[ARM][RDA] Allow multiple killed users

[samparker]

Summary

In RDA, check against the already decided dead instructions when looking at users. This allows an instruction to be removed if it has multiple users, but they're all dead.
This means that IT instructions can be considered killed once all the itstate using instructions are dead.

[SjoerdMeijer]

Looks like this needs a rebase?

Are the test changes in D75167?

[samparker]

The idea here is that the because we allow multiple users, it allows us to remove the multi-instruction IT blocks without having to manually delete them.

[samparker]

Rebased!

[SjoerdMeijer]

LGTM

llvm/lib/CodeGen/ReachingDefAnalysis.cpp

   }
   ToRemove.insert(MI);
   return true;
 }
 
 void ReachingDefAnalysis::collectLocalKilledOperands(MachineInstr *MI,
                                                      InstSet &Dead) const {
   Dead.insert(MI);
-  auto IsDead = [this](MachineInstr *Def, int PhysReg) {
+  auto IsDead = [this, &Dead](MachineInstr *Def, int PhysReg) {
     unsigned LiveDefs = 0;
     for (auto &MO : Def->operands()) {
       if (!MO.isReg() || !MO.getReg() || MO.isUse())
         continue;
       if (!MO.isDead())
         ++LiveDefs;
     }
 
     if (LiveDefs > 1)
       return false;
 
     SmallPtrSet<MachineInstr*, 4> Uses;
     getGlobalUses(Def, PhysReg, Uses);
-    return Uses.size() == 1;
+    for (auto *Use : Uses)
+      if (!Dead.count(Use))
+        return false;
+    return true;
   };
 
   for (auto &MO : MI->operands()) {
     if (!MO.isReg() || !MO.getReg() || !MO.isUse() || !MO.isKill())
       continue;
     if (MachineInstr *Def = getReachingLocalMIDef(MI, MO.getReg()))
       if (IsDead(Def, MO.getReg()))
         collectLocalKilledOperands(Def, Dead);

llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp

   }
 
   // Delete the killed instructions only if we don't have any IT blocks that
   // need to be modified because we need to fixup the mask.
   // TODO: Handle cases where IT blocks are modified.
   if (ModifiedITs.empty()) {
     LLVM_DEBUG(dbgs() << "ARM Loops: Will remove iteration count:\n";
                for (auto *MI : Killed)
-                 dbgs() << " - " << *MI;
-               for (auto *MI : DeadITs)
                  dbgs() << " - " << *MI);
     LoLoop.ToRemove.insert(Killed.begin(), Killed.end());
-    LoLoop.ToRemove.insert(DeadITs.begin(), DeadITs.end());
   }
 
   // Collect and remove the users of iteration count.
   SmallPtrSet<MachineInstr*, 4> Ignore = { LoLoop.Start, LoLoop.Dec,
                                            LoLoop.End, LoLoop.InsertPt };
   SmallPtrSet<MachineInstr*, 2> Remove;
   if (RDA->isSafeToRemove(Def, Remove, Ignore))
     LoLoop.ToRemove.insert(Remove.begin(), Remove.end());