cvs commit: src/lib/libalias alias.c

[Bruce Evans]

On Tue, 6 Jul 2004, David Schultz wrote:

> On Tue, Jul 06, 2004, Dag-Erling Smorgrav wrote:
> >   Rewrite twowords() to access its argument through a char pointer and not
> >   a short pointer.  The previous implementation seems to be in a gray zone
> >   of the C standard, and GCC generates incorrect code for it at -O2 or
> >   higher on some platforms.
>
> I don't think this is even a gray zone---the C standard
> specifically disallows aliasing of pointers to different types
> unless one of the types is a 'char *'.

It's grey in the gcc standard.  If there were a gcc standard, then it
should support accesses via pointers of different types if there are no
hardware [1] alignment problems, since Normal C programs do this, but
since there is no gcc standard no one really knows what gcc supports.

The C standard actually specifically allows aliasing of pointers
to different types if the access is through a pointer to a character
type.  The character type also needs to be unsigned char to avoid
the possibility of trap representations.

[1] gcc supports the packig and alignment attributes, so it must support
pointers to misaligned objects, at least if it silently discards attributes
when taking addresses like it does now.

Bruce