[cfe-dev] [RFC] Improving import failure checking strategy inside ASTImporter

Gábor Márton via cfe-dev cfe-dev at lists.llvm.org
Thu Apr 26 02:39:18 PDT 2018


Hi Aleksei,

It is a great and welcoming initiative to identify the weaknesses in
the ASTImporter.

About, handling the error case after a call:
I couldn't agree more that this is a burden to always have a check at
the call site, but this is the price we pay for not having exceptions.
By using macros I have a feeling that the code would be even less
readable because we would hide control flow modifications inside the
macro, also because finally we'd need to create so many different
kinds of macros.
As far as I know, there are cases where an `Import` can return a
`nullptr` or a default even in case of non-error situations, please
correct me if I am wrong. Thus, introducing an optional like return
type would require to discover all these cases. Still, I think this
would worth it, because the enforcement of error checking (as you
mentioned), plus it would make it more explicit which situation is
really an error.
Actually, I think this should be done with a few large commits, e.g.
once we change the return type of `ASTImporter::Import(Decl*)` to
`Optional<Decl*>` then we have to change all call sites.
By introducing an `ImportOrError` and then doing the changes gradually
would result the same final state as if we had done it in one larger
step where we changed all call sites, but until all call sites are
changed we have to maintain both `Import` and `ImportOrError`.


Beside the problem of handling the error case after an `Import` call
we have observed several other weaknesses, and would like to hear your
opinion about them as well.
So, the list of problems so far:

- Do the same things after and before a new AST node is created (::Create)
The original import logic should be really simple, we create the node,
then we mark that as imported, then we recursively import the parts
for that node and then set them on that node.
However, the AST is actually a graph, so we have to handle circles.
If we mark something as imported (Imported()) then we return with the
corresponding To whenever we want to import that node again, this way
circles are handled.
In order to make this algorithm work we must ensure things, which are
not enforced in the current ASTImporter:
* There are no Import() calls in between any node creation (::Create)
and the Imported() call.
* There are no VisitXXX() calls in any other VisitYYY() function, only
call of Import() allowed.
* Before actually creating an AST node (::Create), we must check if
the Node had been imported already, if yes then return with that one.
One very important case for this is connected to templates: we may
start an import both from the templated decl of a template and from
the template itself.
There are good news, the first and third points can be enforced by
providing a variadic forwarding template for the creation, we are
currently preparing to upstream this change from our fork
(https://github.com/Ericsson/clang/pull/354/files) but this is going
to be a huge change.
Still, I have no idea how to enforce the second point. (Maybe a checker?)

- Decl chains are not imported
Currently we import declarations only if there is no available definition.
If in the "From" TU there are both a declaration and a definition we
import only the definition.
Thus we do not import all information from the original AST.
One example:
    struct B { virtual void f(); };
    void B::f() {}
Here, when we import the definition, then the virtual flag is not
getting imported, so finally we import a function as a non-virtual
function.
An other problem is with recursive functions:
    void f(); void f() { f(); }
When we import the prototype then we end up having two identical
definitions for f(), and no prototype.
We again preparing a patch for this, but this solves the problem only
in case of functions.
By not importing the whole decl chain we loose all attributes which
may be present only in declarations.

- Structural Equivalency
There are several holes here. E.g. we do not check the member
functions in a class for equivalency, only members.
Thus, we may end up stating that a forward decl is equivalent with a
class definition if the class does not have data members.
There are no tests at all for structural equivalency.
In our fork we have some tests, and planning to upstream them too.

- Minimal import
This feature is used by LLDB, but not at all in CTU. This feature
makes the code bigger and more complex.
There are 0 unit tests for minimal import, whatever information we
have about this we can get only from the lldb repo.

- Lookup
The way we lookup names during an import of a decl is getting really
complex, especially with respect to friends.
We find an already imported function from an unnamed namespace, thus
we simply skip the import of a function in an other TU if that has the
same name and that is also in an unnamed namespace
(https://github.com/Ericsson/clang/issues/335).
We still use `localUncachedLookup` which can be very inefficient, we
should be using `noload_lookup`.


Thanks,
Gabor

On Wed, Apr 25, 2018 at 5:52 PM, Aleksei Sidorin via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> Hello,
>
> I'd like to discuss the idea of changing the way how we check for import
> issues  in ASTImporter and ASTNodeImporter.
>
> 1. Introduction
>
> ASTImporter is now extensively used in Cross-TU analysis and is in active
> development. However, we find (and introduce) many errors related to how
> import failures are handled now.
>
> Currently, ASTImporter::Import() and ASTNodeImporter::Visit*() methods
> return nullptr for pointers and default value in case of conflict. And this
> approach brings many issues due to the following reasons.
>
> 1. We have to check the return result and take actions explicitly.
>
> The common strategy of failure handling is very simple - just return. While
> it is very simple, almost all the code of ASTImporter consists of such
> explicit checks making it much harder to read.
>
> 2. Both nullptr and default value are valid by itself so we have to place
> explicit checks to check if a failure happened. Example:
>
> Decl *To = Importer.Import(From);
> if (From && !To)
>   return nullptr;
>
> This doesn't look well and it's easy to forget where From can be nullptr and
> where it is impossible.
>
> 3. Error checking is not enforced.
>
> We have to check for import errors every time Import() is called. And it is
> extremely easy to forget placing a check after a call. This error pattern is
> so common in ASTImporter that I was going to write a CSA check for this.
>
> 2. Possible solutions.
>
> The problem with enforcing checks is the most annoying one but also looks to
> be the simplest - we can just replace the return value with Optional or
> ErrorOr so clients will be enforced to perform a failure checking. This
> replacement can include two changes.
>
> 1. A number of large commits that change the return type of
> ASTNodeImporter::Visit*() methods. And I'd like to hear the comments from
> existing users that have their own improvements of ASTImporter because they
> have to change their code not touched by the commit as well - there be some
> pain.
>
> 2. Introducing an alternative method in ASTImporter that returns ErrorOr.
> So, we can migrate to the new, say, ImportOrError(), by small patches.
>
> This replacement also solves the problem (2) because value and error are now
> separated.
>
> The problem of improving readability of these checks is a bit harder. The
> most straightforward approach is to create a macro like:
>
> #define IMPORT_PTR_INTO_VAR(VarName, Type, From) \
>   Type *VarName = nullptr;                                 \
>   {                                                        \
>     auto MacroImportRes = Importer.Import(From);           \
>     if (!MacroImportRes)                                   \
>       return MacroImportRes.getError();                    \
>     VarName = *MacroImportRes;                             \
>   }
>
> Same for non-pointer type. So, the final code will look like this:
>
> ErrorOr<Expr *> ASTNodeImporter::VisitCXXMemberCallExpr(CXXMemberCallExpr
> *E) {
>   IMPORT_QUAL_TYPE_INTO_VAR(T, E->getType())
>   IMPORT_PTR_INTO_VAR(ToFn, Expr, E->getCallee())
>
>   SmallVector<Expr *, 4> ToArgs(E->getNumArgs());
>   IMPORT_CONTAINER(E->arguments(), ToArgs);
>
>   return new (Importer.getToContext()) CXXMemberCallExpr(
>         Importer.getToContext(), ToFn, ToArgs, T, E->getValueKind(),
>         Importer.Import(E->getRParenLoc()));
> }
>
> But macros don't look clean. Unfortunately, I still haven't found a better
> option so any suggestions are very welcome.
>
> PS. The error handling pattern we wish to get is very close to exception
> handling in C++ because usually we don't want to perform local error
> handling but centralized instead. With a possibility to just throw an
> exception and catch it in the ASTImporter entry point (returning an
> ErrorOr), error handling in ASTImporter becomes almost trivial and pretty
> clean. But we don't use exceptions in LLVM and clang and I don't know a
> similar functionality in LLVM utilities.
>
> --
> Best regards,
> Aleksei Sidorin,
> SRR, Samsung Electronics
>
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