[AVR] Don't reserve the frame pointer when spilling registers

This is unnecessary, and required that we added an AVR-specific hack to
the register allocator.

Author: Dylan McKay

lib/CodeGen/RegAllocGreedy.cpp

@@ -135,9 +135,6 @@ class RAGreedy : public MachineFunctionPass,
   std::unique_ptr<Spiller> SpillerInstance;
   PQueue Queue;
   unsigned NextCascade;
-  
-  // AVR specific: have we already unallocated REG_Y after a spill was done?
-  bool IsYReserved;
 
   // Live ranges pass through a number of stages as we try to allocate them.
   // Some of the stages may also create new live ranges:
@@ -425,7 +422,6 @@ class RAGreedy : public MachineFunctionPass,
   void collectHintInfo(unsigned, HintsInfo &);
 
   bool isUnusedCalleeSavedReg(unsigned PhysReg) const;
-  void UndoRegYAllocation();
 };
 } // end anonymous namespace
 
@@ -452,7 +448,7 @@ FunctionPass* llvm::createGreedyRegisterAllocator() {
   return new RAGreedy();
 }
 
-RAGreedy::RAGreedy(): MachineFunctionPass(ID), IsYReserved(false) {
+RAGreedy::RAGreedy(): MachineFunctionPass(ID) {
   initializeLiveDebugVariablesPass(*PassRegistry::getPassRegistry());
   initializeSlotIndexesPass(*PassRegistry::getPassRegistry());
   initializeLiveIntervalsPass(*PassRegistry::getPassRegistry());
@@ -1642,34 +1638,6 @@ RAGreedy::tryInstructionSplit(LiveInterval &VirtReg, AllocationOrder &Order,
   return 0;
 }
 
-// AVR specific code used to handle the reservation of REG_Y if any other
-// register has been spilled.
-// :NOTE: KEEP THIS CONSTANT UPDATED with the backend!
-//   This has to be a define because of linkage problems between libraries.
-#define REG_Y   (51U)
-namespace llvm {
-bool RA_ReserveREG_Y = false;
-bool RA_InSpillerCode = false;
-} // end of namespace llvm
-
-void RAGreedy::UndoRegYAllocation() {
-  // search through all virtual registers where REG_Y has been assigned and
-  // send them back to the work list for reallocation
-  for (unsigned i = 0, e = MRI->getNumVirtRegs(); i != e; ++i) {
-    unsigned VirtReg = TargetRegisterInfo::index2VirtReg(i);
-    if (MRI->reg_nodbg_empty(VirtReg))
-      continue;
-
-    for (MCRegAliasIterator AI(REG_Y, TRI, true); AI.isValid(); ++AI)
-      if (VRM->getPhys(VirtReg) == *AI) {
-        LiveInterval &LI = LIS->getInterval(VirtReg);
-        Matrix->unassign(LI);
-        enqueue(&LI);
-      }
-  }
-}
-#undef REG_Y
-
 
 //===----------------------------------------------------------------------===//
 //                             Local Splitting
@@ -2610,25 +2578,10 @@ unsigned RAGreedy::selectOrSplitImpl(LiveInterval &VirtReg,
     DEBUG(dbgs() << "Do as if this register is in memory\n");
     NewVRegs.push_back(VirtReg.reg);
   } else {
-    RA_InSpillerCode = true;
     NamedRegionTimer T("Spiller", TimerGroupName, TimePassesIsEnabled);
     LiveRangeEdit LRE(&VirtReg, NewVRegs, *MF, *LIS, VRM, this, &DeadRemats);
     spiller().spill(LRE);
     setStage(NewVRegs.begin(), NewVRegs.end(), RS_Done);
-    RA_InSpillerCode = false;
-
-    // AVR specific: If we have reached this point and the backend has notified
-    // it has inserted a spill via ReserveREG_Y, then search for any allocations
-    // of REG_Y in the live intervals and undo them.
-    if (!IsYReserved && RA_ReserveREG_Y) {
-      // do all this work only once
-      IsYReserved = true;
-      // update the reserved register list
-      MRI->freezeReservedRegs(VRM->getMachineFunction());
-      RegClassInfo.runOnMachineFunction(VRM->getMachineFunction());
-      // finally perform the real work
-      UndoRegYAllocation();
-    }
 
     if (VerifyEnabled)
       MF->verify(this, "After spilling");
@@ -2688,8 +2641,5 @@ bool RAGreedy::runOnMachineFunction(MachineFunction &mf) {
   postOptimization();
 
   releaseMemory();
-  
-  IsYReserved = false;
-  RA_ReserveREG_Y = false;
   return true;
 }

lib/Target/AVR/AVRInstrInfo.cpp

@@ -36,13 +36,6 @@
 
 namespace llvm {
 
-// External variable defined inside the register allocator to indicate if
-// we should reserve REG_Y if it is going to be used as the frame pointer.
-extern bool RA_ReserveREG_Y;
-// External variable defined inside the register allocator to indicate if
-// the register allocator is executing code inside the spiller.
-extern bool RA_InSpillerCode;
-
 AVRInstrInfo::AVRInstrInfo()
     : AVRGenInstrInfo(AVR::ADJCALLSTACKDOWN, AVR::ADJCALLSTACKUP), RI() {}
 
@@ -134,10 +127,7 @@ void AVRInstrInfo::storeRegToStackSlot(MachineBasicBlock &MBB,
   MachineFunction &MF = *MBB.getParent();
   AVRMachineFunctionInfo *AFI = MF.getInfo<AVRMachineFunctionInfo>();
 
-  if (RA_InSpillerCode && !AFI->getHasSpills()) {
-    AFI->setHasSpills(true);
-    RA_ReserveREG_Y = true;
-  }
+  AFI->setHasSpills(true);
 
   DebugLoc DL;
   if (MI != MBB.end()) {