[clang] [IRPGO][ValueProfile] Instrument virtual table address that could be used to do virtual table address comparision for indirect-call-promotion. (PR #66825)

[Mingming Liu via cfe-commits]

minglotus-6 wrote:

> > > The work sounds interesting. Can you provide a bit more context about it? Will it be used to improve ICP when it's sufficient to just compare the vtable address instead of the vfunc address?
> > 
> > 
> > yes -- it can not only eliminate vtable load, but also enable target check combining.
> > What is more important is that it can be combined with more aggressive interprocedural type propagation that enables full (unconditional) devirtualization. Example:
> > base->foo(); base->bar(); ==> if (base->vptr == Derived) { Derived::foo(base); // base type is known so virtual calls in foo,bar can further be devirtualized. Derived::bar(base); } else {.. }
> 
> Thanks for the illustration! Have you enabled this in your fleet, and how much performance improvement have you seen?
> 
> We've been also thinking about similar work based on sample PGO, in both the compiler and bolt. cc @WenleiHe

I'm prototyping vtable comparison (to eliminate the additional load conditionally to avoid the downside mentioned https://groups.google.com/g/llvm-dev/c/_1kughXhjIY/m/7fzCmsuVmS8J) and it works for small test programs. I will collect the numbers for macro benchmarks.

https://github.com/llvm/llvm-project/pull/66825