gcc/sort.cc

   1 /* Platform-independent deterministic sort function.
   2    Copyright (C) 2018 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 /* Helper for netsort. Permute, possibly in-place, 2 or 3 elements,
  62    placing E0 to C->OUT, E1 to C->OUT + C->SIZE, and so on.  */
  63 static void
  64 reorder23 (sort_ctx *c, char *e0, char *e1, char *e2)
  65 {
  66 #define REORDER_23(TYPE, STRIDE, OFFSET)                 \
  67 do {                                                     \
  68   TYPE t0, t1;                                           \
  69   memcpy (&t0, e0 + OFFSET, sizeof (TYPE));              \
  70   memcpy (&t1, e1 + OFFSET, sizeof (TYPE));              \
  71   char *out = c->out + OFFSET;                           \
  72   if (likely (c->n == 3))                                \
  73     memmove (out + 2*STRIDE, e2 + OFFSET, sizeof (TYPE));\
  74   memcpy (out, &t0, sizeof (TYPE)); out += STRIDE;       \
  75   memcpy (out, &t1, sizeof (TYPE));                      \
  76 } while (0)
  77
  78   if (likely (c->size == sizeof (size_t)))
  79     REORDER_23 (size_t, sizeof (size_t), 0);
  80   else if (likely (c->size == sizeof (int)))
  81     REORDER_23 (int, sizeof (int), 0);
  82   else
  83     {
  84       size_t offset = 0, step = sizeof (size_t);
  85       for (; offset + step <= c->size; offset += step)
  86         REORDER_23 (size_t, c->size, offset);
  87       for (; offset < c->size; offset++)
  88         REORDER_23 (char, c->size, offset);
  89     }
  90 }
  91
  92 /* Like reorder23, but permute 4 or 5 elements.  */
  93 static void
  94 reorder45 (sort_ctx *c, char *e0, char *e1, char *e2, char *e3, char *e4)
  95 {
  96 #define REORDER_45(TYPE, STRIDE, OFFSET)                 \
  97 do {                                                     \
  98   TYPE t0, t1, t2, t3;                                   \
  99   memcpy (&t0, e0 + OFFSET, sizeof (TYPE));              \
 100   memcpy (&t1, e1 + OFFSET, sizeof (TYPE));              \
 101   memcpy (&t2, e2 + OFFSET, sizeof (TYPE));              \
 102   memcpy (&t3, e3 + OFFSET, sizeof (TYPE));              \
 103   char *out = c->out + OFFSET;                           \
 104   if (likely (c->n == 5))                                \
 105     memmove (out + 4*STRIDE, e4 + OFFSET, sizeof (TYPE));\
 106   memcpy (out, &t0, sizeof (TYPE)); out += STRIDE;       \
 107   memcpy (out, &t1, sizeof (TYPE)); out += STRIDE;       \
 108   memcpy (out, &t2, sizeof (TYPE)); out += STRIDE;       \
 109   memcpy (out, &t3, sizeof (TYPE));                      \
 110 } while (0)
 111
 112   if (likely (c->size == sizeof (size_t)))
 113     REORDER_45 (size_t, sizeof (size_t), 0);
 114   else if (likely(c->size == sizeof (int)))
 115     REORDER_45 (int,  sizeof (int), 0);
 116   else
 117     {
 118       size_t offset = 0, step = sizeof (size_t);
 119       for (; offset + step <= c->size; offset += step)
 120         REORDER_45 (size_t, c->size, offset);
 121       for (; offset < c->size; offset++)
 122         REORDER_45 (char, c->size, offset);
 123     }
 124 }
 125
 126 /* Helper for netsort. Invoke comparator CMP on E0 and E1.
 127    Return E0^E1 if E0 compares less than E1, zero otherwise.
 128    This is noinline to avoid code growth and confine invocation
 129    to a single call site, assisting indirect branch prediction.  */
 130 noinline static intptr_t
 131 cmp1 (char *e0, char *e1, cmp_fn *cmp)
 132 {
 133   intptr_t x = (intptr_t)e0 ^ (intptr_t)e1;
 134   return x & (cmp (e0, e1) >> 31);
 135 }
 136
 137 /* Execute network sort on 2 to 5 elements from IN, placing them into C->OUT.
 138    IN may be equal to C->OUT, in which case elements are sorted in place.  */
 139 static void
 140 netsort (char *in, sort_ctx *c)
 141 {
 142 #define CMP(e0, e1)                   \
 143 do {                                  \
 144   intptr_t x = cmp1 (e1, e0, c->cmp); \
 145   e0 = (char *)((intptr_t)e0 ^ x);    \
 146   e1 = (char *)((intptr_t)e1 ^ x);    \
 147 } while (0)
 148
 149   char *e0 = in, *e1 = e0 + c->size, *e2 = e1 + c->size;
 150   CMP (e0, e1);
 151   if (likely (c->n == 3))
 152     {
 153       CMP (e1, e2);
 154       CMP (e0, e1);
 155     }
 156   if (c->n <= 3)
 157     return reorder23 (c, e0, e1, e2);
 158   char *e3 = e2 + c->size, *e4 = e3 + c->size;
 159   if (likely (c->n == 5))
 160     {
 161       CMP (e3, e4);
 162       CMP (e2, e4);
 163     }
 164   CMP (e2, e3);
 165   if (likely (c->n == 5))
 166     {
 167       CMP (e0, e3);
 168       CMP (e1, e4);
 169     }
 170   CMP (e0, e2);
 171   CMP (e1, e3);
 172   CMP (e1, e2);
 173   reorder45 (c, e0, e1, e2, e3, e4);
 174 }
 175
 176 /* Execute merge sort on N elements from IN, placing them into OUT,
 177    using TMP as temporary storage if IN is equal to OUT.
 178    This is a stable sort if netsort is used only for 2 or 3 elements.  */
 179 static void
 180 mergesort (char *in, sort_ctx *c, size_t n, char *out, char *tmp)
 181 {
 182   if (likely (n <= c->nlim))
 183     {
 184       c->out = out;
 185       c->n = n;
 186       return netsort (in, c);
 187     }
 188   size_t nl = n / 2, nr = n - nl, sz = nl * c->size;
 189   char *mid = in + sz, *r = out + sz, *l = in == out ? tmp : in;
 190   /* Sort the right half, outputting to right half of OUT.  */
 191   mergesort (mid, c, nr, r, tmp);
 192   /* Sort the left half, leaving left half of OUT free.  */
 193   mergesort (in, c, nl, l, mid);
 194   /* Merge sorted halves given by L, R to [OUT, END).  */
 195 #define MERGE_ELTSIZE(SIZE)                     \
 196 do {                                            \
 197   intptr_t mr = c->cmp (r, l) >> 31;            \
 198   intptr_t lr = (intptr_t)l ^ (intptr_t)r;      \
 199   lr = (intptr_t)l ^ (lr & mr);                 \
 200   out = (char *)memcpy (out, (char *)lr, SIZE); \
 201   out += SIZE;                                  \
 202   r += mr & SIZE;                               \
 203   if (r == out) return;                         \
 204   l += ~mr & SIZE;                              \
 205 } while (r != end)
 206
 207   if (likely (c->cmp(r, l + (r - out) - c->size) < 0))
 208     {
 209       char *end = out + n * c->size;
 210       if (sizeof (size_t) == 8 && likely (c->size == 8))
 211         MERGE_ELTSIZE (8);
 212       else if (likely (c->size == 4))
 213         MERGE_ELTSIZE (4);
 214       else
 215         MERGE_ELTSIZE (c->size);
 216     }
 217   memcpy (out, l, r - out);
 218 }
 219
 220 void
 221 gcc_qsort (void *vbase, size_t n, size_t size, cmp_fn *cmp)
 222 {
 223   if (n < 2)
 224     return;
 225   size_t nlim = 5;
 226   bool stable = (ssize_t) size < 0;
 227   if (stable)
 228     nlim = 3, size = ~size;
 229   char *base = (char *)vbase;
 230   sort_ctx c = {cmp, base, n, size, nlim};
 231   long long scratch[32];
 232   size_t bufsz = (n / 2) * size;
 233   void *buf = bufsz <= sizeof scratch ? scratch : xmalloc (bufsz);
 234   mergesort (base, &c, n, base, (char *)buf);
 235   if (buf != scratch)
 236     free (buf);
 237 #if CHECKING_P
 238   qsort_chk (vbase, n, size, cmp);
 239 #endif
 240 }
 241
 242 void
 243 gcc_stablesort (void *vbase, size_t n, size_t size, cmp_fn *cmp)
 244 {
 245   gcc_qsort (vbase, n, ~size, cmp);
 246 }