[LLVMdev] [lld] Implementing the aliasing feature

[Nick Kledzik]

On Sep 13, 2013, at 4:16 PM, Shankar Easwaran <shankare at codeaurora.org> wrote:
> On 9/13/2013 6:06 PM, Nick Kledzik wrote:
>> On Sep 13, 2013, at 3:35 PM, Shankar Easwaran <shankare at codeaurora.org> wrote:
>>> This would work only if an alias is another name for the same symbol(weak symbols).
>> I don’t know what that means. Can you clarify?
> Doesnt this imply that the alias atom is a zero sized atom ?
Yes.  That is what an alias is - another name for something.  An alias has no content of its own.  It is just an alternate name for something.  What do you think alias means?

>>> If what is being aliased is another function definition, which is a non zero sized atom, aliasing will not work.
>> That is the exact scenario I think it *will* work in.  What do you think won’t work.
> If its a non zero sized atom, like for example :-
> 
> definedatoms:
>    - name : fna
>      size : 4
>     ...
>     ...
> 
> definedatoms:
>    - name: fnb
>      size: 4
>     
> If I alias the atom, and add a layoutBefore from fna to fnb, fnb is going to have a seperate virtualaddress from fna.
This sounds like you mean “alias” to mean take one implementation and override another implementation (that has a different name). 

-Nick

> 
> But you essentially wanted fna, fnb to have the same virtual address right ?
> 
> Am I misreading something that you said ?
>> 
>>> 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.
>> I don’t see how those steps will achieve wrapping functionality.  Say you are wrapping malloc.  There will be a malloc seen at build time from libc, and all references to malloc will bind to it.  Adding alternate names won’t stop that binding.
> Yes, thats how ld is behaving, if I have the the function in my .o's, it doesnot override.
> 
> For example :-
> 
> #include <stdio.h>
> 
> int myfn() {
>   return 0;
> }
> 
> void __wrap_myfn()
> {
>   printf("Hello World\n");
> }
> 
> int main() {
>   myfn();
>   return 0;
> }
> $gcc wrap.c -Wl,--wrap,fn
> $./a.out
> $
> 
> Thanks
> 
> Shankar Easwaran
> 
> -- 
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the Linux Foundation

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