[PATCH] SEH exceptions on Win64

[Yaron Keren]

  Nico Reick comment from e-mail thread:

  Unwind emission still suffers from the same problems that the old code
  did. It doesn't handle realigned stacks, and as I summarized in PR16779,
  I believe it's not necessary to change the prologue emission for Win64.
  Currently ilist_node::getPrevNode of MachineInstr is unusable because it
  segfaults on the first node in a list. I raised this issue before on
  llvm-dev and proposed a patch but nobody cared, so you have to do
  something like this:

    static const MachineInstr *getPrevNode(const MachineInstr *MI) {
      if (&*MI->getParent()->instr_begin() == MI)
        return nullptr;
      return MI->getPrevNode();
    }

  Some small nits:

  > +      if (MI->getOperand(1).getImm() != 1 || MI->getOperand(2).getImm() != 0 ||
  > +          MI->getOperand(4).getImm()  != 0)

  Spacing. Also, a comment clarifying that scale/index/segment must be
  unused might be helpful here.

  +  if (MI->getNextNode()) {
  +    const MachineInstr *MI2 = MI->getNextNode();
  +    if (!(MI2->getFlag(MachineInstr::FrameSetup) ||
  +          (MI2->isCFIInstruction() && MI2->getNextNode() &&
  +           MI2->getNextNode()->getFlag(MachineInstr::FrameSetup)))) {
  +      OutStreamer.EmitWin64EHEndProlog();
  +    }
  +  }

  I'd simplify this and just skip over all CFI instructions. If there's
  still a node left, and it lacks the FrameSetup flag, end the prolog.

  > +; WIN64: callq {{__chkstk|___chkstk_ms}}

  This check should really differentiate between MSVC and Mingw. You can
  add additional FileCheck prefixes so the rest does not have to be
  duplicated.

  Overall this needs more tests covering __chkstk, alloca, manual
  over-alignment and various combinations.

http://reviews.llvm.org/D3418