Invalid IR when accessing union

[yugr]

This code

typedef union {                                                                 
  char x;                                                                       
  struct {                                                                      
    short h, l;                                                                 
  } b;                                                                          
} T;                                                                            
                                                                                
void foo(T reg) {                                                               
  reg.b.l;                                                                      
}                                                                               

currently generates invalid IR:

$ ~/src/clangir/build/bin/clang -fclangir-enable -emit-llvm -S tmp.c
loc("tmp.c":4:14): error: 'llvm.getelementptr' op index 1 indexing a struct is out of bounds

It seems that we are missing a bitcast in buildLValueForField and code even has a comment about this:

  // TODO(CIR): CodeGen requires a bitcast here for unions or for structs where 
  // the LLVM type doesn't match the desired type. No idea when the latter might
  // occur, though.                                                             

Indeed adding

  if (rec->isUnion()) {
    auto memTy = getTypes().convertTypeForMem(FieldType);
    addr = builder.createElementBitCast(getLoc(field->getSourceRange()), addr, memTy);
  }

fixes the problem.

[sitio-couto]

@yugr thanks for tracking this.

I'm currently working on refactoring how unions work in CIR. It will likely fix this issue.

[bcardosolopes]

Thanks for sharing @yugr!