[LLVMdev] [lld] Representation of lld::Reference with a fake target

[Rui Ueyama]

The atom model is, compared to the traditional linker model, representing
both symbol and its data as an "atom" and model relocations and other
information as edges between atoms.

I think the atom model doesn't mean nor require all atoms in memory are of
the same type (or class) regardless of architecture/OS. They can be
different subtype as long as output is correct. And that's what we are
doing with LLD. We have many subtypes of atoms or references. A reader for
an architecture creates atoms of some type, and the corresponding writer
for the architecture uses them. Architecture-independent passes work fine
with derived classes of atom/reference because subtypes have compatible
interface with super classes. It seems to be working, except the round-trip
test.

The round-trip test is different from other passes in the sense that it
needs to understand *everything* about given data (please compare this
property with other passes). It needs to be able to write everything about
a given (internal representation of) file and read them back. Otherwise we
will lose data during round-trip. That's what's happening, right?

The point is again we don't really need to save intermediate data. Only the
round-trip passes need to do that. The round-trip tests test the features
for themselves -- not for others. This is just a burden, no?

On Mon, Feb 9, 2015 at 1:05 PM, Shankar Easwaran <shankarke at gmail.com>
wrote:

>
>
> On Mon, Feb 9, 2015 at 2:30 PM, Rui Ueyama <ruiu at google.com> wrote:
>
>> On Mon, Feb 9, 2015 at 12:24 PM, Shankar Easwaran <
>> shankare at codeaurora.org> wrote:
>>
>>>  Hi,
>>>
>>> The round trip passes just tries to load a *complete* object file in
>>> YAML/Native format back.
>>>
>>> The internal state should be the complete object file in native/yaml
>>> format.
>>>
>>> If some state is not recorded and that is really needed in the writer,
>>> we should add that to the Atom model.
>>>
>>
>> Again, not only saying we should do something, but please elaborate why
>> you think we should. Otherwise we can't discuss...
>>
>
> The RoundTrip passes was a method to verify the Atom model. When the pass
> was written / reviewed, this pass was a way to verify that all information
> passed to the writer was passed through an Atom.
>
> I am completely ok to remove the pass as long as there is an alternative
> with which we can verify.
>
> Shankar Easwaran
>
>
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