Safety and legality of optimizations(Re: Optimizing Across && And ||)

David Chase <chase@centerline.com> wrote:
>SSA-transformation, though well-documented and delightfully fast
>and elegant, is not the most obvious thing on the face of the earth.
>There are a couple of instances where it is "too good" -- it's
>a relatively straightforward task to write a dead code eliminator
>that performs "infinite optimization" on infinite loops (that is,
>the loop disappears and control falls through).


Yes, but is it safe to eliminate infinite loops? Consider:


      /* fire the rockets if f is null or acyclic */
      void doit_or_die(foo *f, int n) {
          foo *x;
          for (x=f; x; x = x->next) {}
          /* fire 'em */
              ...
      }


Eliminating the loop or moving code across infinite loop can be argued
as unsafe.


Furthermore, this transformation is based on the notion of control
dependence. And one needs to be extra careful when basing
optimizations like above on the notion of control dependences.


Vinod Grover
--

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