Re: Build failure with probe patch

[Pedro Alves <palves@redhat.com>]

On 02/18/2015 11:57 PM, Sergio Durigan Junior wrote:
> On Wednesday, February 18 2015, Steve Ellcey wrote:
> 
>> Is anyone else seeing this error when building the top-of-tree gdb:
>>
>>
>> cc1: warnings being treated as errors
>> /scratch/sellcey/gdb-bug/src/gdb/gdb/dtrace-probe.c: In function 'dtrace_get_probes':
>> /scratch/sellcey/gdb-bug/src/gdb/gdb/dtrace-probe.c:624: warning: dereferencing type-punned pointer will break strict-aliasing rules
>> make[1]: *** [dtrace-probe.o] Error 1
> 
> Thanks for your report.
> 
> I don't see anything like this in any of our builders here:
> 
>   <http://gdb-build.sergiodj.net/waterfall>
> 
>> I get this when building gdb on a CentOS 5 system with GCC 4.1.2 but I do not
>> see it when building on a Ubuntu 12.04 system with GCC 4.6.3.  I am not sure
>> if this is due to the old GCC or if it is a legitimate error.
> 
> According to:
> 
>   <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=41874>
> 
> This warning has been removed from GCC.  And by looking at the code
> referenced by it, I don't see anything wrong there.  So I'd say you can
> ignore this (and probably update your GCC).
> 

Actually, the code in that GCC bug is valid (char can alias anything),
but the GDB code being warned about is invalid (C99/N1256, 6.5/7) -
the dtrace code is aliasing pointers of incompatible types.

See:
  https://gcc.gnu.org/ml/gcc-help/2013-03/msg00118.html

I fixed up all similar cases I found in the GDB codebase a while ago:

 https://sourceware.org/ml/gdb-patches/2013-03/msg00449.html
 https://sourceware.org/ml/gdb-patches/2013-03/msg00578.html

Compiling with -Wstrict-aliasing=1 shows that dtrace-probe.c warning
with recent GCC's too, but then again, it also dumps a truckload
of false positives.

Thanks,
Pedro Alves