[WIP] Add warning when zero-initializing a union if its first member is not is largest

[leonardchan]

Summary

This may result in the entire union not being initialized. For example:

union U {
  uint32_t x;
  uint64_t y;
};

void func() {
  U u = {};
  ...
}

Some might assume that U u = {} zero-initializes the whole union whereas only the bits that compose U.x are zero-inited.

Oddly enough (mostly from what I've seen personally), this seems to only matter in the case we use -ftrivial-auto-var-init=pattern where llvm undefs are replaced with non-zero values. In any normal case, it seems that these undefs just get lowered to zeros.

See: http://lists.llvm.org/pipermail/cfe-dev/2019-September/063383.html

clang/include/clang/Basic/DiagnosticSemaKinds.td

 def err_builtin_launder_invalid_arg : Error<
   "%select{non-pointer|function pointer|void pointer}0 argument to "
   "'__builtin_launder' is not allowed">;
 
 def err_bit_cast_non_trivially_copyable : Error<
   "__builtin_bit_cast %select{source|destination}0 type must be trivially copyable">;
 def err_bit_cast_type_size_mismatch : Error<
   "__builtin_bit_cast source size does not equal destination size (%0 vs %1)">;
+
+def warn_union_zero_init : Warning<
+  "zero-initializing union '%0' whose first non-static named data member '%1' is not the largest in the union will leave uninitialized bits in the rest of the union">;
 } // end of sema component.

clang/lib/Sema/SemaInit.cpp

if (DeclType->isUnionType() && IList->getNumInits() == 0) {
     // Value-initialize the first member of the union that isn't an unnamed
     // bitfield.
     for (RecordDecl::field_iterator FieldEnd = RD->field_end();
          Field != FieldEnd; ++Field) {
       if (!Field->isUnnamedBitfield()) {
         CheckEmptyInitializable(
             InitializedEntity::InitializeMember(*Field, &Entity),
             IList->getEndLoc());
-        if (StructuredList)
+        if (StructuredList) {
           StructuredList->setInitializedFieldInUnion(*Field);
+
+          ASTContext &Ctx = SemaRef.Context;
+          unsigned UnionSize = Ctx.getTypeSize(Ctx.getRecordType(RD));
+          unsigned FieldSize = Ctx.getTypeSize(Field->getType());
+          if (UnionSize > FieldSize)
+            SemaRef.Diag(IList->getLBraceLoc(), diag::warn_union_zero_init)
+              << RD->getName() << Field->getName();
+        }
         break;
       }
     }
     return;
   }
 
   bool InitializedSomething = false;