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

[Saurabh Jha via cfe-dev]

Hi Florian,

Thank you for the suggestions. I am able to get += and -= work for
matrices and should be able to create a patch for review soon, after I am
done writing lit tests.

Cheers,
Saurabh

On Wed, Mar 3, 2021 at 10:30 AM Florian Hahn <florian_hahn at apple.com> wrote:

> 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 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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