MachineCSE: Clear dead-def flag on CSE.

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

Differential Revision: http://reviews.llvm.org/D7395

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228178 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/MachineInstr.h

@@ -1048,6 +1048,9 @@ class MachineInstr : public ilist_node<MachineInstr> {
   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.

lib/CodeGen/MachineCSE.cpp

@@ -580,8 +580,15 @@ bool MachineCSE::ProcessBlock(MachineBasicBlock *MBB) {
     // 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,

lib/CodeGen/MachineInstr.cpp

@@ -1889,6 +1889,14 @@ bool MachineInstr::addRegisterDead(unsigned Reg,
   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)

test/CodeGen/AArch64/arm64-cse.ll

@@ -1,4 +1,4 @@
-; 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