Why g++ with -O3 segfault with the following LLVM library code

Question

So I want to use the llvm::Twine string chunk class.

I have the following sample:

#include <llvm/ADT/Twine.h>
#include <iostream>

int main()
{
  llvm::Twine twine1 = llvm::Twine("aaaa") + "bbbb" + "cccc" + "dddd";
  llvm::Twine twine2 = llvm::Twine(twine1) + "dddd" + "eeee";
  std::cout << twine1.str() << std::endl;
  std::cout << twine2.str() << std::endl;

  return 0;
}

It runs with clang++ with -O3 and g++ with -O0 but segfault with g++ with -O3. I tried this code parts different versions of clang library from 3.4-3.9 and tried with g++ 4.8.4, g++ 4.8.5 and mingw-5.3.0.

You need the llvm library and link the code with -lLLVMSupport -lLLVMCore and the others from llvm-config --ldflags

score 5 · Accepted Answer · answered Oct 16 '16 at 17:13

From Twine documentation:

A Twine is not intended for use directly and should not be stored, its implementation relies on the ability to store pointers to temporary stack objects which may be deallocated at the end of a statement. Twines should only be used accepted as const references in arguments, when an API wishes to accept possibly-concatenated strings.

In the other words, Twine object doesn't own its parts, so they are destroyed at the end of statement.

A correct usage would be:

#include <llvm/ADT/Twine.h>
#include <iostream>

void bar(const llvm::Twine& twine1, const llvm::Twine2& twine2){
    std::cout << twine1.str() << std::endl;
    std::cout << twine2.str() << std::endl;
}

void foo(const llvm::Twine& twine1){
    bar(twine1, twine1 + "dddd" + "eeee");
}

int main()
{
  foo(llvm::Twine("aaaa") + "bbbb" + "cccc" + "dddd");

  return 0;
}

score -1 · Answer 2 · edited May 23 '17 at 12:33

-1

It looks like you are attempting to compile some code with gcc, and link it with libraries that were built with a different compiler; namely llvm.

Different compilers will often use different internal ABIs. Generally, code compiled by one C++ compiler cannot be linked with code that was built a different compiler, unless there's an explicit ABI compatibility guarantee. TMK, there is none, between gcc and llvm. Even though the actual link might succeed, the results will not have a defined behavior.

In fact, code built by one version of gcc will often not be linkable with code built with a different version of gcc (many versions of gcc do have certain ABI compatibility guarantees, but not with every other version of gcc).

edited May 23 '17 at 12:33

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answered Oct 16 '16 at 14:29

Sam Varshavchik

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I compiled the llvm library with gcc. – Industrial-antidepressant Oct 16 '16 at 14:31
Did you compile every library you linked with, with the same version of gcc? – Sam Varshavchik Oct 16 '16 at 14:32
Yes, but this simple sample only use llvm – Industrial-antidepressant Oct 16 '16 at 14:35
You stated something different in your question: you stated that multiple libraries need to be linked with. In any case, the next step would be to verify that llvm libraries are explicitly documented as being compilable with gcc. Since they are a part of the llvm infrastructure and stack, unless its stated somewhere that they can be compiled and used with gcc, I would not assume that to be true. – Sam Varshavchik Oct 16 '16 at 14:38
Thaks, I will double check these things. The other libraries is pthread, -ldl, libterm and some other generic one. – Industrial-antidepressant Oct 16 '16 at 14:40
Those are C libraries, and they would not be an issue. – Sam Varshavchik Oct 16 '16 at 14:42

Why g++ with -O3 segfault with the following LLVM library code

2 Answers2