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

[Siddhesh Poyarekar <siddhesh@sourceware.org>]

On 2022-11-25 15:40, Andreas Schwab wrote:
> 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).
> 

Hmm, I suppose something like this then?

     for (...)
       {
         ...
         volatile uintptr_t oldp = p;
         void *new_p = realloc (p, newsz)
         if (new_p != oldp)
           ...
       }

Thanks,
Sid