[NFC][Regalloc] Factor out eviction decision from eviction attempt

[mtrofin]

Summary

This splits tryEvict into a const tryFindEvictionCandidate, which
attempts to find a candidate, and the actual eviction (should the former
be successful)

The newly introduced tryFindEvictionCandidate will move subsequently
into the RegAllocEvictionAdvisor.

RFC: https://lists.llvm.org/pipermail/llvm-dev/2021-November/153639.html

llvm/lib/CodeGen/RegAllocGreedy.cpp

void evictInterference(LiveInterval &, MCRegister,
                          SmallVectorImpl<Register> &);
   bool mayRecolorAllInterferences(MCRegister PhysReg, LiveInterval &VirtReg,
                                   SmallLISet &RecoloringCandidates,
                                   const SmallVirtRegSet &FixedRegisters);
 
   MCRegister tryAssign(LiveInterval&, AllocationOrder&,
                      SmallVectorImpl<Register>&,
                      const SmallVirtRegSet&);
+  MCRegister tryFindEvictionCandidate(LiveInterval &, const AllocationOrder &,
+                                      uint8_t, const SmallVirtRegSet &) const;
   MCRegister tryEvict(LiveInterval &, AllocationOrder &,
                     SmallVectorImpl<Register> &, uint8_t,
                     const SmallVirtRegSet &);
   MCRegister tryRegionSplit(LiveInterval &, AllocationOrder &,
                             SmallVectorImpl<Register> &);
   /// Calculate cost of region splitting.
   unsigned calculateRegionSplitCost(LiveInterval &VirtReg,
                                     AllocationOrder &Order,

 bool RAGreedy::isUnusedCalleeSavedReg(MCRegister PhysReg) const {
   MCRegister CSR = RegClassInfo.getLastCalleeSavedAlias(PhysReg);
   if (!CSR)
     return false;
 
   return !Matrix->isPhysRegUsed(PhysReg);
 }
 
-/// tryEvict - Try to evict all interferences for a physreg.
-/// @param  VirtReg Currently unassigned virtual register.
-/// @param  Order   Physregs to try.
-/// @return         Physreg to assign VirtReg, or 0.
-MCRegister RAGreedy::tryEvict(LiveInterval &VirtReg, AllocationOrder &Order,
-                            SmallVectorImpl<Register> &NewVRegs,
-                            uint8_t CostPerUseLimit,
-                            const SmallVirtRegSet &FixedRegisters) {
-  NamedRegionTimer T("evict", "Evict", TimerGroupName, TimerGroupDescription,
-                     TimePassesIsEnabled);
-
+MCRegister RAGreedy::tryFindEvictionCandidate(
+    LiveInterval &VirtReg, const AllocationOrder &Order,
+    uint8_t CostPerUseLimit, const SmallVirtRegSet &FixedRegisters) const {
   // Keep track of the cheapest interference seen so far.
   EvictionCost BestCost;
   BestCost.setMax();
   MCRegister BestPhys;
   unsigned OrderLimit = Order.getOrder().size();
 
   // When we are just looking for a reduced cost per use, don't break any
   // hints, and only evict smaller spill weights.
for (auto I = Order.begin(), E = Order.getOrderLimitEnd(OrderLimit); I != E;
 
     // Best so far.
     BestPhys = PhysReg;
 
     // Stop if the hint can be used.
     if (I.isHint())
       break;
   }
+  return BestPhys;
+}
+
+/// tryEvict - Try to evict all interferences for a physreg.
+/// @param  VirtReg Currently unassigned virtual register.
+/// @param  Order   Physregs to try.
+/// @return         Physreg to assign VirtReg, or 0.
+MCRegister RAGreedy::tryEvict(LiveInterval &VirtReg, AllocationOrder &Order,
+                              SmallVectorImpl<Register> &NewVRegs,
+                              uint8_t CostPerUseLimit,
+                              const SmallVirtRegSet &FixedRegisters) {
+  NamedRegionTimer T("evict", "Evict", TimerGroupName, TimerGroupDescription,
+                     TimePassesIsEnabled);
 
+  MCRegister BestPhys =
+      tryFindEvictionCandidate(VirtReg, Order, CostPerUseLimit, FixedRegisters);
   if (BestPhys.isValid())
     evictInterference(VirtReg, BestPhys, NewVRegs);
   return BestPhys;
 }
 
 //===----------------------------------------------------------------------===//
 //                              Region Splitting
 //===----------------------------------------------------------------------===//