Subzero: Allow delaying Phi lowering until after register allocation.

src/IceRegAlloc.cpp

 //===- subzero/src/IceRegAlloc.cpp - Linear-scan implementation -----------===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements the LinearScan class, which performs the
@@ skipping 87 matching lines @@
 
   // Gather the live ranges of all variables and add them to the
   // Unhandled set.
   const VarList &Vars = Func->getVariables();
   {
     TimerMarker T(TimerStack::TT_initUnhandled, Func);
     Unhandled.reserve(Vars.size());
     for (Variable *Var : Vars) {
       // Explicitly don't consider zero-weight variables, which are
       // meant to be spill slots.
-      if (Var->getWeight() == RegWeight::Zero)
+      if (Var->getWeight() == RegWeight::Zero) {
+        Var->setNeedsStackSlot();
         continue;
+      }
       // Don't bother if the variable has a null live range, which means
       // it was never referenced.
       if (Var->getLiveRange().isEmpty())
         continue;
+      Var->setNeedsStackSlot();
       Var->untrimLiveRange();
       Unhandled.push_back(Var);
       if (Var->hasReg()) {
         Var->setRegNumTmp(Var->getRegNum());
         Var->setLiveRangeInfiniteWeight();
         UnhandledPrecolored.push_back(Var);
       }
     }
     struct CompareRanges {
       bool operator()(const Variable *L, const Variable *R) {
@@ skipping 448 matching lines @@
     Str << "\n";
   }
   Str << "++++++ Inactive:\n";
   for (const Variable *Item : Inactive) {
     dumpLiveRange(Item, Func);
     Str << "\n";
   }
 }
 
 } // end of namespace Ice