IfConversion non-recursive patch

[Nick Lewycky]

Andrew Zhogin wrote:
> Hi all.
>
> I want to propose patch for making IfConversion analyze algorithm
> non-recursive.
>
> Recursive algorithm causes running out of stack space on our system on
> huge functions (with many BasicBlocks).
>
> Could anyone review and apply it?

+    std::stack<MachineBasicBlock *> ParentStack;
+    std::stack<MachineBasicBlock *> ForwardStack;
+    IfConverter &Parent;
+    std::vector<IfConverter::IfcvtToken*> &Tokens;

LLVM has its own ADTs. Please review 
http://llvm.org/docs/ProgrammersManual.html#picking-the-right-data-structure-for-a-task 
and pick some. The most common is to use SmallVector which offers a 
combined "pop_back_val()" method.

+    inline MachineBasicBlock *ExtractFromParent() {
[...]
+        MachineBasicBlock * tmp = ParentStack.top();
[...]
+  if (MachineBasicBlock* extracted = ExtractFromForward()) {

Please be consistent about the placement of the '*' (or '&') versus 
whitespace. The common style in this file is "Type *variable".

+    if (Parent.ValidSimple(FalseBBI, Dups, Prediction.getCompl()) &&
+        Parent.MeetIfcvtSizeLimit(*FalseBBI.BB,
                             FalseBBI.NonPredSize + FalseBBI.ExtraCost,
                             FalseBBI.ExtraCost2, Prediction.getCompl()) &&

Because you adjusted the position of the ( after MeetIfcvtSizeLimit, you 
also need to adjust the spacing on the lines that follow it. You may 
want to consider using clang-format.

Nick