[LLVMdev] [lld] Implementing the aliasing feature

[Shankar Easwaran]

Hi Nick,

This would work only if an alias is another name for the same 
symbol(weak symbols).

If what is being aliased is another function definition, which is a non 
zero sized atom, aliasing will not work.

I was thinking to model this for ELF for the below functionalities :-

a) __wrap

For example : --wrap fn

What I plan to do here is,

create a undefined function fn atom
create a defined weak atom fn
create a alias reference to __wrap_fn which is a undefined atom.

b) --defsym option

For example : --defsym A = B

the defsym option is going to have a undefined atom for A
create a defined weak atom "B" of size 0
create a alias reference to the expression atom, that defines the 
expression.

Both (a), (b) would work if there is an actual function fn, and a symbol 
A that can override.

If there is no such symbol, whatever is being aliased to will take 
preference.

Thanks

Shankar Easwaran

On 9/13/2013 5:26 PM, Nick Kledzik wrote:
> Can an “alias” atom just be a zero sized atom with one reference of kindLayoutBefore to its target?  The layout engine should then place the alias atom right before the real atom, so you wind up with two symbols at one address.
>
> -Nick
>
> On Sep 12, 2013, at 3:02 PM, Shankar Easwaran <shankare at codeaurora.org> wrote:
>
>> Hi Nick,
>>
>> In addition to what you mentioned, I think there needs a special *AliasReference* that need to be created which the DefinedAtom points to.
>>
>> Thanks
>>
>> Shankar Easwaran
>>
>>
>> -------- Original Message --------
>> Subject:	Re: [PATCH] Add a fallback mechanism for undefined atom.
>> Date:	Thu, 29 Aug 2013 15:15:49 -0700
>> From:	kledzik at apple.com <kledzik at apple.com>
>> Reply-To:	reviews+D1550+public+25c98da67f89e22f at llvm-reviews.chandlerc.com
>> To:	shankarke at gmail.com, kledzik at apple.com, bigcheesegs at gmail.com, ruiu at google.com
>> CC:	llvm-commits at cs.uiuc.edu
>>
>>
>>    Interesting feature.
>>
>>    What happens if a.o has an undefine foo which says its fallback is bar, but in b.o an undefine for foo says its fallback is baz?  Is that an error, or does each fallback apply to its original usage.  The problem is that the Resolver is currently coalescing UndefinedAtoms based on their name. I does not know that fallbacks need to match too.
>>
>>    Another variant is to have Undefined return a alternate name as a string, rather than returning a new UndefinedAtom object.
>>
>>    Another approach is to use weak aliases.  That is if a.o has an undefine for foo with a fallback of bar, that when parsing that .o file into atoms produces an UndefinedAtom for foo, but also a DefinedAtom with name=foo, weak (mergeAsWeak), hidden (scopeLinkageUnit), size=0, isAlias=true which aliases to bar.  So, it any definition of foo does show up, it will override the weak alias.  If not the weak alias will be used and change the reference to bar.
>>
>>    I don't think the isAlias support is all wired up in lld.  But weak aliases are something common in ELF. So would should  get it working, and doing so may provide a way to implement this COFF feature.
>>
>>    One issue is how this interacts with static libraries.  The weak alias for foo make look like there is a definition, so any static archives will not be searched for foo.  That seems different than what you want.  I'm not sure what the gnu linker does about this with weak aliases.  We may need a new resolver option as to how to handle this.
>>
>>
>> ================
>> Comment at: lib/Core/Resolver.cpp:214
>> @@ +213,3 @@
>> +      // for COFF "weak external" symbol.
>> +      const UndefinedAtom *fallbackUndefAtom = undefAtom->fallback();
>> +      if (fallbackUndefAtom) {
>> ----------------
>> You need to re-test:
>>    if (!_symbolTable.isDefined(undefName)
>> because searchLibraries() may have already found a definition.
>>
>>
>> http://llvm-reviews.chandlerc.com/D1550
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>>
>>
>>
>>
>>
>


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