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Re: Importance of transformations that turn data dependencies into control dependencies?

On 02/24/2016 05:14 AM, Richard Biener wrote:

Note that if a user writes

   if (p == d)
      ... do lots of stuff via p ...

GCC might rewrite accesses to p as accesses to d and thus expose
those opportunities.  Is that a transform that isn't valid then or is
the code written by the user (establishing the equivalency) to blame?
I think Torvald later clarified this wasn't a problem. If it was, then it wouldn't be hard to disable this for pointers and I doubt the impact would be measurable.

There's a PR where this kind of equivalencies lead to unexpected (wrong?)
points-to results for example.
Yea, I've watched a couple discussions around this issue. I don't recall them reaching any conclusion about the validity of the testcases.

If the final determination is that such equivalence propagation is unsafe for the points-to aliasing system, we can just disable those pointer equivalence tracking bits.

Other potential examples that come to mind are de-virtualization, or
feedback-directed optimizations that has observed at runtime that a
certain pointer is likely to be always equal to some other pointer (eg.,
if p is almost always d[0], and specializing for that).

That's the cases that are quite important in practice.
I was most concerned about de-virt and feedback stuff that specializes paths based on expected values.


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