[PATCH] D146358: [clang][AST] Print name instead of type when diagnosing uninitialized subobject in constexpr variables

Mon May 15 11:34:39 PDT 2023

aaron.ballman accepted this revision.
aaron.ballman added a comment.
This revision is now accepted and ready to land.

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Comment at: clang/lib/AST/ExprConstant.cpp:2357-2361
+    if (SubobjectDecl) {
+      Info.FFDiag(DiagLoc, diag::note_constexpr_uninitialized) << SubobjectDecl;
+      Info.Note(SubobjectDecl->getLocation(),
+                diag::note_constexpr_subobject_declared_here);
+    }
----------------
hazohelet wrote:
> tbaeder wrote:
> > aaron.ballman wrote:
> > > hazohelet wrote:
> > > > aaron.ballman wrote:
> > > > > Hmm, this breaks one of the contracts of our constexpr evaluation engine, doesn't it? My understanding is that if constexpr evaluation fails, we will have emitted a note diagnostic for why it failed. But if the caller doesn't pass a nonnull `SubobjectDecl`, we'll return `false` but we won't issue a diagnostic.
> > > > > 
> > > > > I'm surprised no tests lost notes as a result of this change, that suggests we're missing test coverage for the cases where nullptr is passed in explicitly to this function.
> > > > Yeah, I was looking into when `SubobjectDecl` can be null here. I `assert`ed the non-nullness of `SubobjectDecl` before and found that there exists two lines of code (https://github.com/llvm/llvm-project/blob/22a3f974d35da89247c0396594f2e4cd592742eb/clang/test/SemaCXX/attr-weak.cpp#L49 and https://github.com/llvm/llvm-project/blob/abf4a8cb15d4faf04ee0da14e37d7349d3bde9a1/clang/test/CodeGenCXX/weak-external.cpp#L97) in the test codes that nulls here.
> > > > It seems they are doing the same thing, doing comparison against a pointer to a `[[gnu::weak]]` member. A simple reproducing code is here: https://godbolt.org/z/qn997n85n
> > > > As you can see from the compiler explorer, there's no note emitted here before the patch.
> > > > I inserted some `printf` into the code before this patch  and confirmed `Info.FFDiag(DiagLoc, diag::note_constexpr_uninitialized) << true << Type` was actually called when compiling the reproducing code and that somehow it is ignored. FWIW, `SubobjectLoc.isValid()` was `false` here.
> > > > It seems they are doing the same thing, doing comparison against a pointer to a [[gnu::weak]] member. A simple reproducing code is here: https://godbolt.org/z/qn997n85n
> > > > As you can see from the compiler explorer, there's no note emitted here before the patch.
> > > 
> > > I see a note generated there:
> > > ```
> > > <source>:4:41: note: comparison against pointer to weak member 'Weak::weak_method' can only be performed at runtime
> > > constexpr bool val = &Weak::weak_method != nullptr;
> > >                                         ^
> > > ```
> > > The situations I'm concerned about are the changes to ExprConstant.cpp:2270 or line 2399 and so on.
> > The `FFDiag` call can just stay as it was, right? And then only the `Info.Note` call needs to be conditional for whether we have  a `SubobjectDecl`.
> In my understanding, the concern is that there could be note loss when `Value.hasValue()` evaluates to `false` and at the same time `SubobjectDecl` is `nullptr` explicitly passed in this change. 
> `[[gnu::weak]]` member pointer comparison is the only example of this happening in the clang test codes where `Subobject` is `nullptr` passed at ExprConstant.cpp:2454 in `CheckFullyInitialized`. In this case the "uninitialized subobject" note was not displayed before this patch as well because there is another note issued elsewhere. Thus, the note loss does not happen.
> 
> BTW, I checked where the `[[gnu::weak]]` note is issued and I think there might be a mistake in the return statements. The `return true` should be `return false` because the constexpr evaluator fails to evaluate the expression, right? Correct me if I am wrong. Here's the reference:
> https://github.com/llvm/llvm-project/blob/e58a49300e757ff61142f6abd227bd1437c1cf87/clang/lib/AST/ExprConstant.cpp#L13140-L13151
> 
> If we change them to `false`, `CheckFullyInitialized` will not be called in this scenario, and we will  have no test cases where `SubobjectDecl` is `nullptr` and at the same time `Value.hasValue()` is `false`. I am unsure whether this is due to the lack of test coverage or it is guaranteed that this condition does not hold here.
> 
> > The `FFDiag` call can just stay as it was, right? And then only the `Info.Note` call needs to be conditional for whether we have  a `SubobjectDecl`.
> If `SubobjectDecl` is null, it would cause segfault when the `FFDiag` note is displayed, I think.
> 
> The best way I can think of would be to `assert(SubobjectDecl)` here, but I am still struggling and would like to ask for comments.
> In my understanding, the concern is that there could be note loss when Value.hasValue() evaluates to false and at the same time SubobjectDecl is nullptr explicitly passed in this change.

Correct.

> BTW, I checked where the [[gnu::weak]] note is issued and I think there might be a mistake in the return statements. The return true should be return false because the constexpr evaluator fails to evaluate the expression, right? Correct me if I am wrong. Here's the reference:
https://github.com/llvm/llvm-project/blob/e58a49300e757ff61142f6abd227bd1437c1cf87/clang/lib/AST/ExprConstant.cpp#L13140-L13151

Agreed, that's a bug, good catch and thank you for the fix!

> The best way I can think of would be to assert(SubobjectDecl) here, but I am still struggling and would like to ask for comments.

That's the approach I would try initially as well -- unfortunately, precommit CI is currently broken due to configuration issues, so we might only learn about issue post-commit.

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  https://reviews.llvm.org/D146358/new/

https://reviews.llvm.org/D146358