  */
 214   mergesort (in, c, nl, l, mid);
 215   /* Merge sorted halves given by L, R to [OUT, END).  */
 216 #define MERGE_ELTSIZE(SIZE)                     \
 217 do {                                            \
 218   intptr_t mr = c->cmp (r, l) >> 31;            \
 219   intptr_t lr = (intptr_t)l ^ (intptr_t)r;      \
 220   lr = (intptr_t)l ^ (lr & mr);                 \
 221   out = (char *)memcpy (out, (char *)lr, SIZE); \
 222   out += SIZE;                                  \
 223   r += mr & SIZE;                               \
 224   if (r == out) return;                         \
 225   l += ~mr & SIZE;                              \
 226 } while (r != end)
 227
 228   if (likely (c->cmp(r, l + (r - out) - c->size) < 0))
 229     {
 230       char *end = out + n * c->size;
 231       if (sizeof (size_t) == 8 && likely (c->size == 8))
 232         MERGE_ELTSIZE (8);
 233       else if (likely (c->size == 4))
 234         MERGE_ELTSIZE (4);
 235       else
 236         MERGE_ELTSIZE (c->size);
 237     }
 238   memcpy (out, l, r - out);
 239 }
 240
 241 #if CHECKING_P
 242 /* Adapter for using two-argument comparators in functions expecting the
 243    three-argument sort_r_cmp_fn type.  */
 244 static int
 245 cmp2to3 (const void *a, const void *b, void *c)
 246 {
 247   return ((cmp_fn *)c) (a, b);
 248 }
 249 #endif
 250
 251 /* Replacement for C qsort.  */
 252 void
 253 gcc_qsort (void *vbase, size_t n, size_t size, cmp_fn *cmp)
 254 {
 255   if (n < 2)
 256     return;
 257   size_t nlim = 5;
 258   bool stable = (ssize_t) size < 0;
 259   if (stable)
 260     nlim = 3, size = ~size;
 261   char *base = (char *)vbase;
 262   sort_ctx c = {cmp, base, n, size, nlim};
 263   long long scratch[32];
 264   size_t bufsz = (n / 2) * size;
 265   void *buf = bufsz <= sizeof scratch ? scratch : xmalloc (bufsz);
 266   mergesort (base, &c, n, base, (char *)buf);
 267   if (buf != scratch)
 268     free (buf);
 269 #if CHECKING_P
 270   qsort_chk (vbase, n, size, cmp2to3, (void*)cmp);
 271 #endif
 272 }
 273
 274 /* Substitute for Glibc qsort_r.  */
 275 void
 276 gcc_sort_r (void *vbase, size_t n, size_t size, sort_r_cmp_fn *cmp, void *data)
 277 {
 278   if (n < 2)
 279     return;
 280   size_t nlim = 5;
 281   bool stable = (ssize_t) size < 0;
 282   if (stable)
 283     nlim = 3, size = ~size;
 284   char *base = (char *)vbase;
 285   sort_r_ctx c = {data, cmp, base, n, size, nlim};
 286   long long scratch[32];
 287   size_t bufsz = (n / 2) * size;
 288   void *buf = bufsz <= sizeof scratch ? scratch : xmalloc (bufsz);
 289   mergesort (base, &c, n, base, (char *)buf);
 290   if (buf != scratch)
 291     free (buf);
 292 #if CHECKING_P
 293   qsort_chk (vbase, n, size, cmp, data);
 294 #endif
 295 }
 296
 297 /* Stable sort, signature-compatible to C qsort.  */
 298 void
 299 gcc_stablesort (void *vbase, size_t n, size_t size, cmp_fn *cmp)
 300 {
 301   gcc_qsort (vbase, n, ~size, cmp);
 302 }
 303
 304 /* Stable sort, signature-compatible to Glibc qsort_r.  */
 305 void
 306 gcc_stablesort_r (void *vbase, size_t n, size_t size, sort_r_cmp_fn *cmp,
 307                   void *data)
 308 {
 309   gcc_sort_r (vbase, n, ~size, cmp, data);
 310 }