Re: [Fwd: basic_string<> - useless because of its poor performance]

[Carlo Wood <carlo at alinoe dot com>]

On Fri, Jul 06, 2001 at 02:35:19AM -0500, Loren James Rittle wrote:
> +       // This version assumes the malloc implementation prefers to
> +       // align allocations over the size of a page to a page boundary
> +       // and under the size of a page to a power of 2.
> +       const size_t __pagesize = 4096; // This magic constant, from OS.
> +       const size_t __malloc_header_size = 0; // This one, from malloc.
> +       if ((__size + __malloc_header_size) > __pagesize)
> + 	{
> + 	  size_t __extra =
> + 	    (__pagesize - ((__size + __malloc_header_size) % __pagesize))
> + 	    % __pagesize;
> + 	  __capacity += __extra / sizeof(_CharT);
> + 	  __size = (__capacity + 1) * sizeof(_CharT) + sizeof(_Rep);
> + 	}
> +       else if (__size > 128) // This magic constant is from stl_alloc.h.
> + 	{
> + 	  size_t __extra =
> + 	    (256 - ((__size + __malloc_header_size) % 256)) % 256;
> + 	  __capacity += __extra / sizeof(_CharT);
> + 	  __size = (__capacity + 1) * sizeof(_CharT) + sizeof(_Rep);
> + 	}
> + 

The comments are not consistant with the code.  It does not align to a
power of 2 for small sizes, does it?  It seems to align to a boundary of
256 bytes (256, 512, 768, 1024, 1280, ...) for sizes larger than 128 and
not touch the size at all for sizes smaller or equal than 128.

-- 
Carlo Wood <carlo@alinoe.com>