[Lldb-commits] [PATCH] D40537: Simplify UUID::IsValid()

[Zachary Turner via lldb-commits]

Eh, that actually just makes me think the compiler *can* check it.  For
example, right now you can have mach-o files with 20 byte UUIDs.  But just
in the code, not in practice.  You could have a bug in your code that
accidentally wrote the wrong number of bytes from a dynamic buffer.

You could enforce this at the compiler level by saying:

class ObjectFileMachO {
  UUID<16> uuid;
};

Not only is this more correct, but it is less error prone and is also nice
documentation to the reader who may be just learning about MachO that this
UUID is always 16 bytes.

For the case of ELF, it sounds like you either have a 20 byte UUID or a 4
byte UUID, but never both, and never any other size.  That makes me think
of:

class ObjectFileELF {
  union {
    UUID<20> BuildId;
    UUID<4> DebugCrc;
  }
};

And now the person reading this code can immediately tell that there will
either be one or the other, and depending on which one it is, he/she
already knows something about it, like how many bytes it is and what it
represents.

To me this is much more clear than:

class ObjectFileELF {
  // This might not actually be a build id, and it could be a variable
size, and you also have to be careful
  // not to put some strange number of bytes in it that we don't recognize,
but it's up to the user
  // to know under what circumstances it should be a certain number of
bytes, and you should also always
  // be careful ensure that there's no buffer overruns since you'll be
working with dynamically sized buffers
  // and the compiler can't warn you when you're doing something wrong.
  UUID BuildIdOrDebugCrc;
};

On Tue, Nov 28, 2017 at 11:06 AM Greg Clayton <clayborg at gmail.com> wrote:

>
> On Nov 28, 2017, at 10:24 AM, Zachary Turner <zturner at google.com> wrote:
>
>
>
> On Tue, Nov 28, 2017 at 10:18 AM Greg Clayton <clayborg at gmail.com> wrote:
>
>> On Nov 27, 2017, at 10:11 PM, Zachary Turner <zturner at google.com> wrote:
>>
>> As an aside, I don't really like this class.  For example, You can
>> currently assign a UUID[16] to a UUID[20].  That doesn't make a lot of
>> sense to me.
>>
>>
>> What about an invalid UUID[0] being assigned with a valid UUID[16] or
>> UUID[20]? Why doesn't this make sense? I don't follow.
>>
>
> Nothing is invalid, I just think it's better and expresses the intent more
> clearly if you can only assign between UUIDs of the same size.  For
> example, If the UUID class were templated on size, then there would not
> even be such thing as a UUID[0] or a "universally invalid UUID".  There
> would be an "invalid 16-byte UUID" and an "invalid 20-byte UUID", and those
> would be different things.
>
>
>>
>>
>> As a future cleanup, I think this class should probably be a template
>> such as UUID<N>, and then internally it can store a std::array<uint8_t,
>> N>.  And we can static_assert that N is of a known size if we desire.
>>
>>
>> UUID values are objects contained as members inside of other objects.
>> They all default to start with no preconceived notion of what the UUID
>> should be. IMHO the UUID class is just fine and needs to be able to
>> represent any UUID, from empty uninitialized ones, and be able to be
>> assigned and changed at will.
>>
>>
> Is there ever a use case for changing the number of bytes in a UUID?  If
> you're working with 16-byte UUIDs, does it ever actually happen that now
> you have a 20-byte UUID?  Can you imagine a use case currently where an
> N-byte UUID is being compared against an M-byte UUID in a real-world
> scenario?  If the answer is no, then it may as well be enforced by the
> compiler.
>
>
> The ObjectFile class has a "UUID m_uuid;" member that any object file can
> fill in. Right now mach-o files have 16 byte UUIDs. ELF files can have 20
> bytes UUIDs (build ID) or 4 byte UUIDs (debug info CRC if no build ID is
> around, and these are current represented as 20 byte UUIDs with just the
> first 4 bytes filled in. So no, we can't enforce this using the compiler. I
> don't see a need to change way from a byte buffer that has the max number
> of bytes needed for any currently supported UUID (20 right now).
>
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