Re: Is this reordering valid?

From:

"James Kanze" <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

29 Oct 2006 10:44:14 -0500

Message-ID:

<1162122851.713377.34010@m7g2000cwm.googlegroups.com>

Joshua Lehrer wrote:

pongba@gmail.com wrote:

1. allocation of raw memory
2. construction of C
3. assignment

struct C{
  C()
  {
    throw std::runtime_exception("...");
  }
};

C* pc = 0;
int main(){
try{
pc = new C();
}
catch(...)
{
if(pc) some_library_IO_func(); // observable behavior
}
}

Yes, the compiler can reorder by doing:

1: fetch memory
2: assign to pc
3: call constructor
4: if exception, clear pc, rethrow

The observable behavior remains the same.

Even if pc is a global variable, and accessed in the constructor
of C?

Also, keep in mind that even if tihs was not a valid reorder,
double checked locking still fails due to CPU reording of
instructions, cache coherency, and other issues.

That's at least part of the key here. The problems go well
beyond what the compiler is allowed to do or not.

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