Directly access a compiler-generated Thread Local Storage guard variable

tls_guardvar.cpp

// Tested with clang-6.0 on Ubuntu 14.04 using:
//   clang --std=c++14 -stdlib=libstdc++ -lstcd++ -fasm-blocks -O3 tls_guardvar.cpp -o tls_guardvar
// I will be genuinely surprised if this works in any other circumstance.

#include <iostream>

struct S {
  int x;
  S() { x = 42; }
};

int main() {
  uint32_t value1;
  uint32_t value2;

  __asm {
    mov eax, fs:[0xfffffffffffffffc]
    mov value1, eax
  }

  static thread_local S s;

  __asm {
    mov eax, fs:[0xfffffffffffffffc]
    mov value2, eax
  }

  std::cout << "value1=" << value1 << ", value2=" << value2 << ", s.x=" << s.x << std::endl;

  return 0;
}