sort.c

/* sort.c -- Sort without allocating memory
   Copyright (C) 2012-2024 Free Software Foundation, Inc.
   Written by Ian Lance Taylor, Google.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

    (1) Redistributions of source code must retain the above copyright
    notice, this list of conditions and the following disclaimer.

    (2) Redistributions in binary form must reproduce the above copyright
    notice, this list of conditions and the following disclaimer in
    the documentation and/or other materials provided with the
    distribution.

    (3) The name of the author may not be used to
    endorse or promote products derived from this software without
    specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE.  */

#include "config.h"

#include <stddef.h>
#include <string.h>
#include <sys/types.h>

#include "backtrace.h"
#include "internal.h"

/* The GNU glibc version of qsort allocates memory, which we must not
   do if we are invoked by a signal handler.  So provide our own
   sort.

   Quicksort alone has no worst-case bound, and real symbol tables do
   reach it: on one 300 MB Go binary the partition work is 122 times
   n*log2(n), which is twelve seconds.  Cap the recursion depth and
   finish with an in-place heapsort past that, bounding the worst case
   at O(n log n) for any input.  Heapsort allocates nothing, so the
   reason this file exists is preserved.  */

static void
swap (char *a, char *b, size_t size)
{
  size_t i;

  /* Swap a machine word at a time when the element size permits.  The
     byte loop below cannot be widened by the compiler, as size is a
     runtime value and the pointers are char *.  memcpy rather than a
     cast, so that no aliasing assumption is introduced; the compiler
     emits a plain load and store for each word.  */
  if (size % sizeof (uintptr_t) == 0)
    {
      size_t nwords = size / sizeof (uintptr_t);

      for (i = 0; i < nwords; i++)
	{
	  uintptr_t ta, tb;

	  memcpy (&ta, a + i * sizeof ta, sizeof ta);
	  memcpy (&tb, b + i * sizeof tb, sizeof tb);
	  memcpy (a + i * sizeof tb, &tb, sizeof tb);
	  memcpy (b + i * sizeof ta, &ta, sizeof ta);
	}
      return;
    }

  for (i = 0; i < size; i++, a++, b++)
    {
      char t;

      t = *a;
      *a = *b;
      *b = t;
    }
}

/* Sift the element at index I down into the heap rooted at BASE.  */

static void
sift_down (char *base, size_t i, size_t count, size_t size,
	   int (*compar) (const void *, const void *))
{
  for (;;)
    {
      size_t child = 2 * i + 1;

      if (child >= count)
	break;
      if (child + 1 < count
	  && (*compar) (base + child * size, base + (child + 1) * size) < 0)
	++child;
      if ((*compar) (base + i * size, base + child * size) >= 0)
	break;
      swap (base + i * size, base + child * size, size);
      i = child;
    }
}

/* In-place heapsort, used once the quicksort recursion gets too deep.
   Allocates nothing.  */

static void
heap_sort (char *base, size_t count, size_t size,
	   int (*compar) (const void *, const void *))
{
  size_t i;

  if (count < 2)
    return;
  for (i = count / 2; i > 0; i--)
    sift_down (base, i - 1, count, size, compar);
  for (i = count - 1; i > 0; i--)
    {
      swap (base, base + i * size, size);
      sift_down (base, 0, i, size, compar);
    }
}

static void
qsort_limited (char *base, size_t count, size_t size,
	       int (*compar) (const void *, const void *), unsigned depth)
{
  size_t i;
  size_t mid;

 tail_recurse:
  if (count < 2)
    return;

  if (depth == 0)
    {
      heap_sort (base, count, size, compar);
      return;
    }
  --depth;

  /* The symbol table and DWARF tables, which is all we use this
     routine for, tend to be roughly sorted.  Pick the middle element
     in the array as our pivot point, so that we are more likely to
     cut the array in half for each recursion step.  */
  swap (base, base + (count / 2) * size, size);

  mid = 0;
  for (i = 1; i < count; i++)
    {
      if ((*compar) (base, base + i * size) > 0)
	{
	  ++mid;
	  if (i != mid)
	    swap (base + mid * size, base + i * size, size);
	}
    }

  if (mid > 0)
    swap (base, base + mid * size, size);

  /* Recurse with the smaller array, loop with the larger one.  That
     ensures that our maximum stack depth is log count.  */
  if (2 * mid < count)
    {
      qsort_limited (base, mid, size, compar, depth);
      base += (mid + 1) * size;
      count -= mid + 1;
      goto tail_recurse;
    }
  else
    {
      qsort_limited (base + (mid + 1) * size, count - (mid + 1),
		     size, compar, depth);
      count = mid;
      goto tail_recurse;
    }
}

void
backtrace_qsort (void *basearg, size_t count, size_t size,
		 int (*compar) (const void *, const void *))
{
  unsigned depth = 0;
  size_t c = count;

  while (c > 1)
    {
      ++depth;
      c >>= 1;
    }
  qsort_limited ((char *) basearg, count, size, compar, 2 * depth);
}