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Re: [Mips}Using DT tags for handling local ifuncs

On Sun, 12 Jan 2014, Richard Sandiford wrote:

> >  I'm not sure what to do about sections though -- they are not required in 
> > final ELF binaries and not interpreted by, but we keep them and 
> > therefore have to decide how to handle them.  We could merge all the 
> > original sections into .got, but that could be confusing to some.  We 
> > could keep original .lit4, .sdata, etc. section names, keeping .got for 
> > the legacy GOT part and choosing a new name for the explicitly relocated 
> > GOT part.  But then the reserved entries wouldn't fit anywhere.
> Yeah, I was wondering this too.  Things like .lit4 could be handled even
> in a multigot object, since there's no ODR problem with duplicating the
> contents in each GOT that needs them (i.e. it's not valid to rely on
> address equality for .lit4 entries).  So for those I think we could
> end up with the contents being spread across several GOTs.  And in
> that case just putting them in .got might be easiest.
> Obviously that isn't possible for .sdata and .sbss: we need to keep
> the original link order.  But in principle we could still put .sdata
> in a single secondary GOT.
> It wouldn't be trivial to do any of this and to make it coexist with
> linker scripts though.  I'm not sure it's worth spending too much time
> thinking about it unless someone's actually ready to implement it.

 Agreed, I'm fine as long as the design is compatible with such an 
extension in the future so that we do not need to invent more dynamic tags 
or whatever.  I do not plan to commence any work in that direction at the 

> And I'm not sure whether .sdata and .sbss would be much of a win in
> practice.  It would only help with PIEs and DSOs that make relatively
> heavy use of a small amount of global state.  How many modern DSOs have
> that pattern?

 I haven't made any assessment on that, I'd leave it up to people to 
figure out whether they want to use a non-zero -G value for SVR4 code.  
I've been asked about the feature a couple times though so it looks like 
there's some demand or at least people think they might benefit from it -- 
whether they're right or wrong about it.

 One use might be teaching the compiler to put 64-bit integer immediates 
out of line with the new ABIs -- that'd still be .lit8 though I suppose.


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