[lld] r190608 - [lld][LayoutPass] Order the atoms that are in the same chain

[Rui Ueyama]

On Fri, Sep 13, 2013 at 4:56 PM, Nick Kledzik <kledzik at apple.com> wrote:

>
> On Sep 13, 2013, at 4:40 PM, Rui Ueyama <ruiu at google.com> wrote:
>
> I know we are using SimpleFile for GOT/PLT atoms, for example, and I doubt
> that's a good design. These atoms are not read from any files but produced
> by the linker after the core liking is done so semantically they don't
> belong to any file. A SimpleFile is used only as a dummy value because it's
> mandatory. We might be able to simplify the thing by making an optional
> thing optional.
>
> So I'd want to make _file field optional for the atom. What do you guys
>>> think?
>>>
>> I dont think we do this.
>>
>
> Can you elaborate?
>
> In the darwin linker (also atom based), the file() method is optional.
>  And that has been a big source of (crasher) bugs.  Because to be truly
> optional, every place in the linker that access the file() method must be
> prepared for it to return NULL and do something.  That is a pain to get
> right.
>
> Having a dummy file with some useful name (e.g. “GOT pass”) simplifies all
> clients (don’t need to check if file exists), and makes error messages more
> useful.
>

Atom::atom() is defined to return a reference to File, so it cannot return
NULL. How does the Darwin linker made it optional? Your point makes sense
though.
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