[PATCH] D60047: [CaptureTracking] Pointer comparisons cannot escape

[Ayke via Phabricator via llvm-commits]

aykevl added a comment.

Oh, I see. Thank you for explaining the actual behavior of getelementptr.

So I'm thinking about how to implement this feature. Would it be possible to infer the `nocapture` flag for the ptr argument in the following function, if it is also marked with `dereferenceable_or_null(4)`?

  int deref(int *ptr) {
      if (ptr == NULL) {
          abort(); // does not return
      }
      return *ptr;
  }

My understanding is that `dereferenceable_or_null(N)` implies that it is either a valid pointer (no out-of-bounds GEP allowed) or null, meaning that icmp cannot be used to let the pointer escape with a comparison against null.
In other words, it seems to me that if a pointer comes from a `dereferenceable_or_null(N)` argument (possibly through bitcast, inbounds GEP, addrspacecast, or other 'inbounds' operations) an icmp against null cannot let this pointer escape.

My goal is to make CaptureTracking (used in the functionattrs pass) powerful enough to be usable for escape analysis. The source language is Go, which has pointer semantics similar to Java.


Repository:
  rL LLVM

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