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

[David Li via cfe-commits]

david-xl 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 {..
}
   

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