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From: "matt at godbolt dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/68148] New: Devirtualization only applies to last of multiple successive calls
Date: Thu, 29 Oct 2015 15:49:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68148

            Bug ID: 68148
           Summary: Devirtualization only applies to last of multiple
                    successive calls
           Product: gcc
           Version: 5.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: matt at godbolt dot org
  Target Milestone: ---

Given the code:

----
struct Interface {
  virtual ~Interface() {}
  virtual void virtualFunc() = 0;
  virtual void virtualFunc2() = 0;
};

struct Concrete : Interface {
  int counter_;
  Concrete() : counter_(0) {}
  void virtualFunc() { counter_++; }
  void virtualFunc2() { counter_++; }
};

void test(Interface &c) {
  c.virtualFunc();
  c.virtualFunc2();
}
----

(Compiled at -O3 -fdevirtualize-speculatively)

Speculative devirtualization is applied to the call to virtualFunc2, but not to
virtualFunc. (See https://goo.gl/Vtx5Fe). If one comments out the call to
virtualFunc2, then the virtualFunc() call *is* speculatively devirtualized
(https://goo.gl/G8f505).

It seems to me that either both should be spec devirtualized, or none. Or
perhaps even more generally, if the vtable pointer is that of "Concrete" then
both calls can be inlined in one and converted to counter+=2 (provided
inspection proved that Concrete's virtualFunc() does not modify the vtable,
which I believe is otherwise a barrier to this kind of optimization).

Am I missing something here, or is this a missed opportunity?

Thanks, Matt