[ExecutionDepsFix] Don't revisit true dependencies

[loladiro]

Summary

If an instruction has a true dependency, it makes sense for to use that
register for any undef read operands in the same instruction (we'll have
to wait for that register to become available anyway). This logic
was already implemented. However, the code would then still try to
revisit that instruction and break the dependency (and always fail,
since by definition a true dependency has to be live before the
instruction). Avoid revisiting such instructions as a performance
optimization. No functional change.

lib/CodeGen/ExecutionDepsFix.cpp

private:
   void leaveBasicBlock(MachineBasicBlock*);
   bool isBlockDone(MachineBasicBlock *);
   void processBasicBlock(MachineBasicBlock *MBB, bool PrimaryPass);
   void updateSuccessors(MachineBasicBlock *MBB, bool PrimaryPass);
   bool visitInstr(MachineInstr *);
   void processDefs(MachineInstr *, bool breakDependency, bool Kill);
   void visitSoftInstr(MachineInstr*, unsigned mask);
   void visitHardInstr(MachineInstr*, unsigned domain);
-  void pickBestRegisterForUndef(MachineInstr *MI, unsigned OpIdx,
+  bool pickBestRegisterForUndef(MachineInstr *MI, unsigned OpIdx,
                                 unsigned Pref);
   bool shouldBreakDependence(MachineInstr*, unsigned OpIdx, unsigned Pref);
   void processUndefReads(MachineBasicBlock*);
 };
 }
 
 char ExeDepsFix::ID = 0;
 
bool ExeDepsFix::visitInstr(MachineInstr *MI) {
   }
 
   return !DomP.first;
 }
 
 /// \brief Helps avoid false dependencies on undef registers by updating the
 /// machine instructions' undef operand to use a register that the instruction
 /// is truly dependent on, or use a register with clearance higher than Pref.
-void ExeDepsFix::pickBestRegisterForUndef(MachineInstr *MI, unsigned OpIdx,
+/// Returns true if it was able to find a true dependency, thus not requiring
+/// a dependency breaking instruction regardless of clearance.
+bool ExeDepsFix::pickBestRegisterForUndef(MachineInstr *MI, unsigned OpIdx,
                                           unsigned Pref) {
   MachineOperand &MO = MI->getOperand(OpIdx);
   assert(MO.isUndef() && "Expected undef machine operand");
 
   unsigned OriginalReg = MO.getReg();
 
   // Update only undef operands that are mapped to one register.
   if (AliasMap[OriginalReg].size() != 1)
-    return;
+    return false;
 
   // Get the undef operand's register class
   const TargetRegisterClass *OpRC =
       TII->getRegClass(MI->getDesc(), OpIdx, TRI, *MF);
 
   // If the instruction has a true dependency, we can hide the false depdency
   // behind it.
   for (MachineOperand &CurrMO : MI->operands()) {
     if (!CurrMO.isReg() || CurrMO.isDef() || CurrMO.isUndef() ||
         !OpRC->contains(CurrMO.getReg()))
       continue;
     // We found a true dependency - replace the undef register with the true
     // dependency.
     MO.setReg(CurrMO.getReg());
-    return;
+    return true;
   }
 
   // Go over all registers in the register class and find the register with
   // max clearance or clearance higher than Pref.
   unsigned MaxClearance = 0;
   unsigned MaxClearanceReg = OriginalReg;
   ArrayRef<MCPhysReg> Order = RegClassInfo.getOrder(OpRC);
   for (auto Reg : Order) {
     assert(AliasMap[Reg].size() == 1 &&
            "Reg is expected to be mapped to a single index");
     int RCrx = *regIndices(Reg).begin();
     unsigned Clearance = CurInstr - LiveRegs[RCrx].Def;
     if (Clearance <= MaxClearance)
       continue;
     MaxClearance = Clearance;
     MaxClearanceReg = Reg;
 
     if (MaxClearance > Pref)
       break;
   }
 
   // Update the operand if we found a register with better clearance.
   if (MaxClearanceReg != OriginalReg)
     MO.setReg(MaxClearanceReg);
+
+  return false;
 }
 
 /// \brief Return true to if it makes sense to break dependence on a partial def
 /// or undef use.
 bool ExeDepsFix::shouldBreakDependence(MachineInstr *MI, unsigned OpIdx,
                                        unsigned Pref) {
   unsigned reg = MI->getOperand(OpIdx).getReg();
   for (int rx : regIndices(reg)) {
void ExeDepsFix::processDefs(MachineInstr *MI, bool breakDependency,
                              bool Kill) {
   assert(!MI->isDebugValue() && "Won't process debug values");
 
   // Break dependence on undef uses. Do this before updating LiveRegs below.
   unsigned OpNum;
   if (breakDependency) {
     unsigned Pref = TII->getUndefRegClearance(*MI, OpNum, TRI);
     if (Pref) {
-      pickBestRegisterForUndef(MI, OpNum, Pref);
-      if (shouldBreakDependence(MI, OpNum, Pref))
+      bool HadTrueDependency = pickBestRegisterForUndef(MI, OpNum, Pref);
+      // We don't need to bother trying to break a dependency if this
+      // instruction has a true dependency on that register through another
+      // operand - we'll have to wait for it to be available regardless.
+      if (!HadTrueDependency && shouldBreakDependence(MI, OpNum, Pref))
         UndefReads.push_back(std::make_pair(MI, OpNum));
     }
   }
   const MCInstrDesc &MCID = MI->getDesc();
   for (unsigned i = 0,
          e = MI->isVariadic() ? MI->getNumOperands() : MCID.getNumDefs();
          i != e; ++i) {
     MachineOperand &MO = MI->getOperand(i);