Add a set of comments and question on PDG construction #1092
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| // Question: this takes a pointer as input and produces another? | ||
| Field(ptr, ..) => ptr, |
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Question: this takes a pointer as input and produces another?
That's what I would expect. In the static analysis, we have a similar notion of PlaceElem::Field as a "pointer projection" that takes a pointer to the struct and produces a pointer to the field. This is similar to (some uses of) offset, which takes a pointer to (the first element of) an array and produces a pointer to an element, and also similar to LLVM's GetElementPtr operation.
| // Question: same, is the base pointer enough? | ||
| Offset(ptr, _, _) => ptr, |
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is the base pointer enough?
At one point, we planned to distinguish forward offsets (OFFSET_ADD) from backward ones (OFFSET_SUB). Offsets that statically only go forward can be implemented on slices with a subslice operation, but offsets that might go backward need a different type that tracks how far back it can go and remain in bounds.
| // Question: is "provenance" exactly tied to the memory address of this pointer? | ||
| // Why don't we just use that for everything? | ||
| // Answer attempt: provenance only covers the original | ||
| // source of the pointer, and not any copies like LoadValue |
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I don't have an answer for this case, but in general it's possible for two pointers to have the same address but different provenance. In C:
int x;
int y;
int* p = &x + 1;
int* q = &y;It's possible that p == q, but q has &y's provenance and is legal to dereference, while p has &x's provenance and dereferencing it is UB (it's out of bounds for the object it points to).
I wrote my questions on the PDG builder algorithm in the form of comments. It might be useful to answer (some of) these and leave them in as comments, for future reference.