LLVM cannot devirtualize with MSVC-style RTTI


Bugzilla Link	PR49765
Status	CONFIRMED
Importance	P enhancement
Reported by	Reid Kleckner (rnk@google.com)
Reported on	2021-03-29 15:46:19 -0700
Last modified on	2021-06-06 22:19:27 -0700
Version	trunk
Hardware	PC All
CC	aeubanks@google.com, llvm-bugs@lists.llvm.org, peter@pcc.me.uk
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Consider this simple case when devirtualization should be possible after
inlining the constructor:
https://gcc.godbolt.org/z/osGe67s9b

struct Foo {
  Foo() {}
  virtual ~Foo() {}
  virtual void f() {}
  int x;
};
int main() {
  Foo *p = new Foo();
  p->f();
  delete p;
}

When RTTI data is disabled (/GR- / -fno-rtti-data), we cannot devirtualize the
call to Foo::f. We can optimize the vptr load, but the vtable alias inhibits
full devirtualization. See this instruction:
  call qword ptr [rip + "??_7Foo@@6B@"+8]

LLVM knows exactly which vtable we are loading from, but because of the MSVC
RTTI alias code pattern, it refuses to optimize away the load.

We should either make the optimize more aggressive, or find a better IR pattern
for MSVC-style RTTI.

Arthur had a patch to address this, but maybe it isn't sound:
https://reviews.llvm.org/D99240

Quuxplusone / LLVMBugzillaTest

LLVM cannot devirtualize with MSVC-style RTTI #48734