[Bug tree-optimization/66868] New: [5/6 Regression] wrong code generated with -O3 (dead code removal?)

[doko at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66868

            Bug ID: 66868
           Summary: [5/6 Regression] wrong code generated with -O3 (dead
                    code removal?)
           Product: gcc
           Version: 5.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: doko at gcc dot gnu.org
  Target Milestone: ---

Created attachment 35975
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35975&action=edit
preprocessed source

[forwarded from https://bugs.launchpad.net/ubuntu/+source/gcc-5/+bug/1473674]

seen in the apt package manager, when built with -O3. Works with -O2 or -O3
-fno-inline-functions -finline-small-functions, on at least x86_64 and
powerpc64le. The test CDROMTest.FindPackages from the apt testsuite then fails.

built with g++ -c -g -O3 -fPIE -fstack-protector-strong -fPIC 

upstream writes:

There is a Cdrom wrapper:
"""
class Cdrom : public pkgCdrom {
   public:
      bool FindPackages(std::string const &CD,
     std::vector<std::string> &List,
     std::vector<std::string> &SList,
     std::vector<std::string> &SigList,
     std::vector<std::string> &TransList,
     std::string &InfoDir) {
  std::string const startdir = SafeGetCWD();
  EXPECT_FALSE(startdir.empty());
  bool const result = pkgCdrom::FindPackages(CD, List, SList, SigList,
TransList, InfoDir, NULL, 0);
...
}
"""

and a unittest that calls it:
"""
TEST(CDROMTest,FindPackages)
{
   Cdrom cd;
  std::string InfoDir;
  EXPECT_TRUE(cd.FindPackages(path, Packages, Sources, Signatur, Translation,
InfoDir));
...
  EXPECT_EQ(path + "/.disk/", InfoDir);
}
"""
The actual code for this is apt-pkg/cdrom.cc:
"""
bool pkgCdrom::FindPackages(string CD,
       vector<string> &List,
       vector<string> &SList,
       vector<string> &SigList,
       vector<string> &TransList,
       string &InfoDir, pkgCdromStatus *log,
       unsigned int Depth)
{
...
   if (DirectoryExists(".disk") == true)
   {
      if (InfoDir.empty() == true)
  InfoDir = CD + ".disk/";
   }
...
"""

So I suspect that the optimizer gets confused that InfoDir is a reference or it
gets confused because InfoDir is not used in FindPackages anymore and it
assumes its dead code.

I tried to create a simplified testcase but failed so far. Whats interessting
is that if I add a std::cerr << "debug" line into cdrom.cc lines (or even a "CD
= CD"):
"""
   if (DirectoryExists(".disk") == true)
   {
      if (InfoDir.empty() == true) {
std::cerr << "debug" << std::endl;
  InfoDir = CD + ".disk/";
}
   }
"""
it works (which indicates dead-code elimination to me).