On Mon, Jan 14, 2019 at 2:46 PM H.J. Lu <hongjiu...@intel.com> wrote:
>
> This patch adds -Waddress-of-packed-member to GCC 9 porting guide.
>
> OK to install?
The docs fail to mention what to do when the unaligned pointer is _not_
safe to use. That is, how do I fix
struct { char c; int i[4]; } s __attribute__((packed));
int foo()
{
int *p = s.i;
return bar (p);
}
int bar (int *q)
{
return *q;
}
for the cases where eliding the pointer isn't easily possible?
Please also mention the new warning in changes.html
(it seems to be enabled by default even?).
IIRC the frontends themselves build "bogus" pointer types
to aligned data from a simple &s.i[1] because the FIELD_DECLs
types are naturally aligned.
Richard.
> Thanks.
>
> H.J.
> ---
> Index: gcc-9/porting_to.html
> ===================================================================
> RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-9/porting_to.html,v
> retrieving revision 1.1
> diff -u -r1.1 porting_to.html
> --- gcc-9/porting_to.html 11 Jan 2019 18:21:45 -0000 1.1
> +++ gcc-9/porting_to.html 14 Jan 2019 13:46:07 -0000
> @@ -56,13 +56,36 @@
> }
> </code></pre>
>
> +<h2 id="c/cxx">C/C++ language issues</h2>
> +
> +<h3 id="Waddress-of-packed-member"><code>-Waddress-of-packed-member</code>
> +is enabled by default</h3>
> +
> +<p>
> + When address of packed member of struct or union is taken, it may result
> + in an unaligned pointer value. A new warning
> + <code>-Waddress-of-packed-member</code> was added to check alignment at
> + pointer assignment. It warns both unaligned address and unaligned
> + pointer.
> +</p>
> +
> +<p>
> + If the pointer value is safe to use, you can suppress
> + <code>-Waddress-of-packed-member</code> warnings by using pragmas:
> +</p>
> + <pre><code>
> + #pragma GCC diagnostic push
> + #pragma GCC diagnostic ignored "-Waddress-of-packed-member"
> + /* (code for which the warning is to be disabled) */
> + #pragma GCC diagnostic pop
> + </code></pre>
> +
> <!--
> <h2 id="cxx">C++ language issues</h2>
> -->
>
> <!--
> <h2 id="fortran">Fortran language issues</h2>
> --->
>
> <!--
> <h2 id="links">Links</h2>
