Re: Document __builtin_*_overflow

[Richard Biener <richard dot guenther at gmail dot com>]

On Wed, Nov 12, 2014 at 2:13 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> Hi!
>
> This patch mentions __builtin_*_overflow in gcc-5/changes.html.
> Ok for CVS?

Ok.

Thanks,
Richard.

> --- gcc-5/changes.html  27 Oct 2014 18:05:26 -0000      1.20
> +++ gcc-5/changes.html  12 Nov 2014 13:09:01 -0000
> @@ -84,7 +84,36 @@
>          of the standard directive <code>#include</code>
>          and the extension <code>#include_next</code> respectively.
>      </li>
> -
> +    <li>A new set of built-in functions for arithmetics with overflow checking
> +       has been added: <code>__builtin_add_overflow</code>,
> +       <code>__builtin_sub_overflow</code> and <code>__builtin_mul_overflow</code>
> +       and for compatibility with clang also other variants.
> +       These builtins have two integral arguments (which don't need to have
> +       the same type), the arguments are extended to infinite precision
> +       signed type, <code>+</code>, <code>-</code> or <code>*</code>
> +       is performed on those and the result is stored into some integer
> +       variable pointed by the last argument.  If the stored value is equal
> +       to the infinite precision result, the built-in functions return
> +       <code>false</code>, otherwise <code>true</code>.  The type of
> +       the integer variable that will hold the result can be different from
> +       the types of arguments.  The following snippet demonstrates how
> +       this can be used in computing the size for the <code>calloc</code>
> +       function:
> +<blockquote><pre>
> +void *
> +calloc (size_t x, size_t y)
> +{
> +  size_t sz;
> +  if (__builtin_mul_overflow (x, y, &amp;sz)
> +    return NULL;
> +  void *ret = malloc (sz);
> +  if (ret) memset (res, 0, sz);
> +  return ret;
> +}
> +</pre></blockquote>
> +       On e.g. i?86 or x86-64 the above will result in <code>mul</code>
> +       instruction followed by jump on overflow.
> +    </li>
>    </ul>
>
>  <h3 id="c">C</h3>
>
>         Jakub