Re: type based aliasing again

[Mark Mitchell <mark@codesourcery.com>]

>>>>> "Joe" == Joe Buck <jbuck@synopsys.COM> writes:

    >> One of your assumptions is that the only times we ask `does x
    >> alias y' are when it is sensible to do so. :-) We likely do so
    >> at other times as well; I didn't point this out, but should
    >> have.  Consider:
    >> 
    >> void *v = malloc (4); if (p) * ((float *) v) = 3; else * ((int
    >> *) v) = 7;
    >> 
    >> This code is conforming.  I don't mind terribly if we warn on
    >> this code; I bet the number of false positives will be small.
    >> But, we shouldn't force the compiler to go slower in this case.

    Joe> I guess I am missing something in this example.  Could you
    Joe> explain why we would get a warning?  The accesses are legal.
    Joe> Yes, if you check whether the two assignments alias there
    Joe> would be the possibility of a false warning if you aren't
    Joe> careful.

As you presented it, the algorithm was:

  o Whenever we check if two things alias,
  o First check to see if they definitely do,
  * Then, if there types would say they cannot, complain.

So, here, to avoid the warning/error, we must never check.  That's a
tough burden for the compiler.  

For example, it might be sensible to precompute an all-pairs aliasing
matrix for the RTL in the function, if we found that we were
recomputing the same data all the time.  (The (n,m) entry is whether
or not the Nth one can alias the Mth one.)  Merely computing that
matrix would trigger the warning.

    Joe> So, if I can only have your amended proposal, I suppose I'll
    Joe> take it.  It's better than nothing.

I agree.  Perhaps someone will implement it? :-)

--
Mark Mitchell                   mark@codesourcery.com
CodeSourcery, LLC               http://www.codesourcery.com