llvm dev - Dec 2019 - RFC: Safe Whole Program Devirtualization Enablement

(cc list this time)

Hi Teresa,

Apologies if this has been discussed before but ...
> The LTO visibility of a class is derived at compile time from the class’s
symbol visibility.
> Generally, only classes that are internal at the source level (e.g.
declared in an anonymous namespace) receive hidden LTO visibility.
> Compiling with -fvisibility=hidden tells the compiler that, unless 
> otherwise marked, symbols are assumed to have hidden visibility, which 
> also implies that all classes have hidden LTO visibility (unless decorated
with a public visibility attribute).
> This results in much more aggressive devirtualization.
Note that by default, unlike GCC, LLVM is liberal on visibility-constrained
optimizations. In particular it freely performs inlining, IPA and cloning on
them (see https://lists.llvm.org/pipermail/llvm-dev/2016-November/107625.html
which also suggested adding -fsemantic-interposition to actually respect
visibility in optimizations). It's unclear why devirtualization should
behave differently than other optimizations (at least by default).

-I

> Are you suggesting that we should be more aggressive by default (i.e.
without -fvisibility=hidden or any new options)?
> I believe that will be too aggressive for class LTO visibility.
> It is common to override a virtual functions across shared library
boundaries
I'm myself a fan of GCC's conservative approach so I'd agree with
you on that. But regardless of my personal opinions, LLVM already optimizes
normal functions in shared libraries (thus mercilessly breaking overrides via
LD_PRELOAD, etc.) and I don't see how virtual functions are any different. I
think default LLVM behavior needs to be consistent for all inter-procedural
optimizations (be it inlining, devirtualization or cloning).

Maybe it's time to resurrect Hal's -fsemantic-interposition flag and use
it consistently throughout compiler? Users who need GCC-like semantics will be
able to employ this flag to prevent unsafe optimizations.

-I