[NFC][Debug] Fix unnecessary deep-copy for vector to save compiling time

[steven.zhang]

Summary

We saw some big compiling time impact after enabling the debug entry value feature for X86 platform(D73534). Compiling time goes from 900s->1600s with our testcase. It is caused by allocating/freeing the memory busily.

using FwdRegWorklist = MapVector<unsigned, SmallVector<FwdRegParamInfo, 2>>;

The value for this map vector is vector, and we miss the reference when access the element. The same happens for `auto CalleesMap = MF->getCallSitesInfo();` which is a DenseMap.

[steven.zhang]

Hi, @djtodoro, do you have any concern on this ? If not, I will land this patch days later.

[djtodoro]

Does this solve the problem?

[steven.zhang]

In D100162#2682340, @djtodoro wrote:

Does this solve the problem?

Yes.

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

static void interpretValues(const MachineInstr *CurMI,
   auto getForwardingRegsDefinedByMI = [&](const MachineInstr &MI,
                                           SmallSetVector<unsigned, 4> &Defs) {
     if (MI.isDebugInstr())
       return;
 
     for (const MachineOperand &MO : MI.operands()) {
       if (MO.isReg() && MO.isDef() &&
           Register::isPhysicalRegister(MO.getReg())) {
-        for (auto FwdReg : ForwardedRegWorklist)
+        for (auto &FwdReg : ForwardedRegWorklist)
           if (TRI.regsOverlap(FwdReg.first, MO.getReg()))
             Defs.insert(FwdReg.first);
       }
     }
   };
 
   // Set of worklist registers that are defined by this instruction.
   SmallSetVector<unsigned, 4> FwdRegDefs;
static void interpretValues(const MachineInstr *CurMI,
   }
 
   // Remove all registers that this instruction defines from the worklist.
   for (auto ParamFwdReg : FwdRegDefs)
     ForwardedRegWorklist.erase(ParamFwdReg);
 
   // Now that we are done handling this instruction, add items from the
   // temporary worklist to the real one.
-  for (auto New : TmpWorklistItems)
+  for (auto &New : TmpWorklistItems)
     addToFwdRegWorklist(ForwardedRegWorklist, New.first, EmptyExpr, New.second);
   TmpWorklistItems.clear();
 }
 
 static bool interpretNextInstr(const MachineInstr *CurMI,
                                FwdRegWorklist &ForwardedRegWorklist,
                                ParamSet &Params) {
   // Skip bundle headers.
static bool interpretNextInstr(const MachineInstr *CurMI,
   return true;
 }
 
 /// Try to interpret values loaded into registers that forward parameters
 /// for \p CallMI. Store parameters with interpreted value into \p Params.
 static void collectCallSiteParameters(const MachineInstr *CallMI,
                                       ParamSet &Params) {
   const MachineFunction *MF = CallMI->getMF();
-  auto CalleesMap = MF->getCallSitesInfo();
+  const auto &CalleesMap = MF->getCallSitesInfo();
   auto CallFwdRegsInfo = CalleesMap.find(CallMI);
 
   // There is no information for the call instruction.
   if (CallFwdRegsInfo == CalleesMap.end())
     return;
 
   const MachineBasicBlock *MBB = CallMI->getParent();
 
   // Skip the call instruction.
   auto I = std::next(CallMI->getReverseIterator());
 
   FwdRegWorklist ForwardedRegWorklist;
 
   const DIExpression *EmptyExpr =
       DIExpression::get(MF->getFunction().getContext(), {});
 
   // Add all the forwarding registers into the ForwardedRegWorklist.
-  for (auto ArgReg : CallFwdRegsInfo->second) {
+  for (auto &ArgReg : CallFwdRegsInfo->second) {
     bool InsertedReg =
         ForwardedRegWorklist.insert({ArgReg.Reg, {{ArgReg.Reg, EmptyExpr}}})
             .second;
     assert(InsertedReg && "Single register used to forward two arguments?");
     (void)InsertedReg;
   }
 
   // Do not emit CSInfo for undef forwarding registers.
if (!interpretNextInstr(&*I, ForwardedRegWorklist, Params))
       return;
   }
 
   // Emit the call site parameter's value as an entry value.
   if (ShouldTryEmitEntryVals) {
     // Create an expression where the register's entry value is used.
     DIExpression *EntryExpr = DIExpression::get(
         MF->getFunction().getContext(), {dwarf::DW_OP_LLVM_entry_value, 1});
-    for (auto RegEntry : ForwardedRegWorklist) {
+    for (auto &RegEntry : ForwardedRegWorklist) {
       MachineLocation MLoc(RegEntry.first);
       finishCallSiteParams(MLoc, EntryExpr, RegEntry.second, Params);
     }
   }
 }
 
 void DwarfDebug::constructCallSiteEntryDIEs(const DISubprogram &SP,
                                             DwarfCompileUnit &CU, DIE &ScopeDIE,