Re: Visual C++ aliasing issue

From:

"Bo Persson" <bop@gmb.dk>

Newsgroups:

microsoft.public.vc.language

Date:

Tue, 21 Apr 2009 22:42:51 +0200

Message-ID:

<756pc8F16snbvU1@mid.individual.net>

{} wrote:

Hello,

Here is an example, where Visual Studio 2008 produces a code, that
does not work right when compiled with the default Release
configuration. The code works correctly in Debug.
Looks like SP1 has the same problem.

The code was working in VS2003.

The program copies an array of BGR elements into an array of BGRA
elements in place backwards (starting from the last element).

The variable tmp in the loop is introduced specially to prevent the
aliasing issue. The compiler optimizes the variable out however.

{
   Pixel3 tmp = *(Pixel3*)pBufSrc; //copy to temp variable
   *(Pixel3*)pBufDst = tmp;
   ((Pixel4*)pBufDst)->a = 255;
}

Around here you first tell the compiler that the pointer points to a
Pixel3, and then that it points to a Pixel4 as well. This is just
impossible (unless one is a subobject of the other).

As you have now lied to the compiler, you have broken the contract and
it is free to do whatever it wants. Victor called that "undefined
behavior" - there is no rule for what should happen.

Bo Persson