[PATCH] D65267: [MachineCopyPropagation] Remove redundant copies after TailDup via machine-cp

[Dmitri Gribenko via Phabricator via llvm-commits]

gribozavr added a comment.

Hi,

Unfortunately, with this patch the following program (that I believe to be valid C++) fails under MSan:

  #include <set>
  #include <string>
  #include <vector>
  
  struct Inner {
    __attribute__((noinline)) int Opaque1(int, char) const { return -1; }
    __attribute__((noinline)) void Opaque2(int, char, int) {}
    std::vector<std::set<const std::string*> > output;
    int next = 1;
  };
  
  struct Outer {
    __attribute__((noinline)) void Enter(const std::string&);
    Inner* inner;
  };
  
  void Outer::Enter(const std::string& str) {
    int state = 0;
    int j = 0;
    for (; j < str.size(); j++) {
      int next_state = inner->Opaque1(state, str[j]);
      if (next_state == -1)
        break;
      else
        state = next_state;
    }
    for (; j < str.size(); j++) {
      inner->Opaque2(state, str[j], inner->next);
      state = inner->next;
      inner->next++;
    }
    inner->output[state].insert(&str);
  }
  
  __attribute__((noinline))
  void fault(const std::set<const std::string*>& x) {
    for (const std::string *xx : x) {
      std::string copy = *xx;
    }
  }
  
  int main() {
    std::string TheString = "a";
    Outer outer;
    outer.inner = new Inner;
    outer.inner->output.resize(2);
    outer.Enter(TheString);
    const std::set<const std::string*>& x = outer.inner->output[1];
    fault(x);
  }



  ==8166==WARNING: MemorySanitizer: use-of-uninitialized-value
      #0 0x7f20d47060e0 in std::__msan::basic_string<char, std::__msan::char_traits<char>, std::__msan::allocator<char> >::__is_long() const include/string:1420:39
      #1 0x7f20d47060e0 in std::__msan::basic_string<char, std::__msan::char_traits<char>, std::__msan::allocator<char> >::basic_string(std::__msan::basic_string<char, std::__msan::char_traits<char>, std::__msan::allocator<char> > const&) include/string:1835:16
      #2 0x7f20d58209ad in fault(std::__msan::set<std::__msan::basic_string<char, std::__msan::char_traits<char>, std::__msan::allocator<char> > const*, std::__msan::less<std::__msan::basic_string<char, std::__msan::char_traits<char>, std::__msan::allocator<char> > const*>, std::__msan::allocator<std::__msan::basic_string<char, std::__msan::char_traits<char>, std::__msan::allocator<char> > const*> > const&) test.cc:38:24
      #3 0x7f20d5820b21 in main test.cc:49:3
      #4 0x7f20d3e6dbbc in __libc_start_main
      #5 0x563387ed2d08 in _start

I'm suspecting a miscompile in `Outer::Enter`. I have verified that the program works without this patch, and fails with this patch. Please let me know if I could help more you to reproduce the problem.

I'm reverting this patch for now, sorry (r371421).


Repository:
  rL LLVM

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  https://reviews.llvm.org/D65267/new/

https://reviews.llvm.org/D65267