Re: Aligned blocks management: obsolete?

[Dmitry Antipov <dmantipov@yandex.ru>]

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On 06/20/2012 04:48 PM, Stefan Monnier wrote:

> I'm not completely sure the test is sufficiently thorough to catch all
> problems, and more importantly it doesn't test the efficiency
> of memalign.

If it's just a matter of configure test, different methods may be used.
For example, attached code checks whether memalign tends to create holes
while allocating (1024 - 8) blocks aligned at 1024-bytes boundary; optimal
pad (BLOCK_PADDING in current code) may be detected with the similar way.

Dmitry

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/* Simple check whether memalign tends to waste some space.  */

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>

int
main (int argc, char *argv[])
{
  int i, j, bad = 0;
  const size_t blksz = 1024, padsz = 8;
  void *p, *prev;

  /* make some chaos in the heap */
  for (i = 0; i <= 1000; i++)
    for (j = 8; j <= 1024; j += 8)
      {
	p = malloc (j);
	if (((i + j) & 7) == 0)
	  free (p);
      }

  /* test asks for BLKSZ - PADSZ bytes aligned at BLKSZ boundary */
  for (i = 0, prev = NULL; i < 16; i++)
    {
      p = memalign (blksz, blksz - padsz);
      if (!p)
	bad++;
      else if (prev && p - prev > blksz)
	bad++;
      prev = p;
    }

  /* make even more chaos with larger chunks */
  for (i = 0; i <= 1000; i++)
    for (j = 8; j <= 1024; j += 8)
      {
	p = malloc (j * 16);
	if (((i + j) & 15) == 0)
	  free (p);
      }

  /* test again */
  for (i = 0, prev = NULL; i < 16; i++)
    {
      p = memalign (blksz, blksz - padsz);
      if (!p)
	bad++;
      else if (prev && p - prev > blksz)
	bad++;
      prev = p;
    }
  
  /* zero means memalign looks good enough */
  return bad;
}