[PATCH] Use saturated arithmetic for overflow detection.

[Paul Eggert]

On 10/30/2013 10:45 AM, Ondřej Bílka wrote:

> -  if (BE ((((SIZE_MAX - (sizeof (re_node_set) + sizeof (bitset_t)) * SBC_MAX)
> -        / (3 * sizeof (re_dfastate_t *)))
> -       < ndests),
> -      0))
> -    goto out_free;
> +  size_t allocated = (sizeof (re_node_set) + sizeof (bitset_t)) * SBC_MAX;
> +  size_t needed_memory = MUL_S (MUL_S (ndests, 3), sizeof (re_dfastate_t *));
> 
> -  if (__libc_use_alloca ((sizeof (re_node_set) + sizeof (bitset_t)) * SBC_MAX
> -             + ndests * 3 * sizeof (re_dfastate_t *)))
> -    dest_states = (re_dfastate_t **)
> -      alloca (ndests * 3 * sizeof (re_dfastate_t *));
> +  if (__libc_use_alloca (ADD_S (allocated, needed_memory)))
> +    dest_states = (re_dfastate_t **) alloca (needed_memory);

This is slower than the original, since it has four conditional jumps
(MUL_S, MUL_S, ADD_S, __libc_use_alloca) instead of two (<,
__libc_use_alloca)..

All in all it's not clear that this approach is an improvement
overall, as the performance penalty may not be worth the increase in
clarity, and both approaches seem about equally error-prone.