[Statepoint Lowering] Add an option to allow use gc values in regs for landing pad

[skatkov]

Summary

Default value is not changed, so it is NFC actually.

The option allows to use gc values on registers in landing pads.

[dantrushin]

LGTM.
Though, I don't see much point in having it :)

[reames]

LGTM as a testing aid. Should be removed once invoke support is stabilized and fully enabled.

llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp

 STATISTIC(NumOfStatepoints, "Number of statepoint nodes encountered");
 STATISTIC(StatepointMaxSlotsRequired,
           "Maximum number of stack slots required for a singe statepoint");
 
 cl::opt<bool> UseRegistersForDeoptValues(
     "use-registers-for-deopt-values", cl::Hidden, cl::init(false),
     cl::desc("Allow using registers for non pointer deopt args"));
 
+cl::opt<bool> UseRegistersForGCPointersInLandingPad(
+    "use-registers-for-gc-values-in-landing-pad", cl::Hidden, cl::init(false),
+    cl::desc("Allow using registers for gc pointer in landing pad"));
+
 cl::opt<unsigned> MaxRegistersForGCPointers(
     "max-registers-for-gc-values", cl::Hidden, cl::init(0),
     cl::desc("Max number of VRegs allowed to pass GC pointer meta args in"));
 
 cl::opt<bool> AlwaysSpillBase("statepoint-always-spill-base", cl::Hidden,
                               cl::init(true),
                               cl::desc("Force spilling of base GC pointers"));
 
const bool LiveInDeopt =
     SI.StatepointFlags & (uint64_t)StatepointFlags::DeoptLiveIn;
 
   // Decide which deriver pointers will go on VRegs
   unsigned MaxVRegPtrs = MaxRegistersForGCPointers.getValue();
 
   // Pointers used on exceptional path of invoke statepoint.
   // We cannot assing them to VRegs.
   SmallSet<SDValue, 8> LPadPointers;
+  if (!UseRegistersForGCPointersInLandingPad)
     if (auto *StInvoke = dyn_cast_or_null<InvokeInst>(SI.StatepointInstr)) {
       LandingPadInst *LPI = StInvoke->getLandingPadInst();
       for (auto *Relocate : SI.GCRelocates)
         if (Relocate->getOperand(0) == LPI) {
           LPadPointers.insert(Builder.getValue(Relocate->getBasePtr()));
           LPadPointers.insert(Builder.getValue(Relocate->getDerivedPtr()));
         }
     }
 
   LLVM_DEBUG(dbgs() << "Deciding how to lower GC Pointers:\n");
 
   // List of unique lowered GC Pointer values.
   SmallSetVector<SDValue, 16> LoweredGCPtrs;
   // Map lowered GC Pointer value to the index in above vector
   DenseMap<SDValue, unsigned> GCPtrIndexMap;