MachineCSE: Clear dead-def flag on CSE.

[MatzeB]

Summary

In case CSE reuses a previoulsy unused register the dead-def flag has to
be cleared on the def operand, as exposed by the arm64-cse.ll test.

This fixes PR22439 and the corresponding rdar://19694987

[hfinkel]

LGTM.

include/llvm/CodeGen/MachineInstr.h

public:
 
   /// addRegisterDead - We have determined MI defined a register without a use.
   /// Look for the operand that defines it and mark it as IsDead. If
   /// AddIfNotFound is true, add a implicit operand if it's not found. Returns
   /// true if the operand exists / is added.
   bool addRegisterDead(unsigned Reg, const TargetRegisterInfo *RegInfo,
                        bool AddIfNotFound = false);
 
+  /// Clear all dead flags on operands defining register @p Reg.
+  void clearRegisterDeads(unsigned Reg);
+
   /// Mark all subregister defs of register @p Reg with the undef flag.
   /// This function is used when we determined to have a subregister def in an
   /// otherwise undefined super register.
   void addRegisterDefReadUndef(unsigned Reg);
 
   /// addRegisterDefined - We have determined MI defines a register. Make sure
   /// there is an operand defining Reg.
   void addRegisterDefined(unsigned Reg,

lib/CodeGen/MachineCSE.cpp

for (unsigned i = 0, e = MI->getNumOperands(); NumDefs && i != e; ++i) {
 
       CSEPairs.push_back(std::make_pair(OldReg, NewReg));
       --NumDefs;
     }
 
     // Actually perform the elimination.
     if (DoCSE) {
       for (unsigned i = 0, e = CSEPairs.size(); i != e; ++i) {
-        MRI->replaceRegWith(CSEPairs[i].first, CSEPairs[i].second);
-        MRI->clearKillFlags(CSEPairs[i].second);
+        unsigned OldReg = CSEPairs[i].first;
+        unsigned NewReg = CSEPairs[i].second;
+        // OldReg may have been unused but is used now, clear the Dead flag
+        MachineInstr *Def = MRI->getUniqueVRegDef(NewReg);
+        assert(Def != nullptr && "CSEd register has no unique definition?");
+        Def->clearRegisterDeads(NewReg);
+        // Replace with NewReg and clear kill flags which may be wrong now.
+        MRI->replaceRegWith(OldReg, NewReg);
+        MRI->clearKillFlags(NewReg);
       }
 
       // Go through implicit defs of CSMI and MI, if a def is not dead at MI,
       // we should make sure it is not dead at CSMI.
       for (unsigned i = 0, e = ImplicitDefsToUpdate.size(); i != e; ++i)
         CSMI->getOperand(ImplicitDefsToUpdate[i]).setIsDead(false);
 
       // Go through implicit defs of CSMI and MI, and clear the kill flags on

lib/CodeGen/MachineInstr.cpp

bool MachineInstr::addRegisterDead(unsigned Reg,
   addOperand(MachineOperand::CreateReg(Reg,
                                        true  /*IsDef*/,
                                        true  /*IsImp*/,
                                        false /*IsKill*/,
                                        true  /*IsDead*/));
   return true;
 }
 
+void MachineInstr::clearRegisterDeads(unsigned Reg) {
+  for (MachineOperand &MO : operands()) {
+    if (!MO.isReg() || !MO.isDef() || MO.getReg() != Reg)
+      continue;
+    MO.setIsDead(false);
+  }
+}
+
 void MachineInstr::addRegisterDefReadUndef(unsigned Reg) {
   for (MachineOperand &MO : operands()) {
     if (!MO.isReg() || !MO.isDef() || MO.getReg() != Reg || MO.getSubReg() == 0)
       continue;
     MO.setIsUndef();
   }
 }
 

test/CodeGen/AArch64/arm64-cse.ll

-; RUN: llc -O3 < %s -aarch64-atomic-cfg-tidy=0 -aarch64-gep-opt=false | FileCheck %s
+; RUN: llc -O3 < %s -aarch64-atomic-cfg-tidy=0 -aarch64-gep-opt=false -verify-machineinstrs | FileCheck %s
 target triple = "arm64-apple-ios"
 
 ; rdar://12462006
 ; CSE between "icmp reg reg" and "sub reg reg".
 ; Both can be in the same basic block or in different basic blocks.
 define i8* @t1(i8* %base, i32* nocapture %offset, i32 %size) nounwind {
 entry:
 ; CHECK-LABEL: t1: