Re: [PATCH 1/4] autotools: Only used -Wcast-align with gcc

[Markus Ongyerth <ell@ongy.net>]

[-- Attachment #1: Type: text/plain, Size: 2758 bytes --]

Hi Marcel,

On 2016/12月/15 07:57, Marcel Holtmann wrote:
> Hi Markus,
> 
> > clang can currently not compile NLMSG_NEXT with -Wcast-align.
> > I asked about this in #llvm on irc.oftc.net, where I was told the best
> > solution is to disable -Wcast-align for now.
> 
> can we get a little bit more background on this. I had also some issues with native gcc usage on Raspberry PI. And yes, the netlink macros are some offenders.
> 
I was mostly going of an answer I got in irc, (I was told it's ok to quote):

/***************************************************************************/
2016-12-14 17:47:14	dblaikie	ongy: Judging by the docs - and this is a bit 
of a guess, GCC is more conservative with this warning, restricting it to only 
platforms where it would be an illegal instruction (I haven't actually tested 
this, because I don't have a GCC cross compiler and don't remember which 
targets have this behavior - x86 just has reduced performance)
/***************************************************************************/

The docs they reference here are probably [1] . Here the relevant lines are:

/***************************************************************************/
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of the target
is increased. For example, warn if a char * is cast to an int * on machines 
where integers can only be accessed at two- or four-byte boundaries. 
/***************************************************************************/

Judging from [2] this is the case on older ARM processors.
If I read this correctly, The Raspberry PI should support unaligned access (at 
least version 2 and 3). It may be worth to see what architecture gcc targets 
and make it more specific.


[1] https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
[2] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.faqs/ka15414.html
> > ---
> > acinclude.m4 | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/acinclude.m4 b/acinclude.m4
> > index 8aab4ee..b1dbabe 100644
> > --- a/acinclude.m4
> > +++ b/acinclude.m4
> > @@ -58,6 +58,8 @@ AC_DEFUN([COMPILER_FLAGS], [
> > 		CFLAGS+=" -Wdeclaration-after-statement"
> > 		CFLAGS+=" -Wmissing-declarations"
> > 		CFLAGS+=" -Wredundant-decls"
> > -		CFLAGS+=" -Wcast-align"
> > +		if ( $CC -v 2>/dev/null | grep "gcc version" ); then
> > +			CFLAGS+=" -Wcast-align"
> > +		fi
> 
> Are you sure there is not a better autoconf macro for checking if we use gcc vs clang.

No I'm not. I'm not really familiar with autotools. This is what came up when 
I searched for compiler specific option in autotools, there may be a better 
way.

Regards,
  Markus