[llvm-dev] How static casts work between Stmt and its inheritors in clang?

[Florian Hahn via llvm-dev]

Hi,


> On Mar 2, 2021, at 20:10, Saurabh Jha via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi LLVM people,
> 

Llvm-dev is usually used to discuss issues related to llvm itself. As this email is related to Clang internals, cfe-dev would be more suitable (added as CC).

> I am new to clang/llvm and have been hacking on it for about three weeks now.  I am trying to implement compound assignment operators, +=, -=, and *=, for matrices. Here are the bug details <https://bugs.llvm.org/show_bug.cgi?id=46164>.
> 

Thank you very much for taking a look at the issue!

> I have a lit test that tries to do a "a += b" for matrices a and b. Here's its trace <https://godbolt.org/z/eM7x1q> (courtesy Florian Hahn). Clang fails on this assertion <https://github.com/llvm/llvm-project/blob/ca5247bb1770a1dfa56b78303d99f1cc9a0a06ee/clang/include/clang/AST/Type.h#L677>. And this is because when we do a static cast from a Stmt instance to a CompoundAssignOperator here <https://github.com/llvm/llvm-project/blob/ca5247bb1770a1dfa56b78303d99f1cc9a0a06ee/clang/include/clang/AST/StmtVisitor.h#L76>, we are not assigning the correct QualType to the LHS. Concretely, here's what I found in my debugging.
> 
> # printing out S
> (lldb) p S
> (std::__add_pointer_helper<clang::Stmt, true>::type) $0 = 0x0000000011621f70
> 
> # cast S to CompoundAssignOperator
> (lldb) p ((CompoundAssignOperator *) S)
> (clang::CompoundAssignOperator *) $1 = 0x0000000011621f70
> 
> # access ComputationLHSType attribute of casted S. The QualType is NULL.
> (lldb) p ((CompoundAssignOperator *) S)->ComputationLHSType
> (clang::QualType) $2 = {
>   Value = (Value = 0)
> }
> 
> # access ComputationResultType attribute of casted S.
> (lldb) p ((CompoundAssignOperator *) S)->ComputationResultType
> (clang::QualType) $3 = {
>   Value = (Value = 291641504)
> }
> 
> So I think it's working correctly for ComputationResultType but is somehow assigning null to ComputationLHSType's QualType.Value. This is interesting because if I try to cast S to something like BinaryOperator and cast its operands to Expr, the types are coming out correctly.

You are correct I think, the key issue is that we assign the wrong type to LHS (null instead of the matrix type of the binop result). The problem is that we do not properly set CompLHSTy for matrix expressions (see https://github.com/llvm/llvm-project/blob/ca5247bb1770a1dfa56b78303d99f1cc9a0a06ee/clang/lib/Sema/SemaExpr.cpp#L10518 <https://github.com/llvm/llvm-project/blob/ca5247bb1770a1dfa56b78303d99f1cc9a0a06ee/clang/lib/Sema/SemaExpr.cpp#L10518> and compare to what’s done for vector types above). I think if that is fixed, things should work out as expected.

Cheers,
Florian

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