[PATCH v2] realloc: Return unchanged if request is within usable size

[DJ Delorie]

Siddhesh Poyarekar via Libc-alpha <libc-alpha@sourceware.org> writes:
> Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

LGTM
Reviewed-by: DJ Delorie <dj@redhat.com>

> diff --git a/malloc/malloc.c b/malloc/malloc.c
> index 2a61c8b5ee..ef8c794fb7 100644
> --- a/malloc/malloc.c
> +++ b/malloc/malloc.c
> @@ -1100,6 +1100,8 @@ static void munmap_chunk(mchunkptr p);
>  static mchunkptr mremap_chunk(mchunkptr p, size_t new_size);
>  #endif
>  
> +static size_t musable (void *mem);

Matches function which occurs later on

> +  /* Return the chunk as is whenever possible, i.e. there's enough usable space
> +     but not so much that we end up fragmenting the block.  We use the trim
> +     threshold as the heuristic to decide the latter.  */
> +  size_t usable = musable (oldmem);
> +  if (bytes <= usable
> +      && (unsigned long) (usable - bytes) <= mp_.trim_threshold)
> +    return oldmem;

Ok.

> diff --git a/malloc/tst-realloc.c b/malloc/tst-realloc.c
> +#include <stdint.h>

Ok.

>  
> +  /* Smoke test to make sure that allocations do not move if they have enough
> +     space to expand in the chunk.  */
> +  for (size_t sz = 3; sz < 256 * 1024; sz += 2048)
> +    {
> +      p = realloc (NULL, sz);

size 3, 2051, 4099... always 3 bytes more than a 2048-boundary

> +      if (p == NULL)
> +	FAIL_EXIT1 ("realloc (NULL, %zu) returned NULL.", sz);
> +      size_t newsz = malloc_usable_size (p);

Ok.

> +      printf ("size: %zu, usable size: %zu, extra: %zu\n",
> +	      sz, newsz, newsz - sz);
> +      uintptr_t oldp = (uintptr_t) p;
> +      void *new_p = realloc (p, newsz);

Should always work; either we're within a few words, or within a page
(mmap).  Ok.

> +      if ((uintptr_t) new_p != oldp)
> +	FAIL_EXIT1 ("Expanding (%zu bytes) to usable size (%zu) moved block",
> +		    sz, newsz);
> +      free (new_p);
> +
> +      /* We encountered a large enough extra size at least once.  */
> +      if (newsz - sz > 1024)
> +	break;

Ok.

> +
>    return 0;
>  }