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

[Andreas Schwab]

On Nov 25 2022, Siddhesh Poyarekar via Libc-alpha wrote:

> +  /* 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);
> +      if (p == NULL)
> +	FAIL_EXIT1 ("realloc (NULL, 31) returned NULL.");

s/31/sz/

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

The compiler is allowed to optimize this to true under the assumption
that new_p == NULL (the only case where p is still a valid pointer).

-- 
Andreas Schwab, schwab@linux-m68k.org
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