[cfe-dev] ASTContext::getParents discussion

Rafael┬ĚStahl via cfe-dev cfe-dev at lists.llvm.org
Mon Apr 8 01:49:04 PDT 2019


Thanks for your comments!

Yes I understand that it would be my responsibility to manage a Parent 
Map if I were to mutate the AST, but in that case I'm just a checker 
user and the CTU mutates the AST via the ASTImporter during analysis.

The use-case is that I just classify a more complex expression, where 
the analyzer callback argument is only a child node. To walk upwards I 
use ASTContext::getParents, which builds the Parent Map for the entire 
TU. When after that a new function gets imported the map will be missing 
the Stmts in that function.

A good suggestion from you is to use function-scoped parent maps or the 
ones from AnalysisDeclContext. This seems to solve my issue, thanks!

In that case I will drop the patch, because of a misuse of 
ASTContext::getParents. Still, you have to know all this and for some 
checker user it might not even be obvious that CTU mutates the AST. So I 
would agree that getParents should probably be moved out of ASTContext. 
Would likely break a lot of user code though.

-Rafael

On 08.04.19 04:07, Artem Dergachev wrote:
> Hmm, i guess that generally it is a problem, if your algorithm mutates 
> the AST, you'll have to deal with stale parent maps somehow and do 
> sophisticated caching, and it's likely that nobody has ever developed 
> a reliable solution to this.
>
> But i don't immediately understand your specific use case. What sort 
> of AST nodes does your checker check? If it's statements within a 
> function body, why do they get mutated when an import of an unrelated 
> function occurs? Are you using the per-function parent maps (such as 
> the ones that live in AnalysisDeclContext), or are you trying to 
> construct a single ParentMap for the whole AST? If you're checking 
> declarations, why aren't you using DeclContext chains instead of 
> parent maps?
>
> Generally, are you using CTU in a manner that's more sophisticated 
> than what normal path-sensitive CTU does? I.e., not only bring in 
> function bodies, but also inject arbitrary stuff into arbitrary stuff?
>
> On 4/4/19 5:24 PM, Rafael┬ĚStahl via cfe-dev wrote:
>> Hi all,
>>
>> while developing a custom checker I encountered the following issue 
>> related to the getParents member function of the ASTContext.
>>
>> My checker was using ACtx->getParents in a callback and I noticed 
>> that sometimes parents were missing. After some debugging it turned 
>> out that this occurs when:
>>
>> - Using the Clang Static Analyzer with Cross Translation Unit (CTU) 
>> analysis
>> - Checker callback is called once, getParent builds a Parent Map to 
>> cache this CPU heavy task
>> - The CTU uses the ASTImporter to import a function defined in 
>> another TU, this modifies the AST, introducing new parents.
>> - Checker callback is called again, now getParents will use the old 
>> Parent Map
>>
>> My fix was to introduce ASTContext::invalidateParents which would be 
>> called by the ASTImporter on every AST mutation. That way, the Parent 
>> Map would only have to be rebuilt if it was invalidated before a 
>> getParents call. This is the patch: https://reviews.llvm.org/D46940
>>
>> Richard expressed some concerns about this patch, to summarize (full: 
>> https://reviews.llvm.org/D46940#1147456 ):
>>
>> - Other entities besides the ASTImporter could mutate the AST, so the 
>> responsibility of invalidation should lie with the consumer of the 
>> Parent Map
>> - The Parent Map should not be in the ASTContext at all, really in 
>> libTooling
>>
>> While I understand these concerns, a solution like that does not 
>> solve my use-case described above. If I create a new Parent Map on 
>> every callback invocation, my checker becomes unusably slow. And if I 
>> cache it, I do not know when it is invalidated.
>>
>> Another idea I had was to introduce invalidation callbacks to the 
>> Checker, Analyzer and ASTImporter interfaces, in order to carry the 
>> invalidation trigger up to a user defined checker. This doesn't seem 
>> very elegant though.
>>
>> I wanted to start a discussion to find a solution to this. Do you 
>> have any ideas or input?
>>
>> -Rafael
>>
>>
>>
>> _______________________________________________
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>> cfe-dev at lists.llvm.org
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>

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