64bit: C++ templates

Ryan Johnson ryan.johnson@cs.utoronto.ca
Mon May 27 15:09:00 GMT 2013


On 27/05/2013 10:57 AM, Corinna Vinschen wrote:
> On May 27 16:39, marco atzeri wrote:
>> Il 5/27/2013 3:22 PM, Corinna Vinschen ha scritto:
>>> On May 27 14:49, marco atzeri wrote:
>>>> Il 5/27/2013 1:15 PM, Corinna Vinschen ha scritto:
>>>>> On May 27 12:59, marco atzeri wrote:
>>>>>> Il 5/27/2013 12:42 PM, Corinna Vinschen ha scritto:
>>>>>>> On May 27 12:22, marco atzeri wrote:
>>>>>>>> Il 5/27/2013 11:53 AM, Corinna Vinschen ha scritto:
>>>>>>>>> And worse, the problem goes away when building without optimization.
>>>>>>>>> There might be another GCC bug lurking in the background...
>>>>>>>>>
>>>>>>>> Probably there is more than one.
>>>>>>>> I hit a gcc-4.8 optimization bug testing slang, the workaround
>>>>>>>> was to build the 64 bit package without any optimization.
>>>>>>> slang is written in c++?
>>>>>> in C
>>>>> And you built it with the default -mcmodel=medium, I assume.  Well,
>>>>> it's not that gcc would be entirely bug free and 4.8.1 is lurking
>>>>> around the corner already.  Any chance to create a simple testcase?
>>>> default -mcmodel=medium
>>>>
>>>> Slang is a small library and the failure is
>>>> during "make check" when UTF8 is not used
>>>>
>>>> http://mailman.jedsoft.org/pipermail/slang-users-l/2013/000788.html
>>> Hmm.  For some reason that reminds me of the problem we had with the
>>> localtime changes two weeks ago.  Just out of curiosity, would you mind
>>> to try an optimized build again, but this time add the -fwrapv option?
>> it works, thanks
> Wow, I haven't actually believed this would help.  Given it's apparent
> dangerousness, wouldn't it make sense to enable the -fwrapv option (or,
> FWIW, disabling the -fstrict-overflow option) by default with -O2, at
> least on x86_64?
I guess lots of folks assume two's complement wraparound for signed 
ints, even though the standard says it's undefined? I guess this is 
destined to turn out like -fstrict-aliasing... when in doubt, disable it.

Ryan



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