[LLVMdev] [lld] Undefined symbols postprocessing

[Shankar Easwaran]

+ Nick

On 2/19/2015 9:00 AM, Shankar Easwaran wrote:
> On 2/19/2015 3:58 AM, Denis Protivensky wrote:
>> Joerg:
>>> 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.
>> Do you want to do that before or after dead code elimination?
>> I think dead code elimination should be performed after all possible 
>> object code modifications done by lld. Therefore, it should be done 
>> after undefines' postprocessing as well.
> Gnu does dead code elimination before undefines are reported. So if a 
> function is not called and it has a undefined reference its would not 
> be an undef.
>>
>> Shankar:
>>> 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.
>> I came across this same problem, and was planning on adding a
>> notifyUndefinedSymbol to the LinkingContext, if the linker wants to add
>> a defined symbol and coalesce it, it would be possible.
>>
>> Do you think this will work for your case too ?
>> With this option, I don't see:
>> - how to postpone processing and reaction on undefines. If the 
>> callback is called from within Resolver::resolve(), you should react 
>> on it immediately, because otherwise the code will still fail in 
>> Resolver::resolve().
>> - how to know if a symbol is needed within the callback body. The 
>> need of any symbol is determined in some other place. So I need to 
>> keep a sort of indication (boolean flags, whatever) to know which 
>> symbols are really needed.
>> - the exact interface of notifyUndefinedSymbol callback. If it 
>> receives `StringRef` name of the undefined symbol, what reaction 
>> should be? Should it return new symbols to add back to the caller as 
>> `const Atom*`?
> notifyUndefinedSymbol will allow the context to coalesce the undefined 
> atom with a defined atom.
>
> Atom *notifyUndefinedSymbol(StringRef name) could be the interface.
>
>> Thanks,
>>    Denis.
>>
>
>


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