[Bug c++/124182] New: gcc-16 optimization regression with -fdevirtualize-speculatively

[frank.mehnert at googlemail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124182

            Bug ID: 124182
           Summary: gcc-16 optimization regression with
                    -fdevirtualize-speculatively
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: frank.mehnert at googlemail dot com
  Target Milestone: ---

Using gcc-16 from Debian/experimental:
$ gcc-16 --version
16-20260208-1) 16.0.1 20260208 (experimental) [trunk r16-7394-gd93c8a679b2]
...

Compiling the L4Re code we get undefined references to extern objects with
g++16. The same code works without any problem with g++15.

Reduced example below. Compiling with gcc-16:
$ g++-16 -O3 -c reduced.cc -o reduced.o && objdump -tC reduced.o|grep marker
0000000000000000         *UND*  0000000000000000 marker

$ g++-15 -O3 -c reduced.cc -o reduced.o && objdump -tC reduced.o|grep marker

$ g++-16 -O3 -c reduced.cc -o reduced.o -fno-devirtualize-speculatively &&
objdump -tC reduced.o|grep marker


Here is the code:

#include <cstdio>

extern unsigned long marker;

struct Logger
{
  unsigned long _mask;
  void log() const
  {
    if (marker & _mask)
      printf("foo bar\n");
  }
};

struct Base
{
  virtual void work() noexcept = 0;
  virtual ~Base() = default;
};

struct DerivedA : Base
{
  void work() noexcept override {}
};

struct DerivedB final : Base
{
  Logger *_dbg;
  void work() noexcept override
  {
    _dbg->log();
  }
};

DerivedA local_obj;
Base *ptr = &local_obj;

void func()
{
  ptr->work();
}