Bug in QualTypeNames.cpp and adding an option to prepend "::" to fully qualified names.

Tue May 12 12:40:13 PDT 2020

Hi,

thanks for your answer.

Just a few remarks:

1. I imagine that we cannot know if people are using getFullyQualifiedName.
In particular, people could have developed their own internal tools, thus
we cannot be aware of all callers. I do not know Clang's policy, but can we
delete or completely change a function without deprecating it first? I was
imagining that the process was to deprecate it, document the case where it
can be incorrect, and that in a next release it gets deleted/replaced (or
someone steps up to fix it).
2. As example of different behaviors:
(a) the qual_type.getAsString() will print implementation namespace details
(e.g. ::std*::__u*::variant instead of std::variant).
(b) It will also display default template arguments, e.g.
template <T = void>
class MyClass
is printed as MyClass (I think) in getFullyQualifiedName, while
getAsString() will use MyClass<void>.
(c) the example of the nested template argument above.

At the end,what matters is that getAsString() is semantically always
correct, even if it can be overly verbose. I tried to
fix getFullyQualifiedName, but I do not understand its code.

3. When the goal we want to reach has been decided, there is also the
question on how to transition from the current state to the next state, and
who can help with that. To be way clearer, I won't be able to fix all of
Google internal usage of this function (I am not at all working on Clang,
Clif, or any tools, I just hit the bug and decided to look into it, and it
happened that I found a fix). Thus, if we can do the changes using
iterative steps, such as
(a) add the new "::" prefix configuration,
(b) Deprecate/document the fact that getFullyQualifiedName has a bug, and
people should move to use qual_type.getAsString(Policy) instead, using
"FullyQualifiedName" and "GlobalScopeQualifiedName". We can for example
provide :

std::string GetFullyQualifiedCanonicalType(QualType qual_type, const
ASTContext &ast,)
  clang::PrintingPolicy print_policy(ast.GetASTContext().getLangOpts());
  print_policy.FullyQualifiedName = 1;
  print_policy.SuppressScope = 0;
  print_policy.PrintCanonicalTypes = 1;
  print_policy.GlobalScopeQualifiedName = 1;
  QualType canonical_type = qual_type.getCanonicalType();
  return canonical_type.getAsString(print_policy);
}
(c) Then, people can upgrade asynchronously, maybe someone will be unhappy
with this behavior and will suggest something else, etc.

If we just blindly delete it, it means for example that we have to fix all
usages in Google to be able to upgrade Clang.  It may be the approach that
is decided we should follow, but I just wanted to make it clear that I will
not be able to do that job^^ While, having an incremental fix in Clang, and
then fix Clif is doable as it does not imply to fix all calls in one go.

I just wanted to develop these points.

Thanks!

On Tue, May 12, 2020 at 7:59 PM David Blaikie <dblaikie at gmail.com> wrote:

> +Mr. Smith for visibility.
>
> I'm /guessing/ the right path might be to change the implementation of
> getFullyQualifiedName to use the type printing/pretty printer approach with
> the extra feature you're suggesting. That way all users get the desired
> behavior
>
> +Sam McCall <sammccall at google.com>  who (if I understand correctly) has a
> lot to do with the Clang Tooling work - looks like Google's got a bunch of
> uses of this function (getFullyQualifiedName) internally in clang tools (I
> wonder why that's the case when there are still zero external callers - is
> that OK? Or are external users doing something different (better? worse?)
> to answer this question - or the tooling uses in LLVM proper just don't
> have the same needs?). So probably best not to leave a buggy implementation
> lying around - either deleting it, or fixing it.
>
> On Mon, May 11, 2020 at 11:28 PM Jean-Baptiste Lespiau via cfe-commits <
> cfe-commits at lists.llvm.org> wrote:
>
>> Hi,
>>
>> *Context and history:*
>>
>> I have found a bug in CLIF <https://github.com/google/clif>, which does
>> not correctly fully qualify templated names when they are nested, e.g.
>>
>> ::tensorfn::Nested< ::std::variant<Tensor, DeviceTensor> >
>>
>> should have been:
>>
>> ::tensorfn::Nested<
>> ::std::variant<::tensorflow::Tensor,::tensorfn::DeviceTensor> >
>>
>> I tracked it down to
>> https://github.com/google/clif/blob/master/clif/backend/matcher.cc#L172
>> which calls
>> https://github.com/llvm-mirror/clang/blob/master/lib/AST/QualTypeNames.cpp
>> and the error is exactly at the line, but I could not really understand
>> why.
>>
>> https://github.com/llvm-mirror/clang/blob/master/lib/AST/QualTypeNames.cpp#L457
>>
>> Historically, it has been added by the developers of CLIF
>> (including Sterling Augustine)
>>
>> https://github.com/llvm/llvm-project/commit/0dd191a5c4bf27cc8a2b6033436b00f0cbdc4ce7
>> .
>> They explained to me, that they pushed this to Clang hoping it would be
>> used by other tools and maintained by the community, but that it kind of
>> failed at that, and it (i.e. QualTypeName.pp) is not much used, and not
>> much maintained.
>>
>> I was not able to understand this file to provide a fix. On the other
>> side, it was easy to understand TypePrinter.cpp and PrettyPrinter.cpp, so I
>> tried extending it to fit my need.
>>
>> *Suggestion*
>>
>>  As I wanted fully qualified types (it's usually more convenient for
>> tools generating code), to prevent some complex errors), I added ~10 lines
>> that add an option to prepend "::" to qualified types (see the patch).
>>
>> In practice, it is still a different display at what QualTypeNames.cpp
>> was doing, as, for example, it displays
>>
>> ::tensorfn::Nested<::std*::__u*::variant<tensorflow::Tensor,
>> ::tensorfn::DeviceTensor>>
>>
>> but I was able to solve my issues. More generally, it is more verbose, as
>> it displays the exact underlying type, including default parameter types in
>> template arguments. So it's verbose, but it's correct (what is best?^^).
>>
>> I am contacting you, so we can discuss:
>>
>> - Whether QualTypeNames.cpp
>> <https://github.com/llvm-mirror/clang/blob/master/lib/AST/QualTypeNames.cpp> is
>> something useful for the Clang project, whether you think we should fix the
>> bug I have found (but I cannot, as I do not understand it), or whether we
>> should deprecate it, or modify the current printing mechanism
>> (TypePrinter.cpp and PrettyPrinter.cpp) to add more options to tune the
>> display in ways people may want to.
>> - Whether adding the CL I have attached is positive, and if yes, what
>> should be done in addition to that (does it need tests? Are there more
>> types that we may want to prepend "::" to its fully qualified name?).
>>
>> Thanks!
>>
>> _______________________________________________
>> cfe-commits mailing list
>> cfe-commits at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
>>
>
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