How bitfields are packed relies on compiler implementation. For example: struct { int x : 31; bool y : 1; }; Above struct consumes 8 bytes in MSVC, where we probably expect it uses only 4 bytes (otherwise why do we even use bitfields?). Unfortunately, we are widely using this pattern in webkit. More information: http://msdn.microsoft.com/en-us/library/hx1b6kkd.aspx "Adjacent bit fields are packed into the same 1-, 2-, or 4-byte allocation unit if the integral types are the same size and if the next bit field fits into the current allocation unit without crossing the boundary imposed by the common alignment requirements of the bit fields. " Ideally, we should be able to assert bitfields are packed with expected size at compile time. Also it would be nice if check-webkit-style can detect such issues.