Re: C strict aliasing rules

[Andrew Johnson <anj@aps.anl.gov>]

Eric Norum wrote:
> static inline void
> writethree(int *p)
> {
>     short *palias = (short *)p;
>    *palias = 3;
> }
>
> int
> testalias()
> {
>     int value = 12345678;
>     writethree(&value);
>     return value;
> }
>
> One might expect that the value returned from testalias() would be  
> 12345678 with either its most-significant 16 bits or least- significant 
> 16 bits set to 3 (on most architectures).
> ....
> One would be wrong, though.
I don't think life is quite as bad as it might seem; if you take off the 
"static inline" keywords from writethree(), I suspect the optimizer may 
not be able to be quite as aggressive in throwing out code - please 
experiment and respond to that suggestion.
Since we don't (shouldn't?) have any C routines in base that are marked 
as "static inline", I don't think our problems are as large as you make 
out them to be.  I suspect the rules for C++ are subtly different, so it 
would also be interesting to see what using the C++ compiler generates.
- Andrew
--
There is considerable overlap between the intelligence of the smartest
bears and the dumbest tourists -- Yosemite National Park Ranger