[PATCH] D29104: Add !associated metadata.

[Evgeniy Stepanov via Phabricator via llvm-commits]

eugenis added a comment.

In https://reviews.llvm.org/D29104#655685, @mehdi_amini wrote:

> In https://reviews.llvm.org/D29104#655669, @eugenis wrote:
>
> > In https://reviews.llvm.org/D29104#655651, @mehdi_amini wrote:
> >
> > > I mean the specific optimizations that won't happen that motivate not using llvm.compiler.used.
> >
> >
> > I'm not sure. Perhaps, link time devirtualization? Without this, ASan would have to add every single global to llvm.compiler.used, which would make globaldce not do anything at all.
>
>
> It'll still operate on every functions. And this is what happens on Darwin today.


Yes, and it works fine. But on Linux we can do better.

Isn't it possible that GC-ing of virtual tables would kill class methods, which can in turn kill more virtual tables and allow devirtualizing of more call sites?

> 
> 
>> If anything, it is bad for the object file size.
> 
> Linker will garbage collect.

Object file size matters, too.

> 
> 
>> The real reason is w/o the GlobalDCE change !associated has strange semantics - it protects a global from linker GC, but only if said global survives compiler GC. I don't see how that is useful.
> 
> I'm not convinced at all by the `!associated` at this point, in particular I haven't seen anything it does which is different from what is done on Darwin.

Darwin is not a problem here. ELF is.
On Darwin, ASan uses the "live_support" linker hack:
https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/Instrumentation/AddressSanitizer.cpp#L1680
SHF_ASSOCIATED is the proposed ELF implementation of roughly the same concept. We need a way to represent it in the IR.


Repository:
  rL LLVM

https://reviews.llvm.org/D29104