Empty union in initialized with undef values in OG and with zeroinitializer in CIR

[bcardosolopes]

There are some differences from the OG LLVM for unions for C++ code...

The empty union in initialized with undef values in OG and with zeroinitializer in CIR:

union U { int a; long long b;};
union U u = {};

// OG
@u = dso_local global { i32, [4 x i8] } { i32 0, [4 x i8] undef }, align 8

// CIR enabled
@u = global { i32, [4 x i8] } zeroinitializer, align  8

This is a difference - and soon or later we may want to fix it.

Originally posted by @gitoleg in #1257 (comment)

[beamandala]

Hi @bcardosolopes, I believe the discrepancy is because of this block (lines 1203 - 1214) in CIRGenModule.cpp.

I'm new to ClangIR and was wondering if this was a good issue to start contributing on.

[bcardosolopes]

I'm new to ClangIR and was wondering if this was a good issue to start contributing on.

Yep, assigning this to you! Thanks

[beamandala]

Hi, I've been looking into this and trying out different changes and have been trying to modify this function to handle unions in a different way but it isn't working as expected.

Inside the if statement checking if T is zero initializable, I added another check to see if T is a union type but this check is always evaluating to false and I'm not sure why (I'm running on the code in the issue).

if (T->isUnionType())

Do you have any idea why this is happening and what I can do to fix it?

[bcardosolopes]

this check is always evaluating to false and I'm not sure why

I recommend running the example with clang under a debugger!

[beamandala]

I've ran it using the debugger but I'm not sure how to determine where the issue is. How can I get details about the T which is a QualType obj and determine what it is since isUnionType() is returning false? Printing doesn't returning anything helpful so I was wondering if you had any tips.

(lldb) p T
(clang::QualType) {
  Value = {
    Value = (Data = "\U00000010\xe9\x85(\U00000001")
  }
}

[smeenai]

Many LLVM and Clang types have a dump function which prints useful information about them. p T.dump() should work here.

[beamandala]

Thank you! That gave me some more information. It looks like by the time it gets to CIRGenModule::emitNullConstant, it's working on initializing the union's fields which is why T->isUnionType() is returning false. I'll look into it more to see where I can better address this.