[llvm-dev] RFC: Safe Whole Program Devirtualization Enablement

[Teresa Johnson via llvm-dev]

On Thu, Dec 19, 2019 at 1:27 AM Iurii Gribov <Iurii.Gribov at ceva-dsp.com>
wrote:

> > -fsemantic-interposition controls whether the compiler may assume that
> symbols are not interposed,
> > and it has nothing to do with the optimization proposed here.
>
> Thanks Peter, you are probably right. I've overestimated the information
> that's available to LTO optimizer at link time.
>
> Leaving shared libraries aside, one might argue that when LTOptimizing
> main executable file compiler/linker is aware of all participating
> libraries and so can decide whether class is not derived from and apply
> devirtualization based on that information (assuming that run-time library
> implementations or dlopen calls do not introduce new inherited classes at
> runtime, that's where -fno-semantic-interposition assumption comes into
> play). But the "decide whether class is not derived from" part here is
> problematic - derived classes in libraries may have hidden visibility and
> will go undetected.
>

Right, this is why we need some guarantee from the user that the LTO link
will see all derived classes. Currently at head the only way to do so is
via -fvisibility=hidden when compiling to bitcode. This proposal adds
another mechanism, pre-enabling the bitcode for WPD and delaying the
guarantee until LTO link time with an option there.

Thanks,
Teresa


> -I
>


-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |
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