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