[PATCH] D32747: [Analyzer] Iterator Checker - Part 3: Invalidation check, first for (copy) assignments

Balogh, Ádám via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Wed Aug 1 01:45:20 PDT 2018

baloghadamsoftware added inline comments.

Comment at: lib/StaticAnalyzer/Checkers/IteratorChecker.cpp:605
+  if (Pos && !Pos->isValid()) {
+    // If I do not put a tag here, some invalidation tests will fail
+    static CheckerProgramPointTag Tag("InvalidatedIteratorChecker",
NoQ wrote:
> baloghadamsoftware wrote:
> > NoQ wrote:
> > > baloghadamsoftware wrote:
> > > > NoQ wrote:
> > > > > This needs investigation, because it may be nasty.
> > > > > 
> > > > > `generateNonFatalErrorNode()` returns null when the exact same non-fatal error node, also produced by the iterator checker with the exact same program state and exact same program point and exact same tag on the program point already exists. As far as i understand, the only difference your tag makes is that the tag is now different, so it is not merged with the existing node. However, it is worth it to try to find out why the node gets merged at all.
> > > > > 
> > > > > This may be caused by an accidental state split. For example, if you are calling `generateNonFatalErrorNode()` twice in the same checker callback without chaining them together (passing node returned by one as an argument to another), this in fact splits states. I'm not immediately seeing such places in the code - you seem to be aware of this problem and avoiding it well. But still, looking at the topology of the exploded graph in the failing test should help finding out what is going on.
> > > > I made some more investigation this time. Unfortunately the case is not what you suggest. Only one non-fatal error node is produced. I tested it with a common tag (a global static so the tag is exactly the same at every `generateNonFatalErrorNode()`, but the tests still pass. I printed out the exploded graph and I found that there are indeed two nodes with the same state ID. The tag is the default tag automatically generated from the name of the checker. The first state is created in function `checkPreStatement()` for `CXXOperatorCallExpr` where I copy the state of the iterator from the formal to the actual `this` parameter. All the test fails happen at the dereference operator call (`*`) of another operator call (`++` or `--`). After this copy, when I call `generateNonFatalErrorNode()` I get `nullptr` because at some point under the hood (I debugged it when I created it originally) the error node is considered as "not new". If I use a custom tag here, the state ID remains, not the node ID changes.
> > > I think i see the problem. The checker subscribes to both `PreCall` and `PreStmt<CallExpr>` (to be exact, `CXXOperatorCallExpr`) and adds transitions in both cases. It results with a predecessor node in `CheckerContext` that's already tagged by the checker. Apparently this never worked, but nobody tried that.
> > > 
> > > Ideally, we should make sure those callbacks use different program points, eg. introduce `PreCall`/`PostCall` program point kinds and use them.
> > > 
> > > Also i wonder why are you using pre- rather than post-statement callback. You model all other operators in `PostCall`, why did those end up here? Maybe merge them? It is generally better to model pre-conditions and look for bugs in `PreStmt`/`PreCall` (before we don't care what happens within the call), and model effects in `PostStmt`/`PostCall` (because effects don't take effect until the call happens).
> > That is what I am trying to to: `Post*` for modelling and `Pre*` for checking. However, this `PreStmt<CXXOperatorCallExpr>()` is special since I have to move the arguments to the context of the called operator //before// the call.
> Ah, it's that thing, `PreCall` then?
Yes! It seems to be working. Thank you for your help!


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