This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: [PATCH] Fix C++ strict-aliasing issues with memcpy folding
On Wed, 3 Feb 2010, Dave Korn wrote:
> On the general subject of constructing a float on top of an int, I wouldn't
> be surprised if the compiler didn't know after "*p = 3;" that the float had
> changed.
Indeed it doesn't. But it would be undefined behavior to inspect
the float value again - it's gone.
> I also wouldn't be surprised if it hoisted "*p = 3;" above any
> stores to the float and really messed things up.
It won't do that. output- and anti-dependence cannot use TBAA with
the middle-end memory model.
> If we can do a bit to avoid
> that by using the concept of dynamic type, all well and good, but maybe we
> shouldn't go too far with it.
There wasn't anything wrong with this original example from the start
and GCC handles it just fine.
What remains undefined behavior is
int i;
*(float *)&i = 1.0;
i = 0;
float f = *(float *)&i;
thus, you can only use an lvalue of a type compatible with the
dynamic type of the object to access it. Unless you change the
dynamic type with that access which you always do if it is a store.
Richard.