[NFC] Make the describeLoadedValue() hook return machine operand objects

[dstenb]

Summary

This changes the ParamLoadedValue pair which the describeLoadedValue()
hook returns so that MachineOperand objects are returned instead of
pointers.

When describing call site values we may need to describe operands which
are not part of the instruction. One such example is zero-materializing
XORs on x86, which I have implemented support for in a child revision.
Instead of having to return a pointer to an operand stored somewhere
outside the instruction, start returning objects directly instead, as
that simplifies the code.

The MachineOperand class only holds POD members, and on x86-64 it is 32
bytes large. That combined with copy elision means that the overhead of
returning a machine operand object from the hook does not become very
large.

I benchmarked this on a 8-thread i7-8650U machine with 32 GB RAM. The
benchmark consisted of building a clang 8.0 binary configured with:

-DCMAKE_BUILD_TYPE=RelWithDebInfo \
-DLLVM_TARGETS_TO_BUILD=X86 \
-DLLVM_USE_SANITIZER=Address \
-DCMAKE_CXX_FLAGS="-Xclang -femit-debug-entry-values -stdlib=libc++"

The average wall clock time increased by 4 seconds, from 62:05 to
62:09, which is an 0.1% increase.

[djtodoro]

Thanks for this! This looks good.

[NikolaPrica]

LGTM!

[vsk]

Lgtm as well, thanks.

llvm/include/llvm/CodeGen/TargetInstrInfo.h

 class RegScavenger;
 class TargetRegisterClass;
 class TargetRegisterInfo;
 class TargetSchedModel;
 class TargetSubtargetInfo;
 
 template <class T> class SmallVectorImpl;
 
-using ParamLoadedValue = std::pair<const MachineOperand*, DIExpression*>;
+using ParamLoadedValue = std::pair<MachineOperand, DIExpression*>;
 
 //---------------------------------------------------------------------------
 ///
 /// TargetInstrInfo - Interface to description of machine instruction set
 ///
 class TargetInstrInfo : public MCInstrInfo {
 public:
   TargetInstrInfo(unsigned CFSetupOpcode = ~0u, unsigned CFDestroyOpcode = ~0u,

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

for (; I != MBB->rend(); ++I) {
     // all of them from the working list.
     for (auto Reg : Defs)
       ForwardedRegWorklist.erase(Reg);
     if (I->getNumDefs() != 1)
       continue;
     unsigned Reg = Defs[0];
 
     if (auto ParamValue = TII->describeLoadedValue(*I)) {
-      if (ParamValue->first->isImm()) {
-        unsigned Val = ParamValue->first->getImm();
+      if (ParamValue->first.isImm()) {
+        unsigned Val = ParamValue->first.getImm();
         DbgValueLoc DbgLocVal(ParamValue->second, Val);
         finishCallSiteParam(DbgLocVal, Reg);
-      } else if (ParamValue->first->isReg()) {
-        Register RegLoc = ParamValue->first->getReg();
+      } else if (ParamValue->first.isReg()) {
+        Register RegLoc = ParamValue->first.getReg();
         unsigned SP = TLI->getStackPointerRegisterToSaveRestore();
         Register FP = TRI->getFrameRegister(*MF);
         bool IsSPorFP = (RegLoc == SP) || (RegLoc == FP);
         if (TRI->isCalleeSavedPhysReg(RegLoc, *MF) || IsSPorFP) {
           DbgValueLoc DbgLocVal(ParamValue->second,
                                 MachineLocation(RegLoc,
                                                 /*IsIndirect=*/IsSPorFP));
           finishCallSiteParam(DbgLocVal, Reg);

llvm/lib/CodeGen/TargetInstrInfo.cpp

 TargetInstrInfo::describeLoadedValue(const MachineInstr &MI) const {
   const MachineFunction *MF = MI.getMF();
   const MachineOperand *Op = nullptr;
   DIExpression *Expr = DIExpression::get(MF->getFunction().getContext(), {});;
   const MachineOperand *SrcRegOp, *DestRegOp;
 
   if (isCopyInstr(MI, SrcRegOp, DestRegOp)) {
     Op = SrcRegOp;
-    return ParamLoadedValue(Op, Expr);
+    return ParamLoadedValue(*Op, Expr);
   } else if (MI.isMoveImmediate()) {
     Op = &MI.getOperand(1);
-    return ParamLoadedValue(Op, Expr);
+    return ParamLoadedValue(*Op, Expr);
   } else if (MI.hasOneMemOperand()) {
     int64_t Offset;
     const auto &TRI = MF->getSubtarget().getRegisterInfo();
     const auto &TII = MF->getSubtarget().getInstrInfo();
     const MachineOperand *BaseOp;
 
     if (!TII->getMemOperandWithOffset(MI, BaseOp, Offset, TRI))
       return None;
 
     Expr = DIExpression::prepend(Expr, DIExpression::DerefAfter, Offset);
     Op = BaseOp;
-    return ParamLoadedValue(Op, Expr);
+    return ParamLoadedValue(*Op, Expr);
   }
 
   return None;
 }
 
 /// Both DefMI and UseMI must be valid.  By default, call directly to the
 /// itinerary. This may be overriden by the target.
 int TargetInstrInfo::getOperandLatency(const InstrItineraryData *ItinData,

llvm/lib/Target/X86/X86InstrInfo.cpp

if (Op && Op->isReg() && Op->getReg() == Op2.getReg() && Coef > 0) {
           Op2.getReg() != X86::NoRegister) {
         Ops.push_back(dwarf::DW_OP_plus);
       }
     }
 
     DIExpression::appendOffset(Ops, Offset);
     Expr = DIExpression::get(MI.getMF()->getFunction().getContext(), Ops);
 
-    return ParamLoadedValue(Op, Expr);;
+    return ParamLoadedValue(*Op, Expr);;
   }
   default:
     return TargetInstrInfo::describeLoadedValue(MI);
   }
 }
 
 /// This is an architecture-specific helper function of reassociateOps.
 /// Set special operand attributes for new instructions after reassociation.