[PATCH] D37478: [analyzer] Implement pointer arithmetic on constants

[Rafael Stahl via Phabricator via cfe-commits]

r.stahl added a comment.

To be honest I was quite surprised that this change in behavior didn't cause more test failures, because for detecting null dereferences the old behavior is definitely more useful. Since it did not, I was convinced that this change is desired.

We use the analyzer for finding dereferences to fixed addresses - very similar to the FixedAddressChecker. For this purpose it is crucial that the execution engine works as perfect as possible, without "swallowing" any arithmetic.

For the struct example you mentioned you can still get the final address by asking the ASTContext if needed, but with pointer arithmetic the information is lost forever. Information is lost either way here. Either you forget that the arithmetic was based on a null pointer or you lose whatever was added to or subtracted from it.

So unless you can somehow tag the information in the SVal when an operation was based on a null pointer, this is pretty difficult. You also could introduce a heuristic that defines all dereferences around zero as null dereferences, but it would be very arbitrary and platform dependent. Or maybe the DereferenceChecker should explicitly break early on all statements that do arithmetic on pointers constrained to null. Overall I don't know enough about the analyzer to suggest more here.

Thanks for the comments, I will address them soon.


https://reviews.llvm.org/D37478