[Bug c++/20607] [3.4 Regression] -fstrict-aliasing causes incorrect scheduling

["mark at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From mark at codesourcery dot com  2005-03-31 00:54 -------
Subject: Re:  [3.4 Regression] -fstrict-aliasing causes incorrect
 scheduling

ebotcazou at gcc dot gnu dot org wrote:
> ------- Additional Comments From ebotcazou at gcc dot gnu dot org  2005-03-30 10:53 -------
> Hum... type-punning simply doesn't work in this case with the C++ compiler, but
> does work with the C compiler.  The problem is that:
> 
> 	union u {
> 	    x_uint64_t first;
> 	    x_uint32_t second;
> 	};
> 	union u conv;
> 	conv.first = arg1;
> 	result = conv.second;
> 
> is translated into:
> 
>   *(&conv.first) = *(&arg1);
>   *(&result) = *(&conv.second);
> 
> and type-punning cannot work in this case (that's the counter-example provided
> in the manual).  I guess the problem comes the fields being of aggregate types.
> 
> Mark, is that fixable or should we document the limitation?

I think it's fixable.  I'm not sure exactly why the C++ front-end is 
inserting the additional operations, but they're certainly not essential 
from a C++ semantics point of view.  Please reassign the bug to me; I'll 
try to take a look.

Thanks,



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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20607