[LLVMdev] [lld] Undefined symbols postprocessing

[Rui Ueyama]

On Mon, Feb 23, 2015 at 12:26 AM, Denis Protivensky <
dprotivensky at accesssoftek.com> wrote:

>  Rui, see inline.
>
>
> On 02/20/2015 10:20 PM, Rui Ueyama wrote:
>
>  On Wed, Feb 18, 2015 at 1:38 AM, Denis Protivensky <
> dprotivensky at accesssoftek.com> wrote:
>
>> Hi everyone,
>>
>> In lld, I need to conditionally add symbols (like GLOBAL_OFFSET_TABLE)
>> during
>> static linking because they may be used by relocations (R_ARM_TLS_IE32) or
>> by some other stuff like STT_GNU_IFUNC symbols.
>> The problem is that now symbols are added in a declarative way by
>> specifying in ExecutableWriter::addDefaultAtoms() override.
>> At that stage, there's no way to determine if additional symbols are
>> required.
>> But libraries providing optimizations like STT_GNU_IFUNC
>> (glibc, for example) expect the GOT symbol to be defined, so the linking
>> process
>> fails in Resolver::resolve() if the symbol is not found.
>>
>> I propose to add the ability to ignore undefined symbols during initial
>> resolution, and then postprocess only those undefines for the second time
>> after the pass manager execution.
>>
>> Technically, this shouldn't be a problem:
>> - there will be a new option in the linking context that should signal
>> that the postprocessing of undefined symbols should be performed.
>> - if postprocessing option is set, newly added symbols will be collected
>> in the MergedFile returned by the Resolver, and then only those new
>> symbols
>> will take part in the resolution process very similar to what
>> Resolver::resolve() does.
>> - available implementations will not break and keep working without use of
>> postprocessing feature.
>>
>
>  I'm fine with the basic idea of allowing undefined symbols in the first
> resolver pass. A few questions about the implementation.
>
>  - How do you know which atom is newly added and which is not? Once an
> atom is added to a MutableFile, there's no easy way to recognize that, I
> guess.
>
> The Resolver returns Resolver::MergedFile type as a result of call to
> resolve(), and we can override its addAtom method to put newly added atoms
> to a special separate collection which then may be examined for undefines.
>
>   - Does the second resolver pass need to be run after all other passes?
> Why don't you run the resolver once, and then call some externally-given
> function (from the resolver) to get a list of atoms that needs to be added
> to the result, and then resolve again, all inside the resolver?
>
> Since we have a chance to determine newly added atoms after resolution, I
> don't see why to complicate the process with external functions and
> additional call dependencies. It all can be done by adding second
> resolve()-like function call in the Driver::link() after PassManager run.
>

If we run the second pass immediately after the resolver runs, even without
returning to the caller of the resolver, the resolver looks like a single
pass process from outside, although internally it iterate twice. It makes
post processing passes simpler if you can get all atoms that needs to be
written to file just by calling the resolver once instead of getting
incomplete set.

I guess you need to run some passes after the second resolver invocation.
For example, you need to call the OrderPass to reorder newly added atoms to
desired places. If resolver resolves everything in one invocation, you
wouldn't think about that, too.

   So my proposal is to move from the declarative style towards imperative
>> and more flexible approach. Of course, there's a downside as the code
>> loses some of its regularity and becomes more volatile, but in the end -
>> we have tests to cover such things and ensure everything works as
>> expected.
>>
>> Any ideas?
>>
>> - Denis Protivensky.
>>
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>>
>
>
>
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