[CodeGen] Allocate RegAllocHints map lazily (#102186)

This hint map is not required whenever a new register is added, in fact,
at -O0, it is not used at all. Growing this map is quite expensive, as
SmallVectors are not trivially copyable.

Grow this map only when hints are actually added to avoid multiple grows
and grows when no hints are added at all.

Author: aengelke

llvm/include/llvm/CodeGen/MachineRegisterInfo.h

@@ -801,6 +801,7 @@ class MachineRegisterInfo {
   /// of an earlier hint it will be overwritten.
   void setRegAllocationHint(Register VReg, unsigned Type, Register PrefReg) {
     assert(VReg.isVirtual());
+    RegAllocHints.grow(Register::index2VirtReg(getNumVirtRegs()));
     RegAllocHints[VReg].first  = Type;
     RegAllocHints[VReg].second.clear();
     RegAllocHints[VReg].second.push_back(PrefReg);
@@ -810,6 +811,7 @@ class MachineRegisterInfo {
   /// vector for VReg.
   void addRegAllocationHint(Register VReg, Register PrefReg) {
     assert(VReg.isVirtual());
+    RegAllocHints.grow(Register::index2VirtReg(getNumVirtRegs()));
     RegAllocHints[VReg].second.push_back(PrefReg);
   }
 
@@ -822,14 +824,17 @@ class MachineRegisterInfo {
   void clearSimpleHint(Register VReg) {
     assert (!RegAllocHints[VReg].first &&
             "Expected to clear a non-target hint!");
-    RegAllocHints[VReg].second.clear();
+    if (RegAllocHints.inBounds(VReg))
+      RegAllocHints[VReg].second.clear();
   }
 
   /// getRegAllocationHint - Return the register allocation hint for the
   /// specified virtual register. If there are many hints, this returns the
   /// one with the greatest weight.
   std::pair<unsigned, Register> getRegAllocationHint(Register VReg) const {
     assert(VReg.isVirtual());
+    if (!RegAllocHints.inBounds(VReg))
+      return {0, Register()};
     Register BestHint = (RegAllocHints[VReg.id()].second.size() ?
                          RegAllocHints[VReg.id()].second[0] : Register());
     return {RegAllocHints[VReg.id()].first, BestHint};
@@ -845,10 +850,10 @@ class MachineRegisterInfo {
 
   /// getRegAllocationHints - Return a reference to the vector of all
   /// register allocation hints for VReg.
-  const std::pair<unsigned, SmallVector<Register, 4>> &
+  const std::pair<unsigned, SmallVector<Register, 4>> *
   getRegAllocationHints(Register VReg) const {
     assert(VReg.isVirtual());
-    return RegAllocHints[VReg];
+    return RegAllocHints.inBounds(VReg) ? &RegAllocHints[VReg] : nullptr;
   }
 
   /// markUsesInDebugValueAsUndef - Mark every DBG_VALUE referencing the

llvm/lib/CodeGen/MachineRegisterInfo.cpp

@@ -47,7 +47,6 @@ MachineRegisterInfo::MachineRegisterInfo(MachineFunction *MF)
                                : MF->getSubtarget().enableSubRegLiveness()) {
   unsigned NumRegs = getTargetRegisterInfo()->getNumRegs();
   VRegInfo.reserve(256);
-  RegAllocHints.reserve(256);
   UsedPhysRegMask.resize(NumRegs);
   PhysRegUseDefLists.reset(new MachineOperand*[NumRegs]());
   TheDelegates.clear();
@@ -147,7 +146,6 @@ MachineRegisterInfo::recomputeRegClass(Register Reg) {
 Register MachineRegisterInfo::createIncompleteVirtualRegister(StringRef Name) {
   Register Reg = Register::index2VirtReg(getNumVirtRegs());
   VRegInfo.grow(Reg);
-  RegAllocHints.grow(Reg);
   insertVRegByName(Name, Reg);
   return Reg;
 }

llvm/lib/CodeGen/TargetRegisterInfo.cpp

@@ -421,13 +421,16 @@ bool TargetRegisterInfo::getRegAllocationHints(
     SmallVectorImpl<MCPhysReg> &Hints, const MachineFunction &MF,
     const VirtRegMap *VRM, const LiveRegMatrix *Matrix) const {
   const MachineRegisterInfo &MRI = MF.getRegInfo();
-  const std::pair<unsigned, SmallVector<Register, 4>> &Hints_MRI =
+  const std::pair<unsigned, SmallVector<Register, 4>> *Hints_MRI =
       MRI.getRegAllocationHints(VirtReg);
 
+  if (!Hints_MRI)
+    return false;
+
   SmallSet<Register, 32> HintedRegs;
   // First hint may be a target hint.
-  bool Skip = (Hints_MRI.first != 0);
-  for (auto Reg : Hints_MRI.second) {
+  bool Skip = (Hints_MRI->first != 0);
+  for (auto Reg : Hints_MRI->second) {
     if (Skip) {
       Skip = false;
       continue;

llvm/tools/llvm-reduce/ReducerWorkItem.cpp

@@ -306,9 +306,10 @@ static std::unique_ptr<MachineFunction> cloneMF(MachineFunction *SrcMF,
       DstMRI->setType(NewReg, RegTy);
 
     // Copy register allocation hints.
-    const auto &Hints = SrcMRI->getRegAllocationHints(Reg);
-    for (Register PrefReg : Hints.second)
-      DstMRI->addRegAllocationHint(NewReg, PrefReg);
+    const auto *Hints = SrcMRI->getRegAllocationHints(Reg);
+    if (Hints)
+      for (Register PrefReg : Hints->second)
+        DstMRI->addRegAllocationHint(NewReg, PrefReg);
   }
 
   const TargetSubtargetInfo &STI = DstMF->getSubtarget();
@@ -530,7 +531,8 @@ static uint64_t computeMIRComplexityScoreImpl(const MachineFunction &MF) {
   const MachineRegisterInfo &MRI = MF.getRegInfo();
   for (unsigned I = 0, E = MRI.getNumVirtRegs(); I != E; ++I) {
     Register Reg = Register::index2VirtReg(I);
-    Score += MRI.getRegAllocationHints(Reg).second.size();
+    if (const auto *Hints = MRI.getRegAllocationHints(Reg))
+      Score += Hints->second.size();
   }
 
   for (const MachineBasicBlock &MBB : MF) {

llvm/tools/llvm-reduce/deltas/ReduceVirtualRegisters.cpp

@@ -23,9 +23,9 @@ static void dropRegisterHintsFromFunction(Oracle &O, MachineFunction &MF) {
   for (unsigned I = 0, E = MRI.getNumVirtRegs(); I != E; ++I) {
     Register Reg = Register::index2VirtReg(I);
 
-    const std::pair<unsigned, SmallVector<Register, 4>> &Hints =
+    const std::pair<unsigned, SmallVector<Register, 4>> *Hints =
         MRI.getRegAllocationHints(Reg);
-    if (Hints.second.empty())
+    if (!Hints || Hints->second.empty())
       continue;
 
     if (!O.shouldKeep())