Re: criteria.html open issues

["Zack Weinberg" <zackw at Stanford dot EDU>]

On Mon, May 28, 2001 at 11:08:40PM -0400, Albert D. Cahalan wrote:

I'm going to skip the parts of the message that Daniel answered
already.  I don't have anything to add to what he said.

> Adding "volatile" didn't work, and there was too much code
> to switch over to unions.
> 
> I think there is/was a pretty clear bug here in fact, even if you
> do love the strict aliasing behavior. Here:
> 
> void foo(volatile float f){
>   volatile unsigned *u = (volatile unsigned *)&f;
>   if(u&WHATEVER){

Volatile means one thing only: the compiler may not assume that the
only thing which can modify this variable is the code it's compiling.
It has zero effect on aliasing rules.  In fact, I bet you there's an
(almost) strictly conforming program that breaks if volatile affects
the aliasing rules.

I have to stick an "almost" in there because any use of volatile
automatically makes a program not strictly conforming.  But if we take
the last sentence of C99 6.7.3 para 6 out and shoot it (does anyone
have the slightest clue what they _intended_ that sentence to mean?
'cos they can't possibly have intended the literal meaning) then its
effect is quite clearly defined.

-- 
zw        This is, no doubt, the rational strategy; quite possibly the
          only one that will work.  But it ignores the exigiencies of
          the tenure system and is therefore impractical.
          	-- Jerry Fodor, _The Mind Doesn't Work That Way_