Re: [Libunwind-devel] [PATCH 06/57] Simplify `sos_alloc()' implementation

[Arun Sharma]

On Fri, Sep 21, 2012 at 4:11 AM, Tommi Rantala <address@hidden> wrote:

>  HIDDEN void *
>  sos_alloc (size_t size)
>  {
> -  char *mem;
> -
> -#ifdef HAVE_CMPXCHG
> -  char *old_mem;
> +  size_t pos;
>
>    size = UNW_ALIGN(size, MAX_ALIGN);
> -  if (!sos_memp)
> -    cmpxchg_ptr (&sos_memp, 0, sos_memory);
> -  do
> -    {
> -      old_mem = sos_memp;
>
> -      mem = (char *) UNW_ALIGN((unsigned long) old_mem, MAX_ALIGN);
> -      mem += size;
> -      assert (mem < sos_memory + sizeof (sos_memory));
> -    }
> -  while (!cmpxchg_ptr (&sos_memp, old_mem, mem));
> +#if defined(__GNUC__)
> +  /* Assume `sos_memory' is suitably aligned. */
> +  assert(((uintptr_t) &sos_memory[0] & (MAX_ALIGN-1)) == 0);
> +#endif
> +
> +#if defined(__GNUC__) && defined(HAVE_FETCH_AND_ADD)
> +  pos = fetch_and_add (&sos_memory_freepos, size);
>  #else
>    static define_lock (sos_lock);
>    intrmask_t saved_mask;
>
> -  size = UNW_ALIGN(size, MAX_ALIGN);
> -
>    lock_acquire (&sos_lock, saved_mask);
>    {
> -    if (!sos_memp)
> -      sos_memp = sos_memory;
> -
> -    mem = (char *) UNW_ALIGN((unsigned long) sos_memp, MAX_ALIGN);
> -    mem += size;
> -    assert (mem < sos_memory + sizeof (sos_memory));
> -    sos_memp = mem;
> +# ifndef __GNUC__
> +    /* No assumptions about `sos_memory' alignment. */
> +    if (sos_memory_freepos == 0)
> +      {
> +       unsigned align = UNW_ALIGN((uintptr_t) &sos_memory[0], MAX_ALIGN)
> +                               - (uintptr_t) &sos_memory[0];
> +       sos_memory_freepos = align;
> +      }
> +# endif
> +    pos = sos_memory_freepos;
> +    sos_memory_freepos += size;
>    }
>    lock_release (&sos_lock, saved_mask);
>  #endif
> -  return mem;
> +
> +  assert (((uintptr_t) &sos_memory[pos] & (MAX_ALIGN-1)) == 0);
> +  assert ((pos+size) <= SOS_MEMORY_SIZE);
> +
> +  return &sos_memory[pos];
>  }
>
>  /* Must be called while holding the mempool lock. */

That's a lot of ifdefs in machine independent code. I feel:

http://paste.ubuntu.com/1250227/

should suffice. This might pessimize the GNUC && !HAVE_FETCH_AND_ADD a
bit. Let me know if you think this is a problem.

 -Arun