diff --git a/llvm/lib/CodeGen/RegisterCoalescer.cpp b/llvm/lib/CodeGen/RegisterCoalescer.cpp
index 1b46256baf2b..19e9310d2210 100644
--- a/llvm/lib/CodeGen/RegisterCoalescer.cpp
+++ b/llvm/lib/CodeGen/RegisterCoalescer.cpp
@@ -1241,6 +1241,9 @@ class JoinVals {
     // Value in the other live range that overlaps this def, if any.
     VNInfo *OtherVNI;
 
+    // Is this value an IMPLICIT_DEF?
+    bool IsImplicitDef;
+
     // True when the live range of this value will be pruned because of an
     // overlapping CR_Replace value in the other live range.
     bool Pruned;
@@ -1249,7 +1252,8 @@ class JoinVals {
     bool PrunedComputed;
 
     Val() : Resolution(CR_Keep), WriteLanes(0), ValidLanes(0),
-            RedefVNI(0), OtherVNI(0), Pruned(false), PrunedComputed(false) {}
+            RedefVNI(0), OtherVNI(0), IsImplicitDef(false), Pruned(false),
+            PrunedComputed(false) {}
 
     bool isAnalyzed() const { return WriteLanes != 0; }
   };
@@ -1385,8 +1389,10 @@ JoinVals::analyzeValue(unsigned ValNo, JoinVals &Other) {
     }
 
     // An IMPLICIT_DEF writes undef values.
-    if (DefMI->isImplicitDef())
+    if (DefMI->isImplicitDef()) {
+      V.IsImplicitDef = true;
       V.ValidLanes &= ~V.WriteLanes;
+    }
   }
 
   // Find the value in Other that overlaps VNI->def, if any.
@@ -1724,22 +1730,29 @@ void JoinVals::pruneValues(JoinVals &Other,
     switch (Vals[i].Resolution) {
     case CR_Keep:
       break;
-    case CR_Replace:
+    case CR_Replace: {
       // This value takes precedence over the value in Other.LI.
       LIS->pruneValue(&Other.LI, Def, &EndPoints);
-      // Remove <def,read-undef> flags. This def is now a partial redef.
-      if (!Def.isBlock()) {
+      // Check if we're replacing an IMPLICIT_DEF value. The IMPLICIT_DEF
+      // instructions are only inserted to provide a live-out value for PHI
+      // predecessors, so the instruction should simply go away once its value
+      // has been replaced.
+      Val &OtherV = Other.Vals[Vals[i].OtherVNI->id];
+      bool EraseImpDef = OtherV.IsImplicitDef && OtherV.Resolution == CR_Keep;
+      if (!EraseImpDef && !Def.isBlock()) {
+        // Remove <def,read-undef> flags. This def is now a partial redef.
         for (MIOperands MO(Indexes->getInstructionFromIndex(Def));
              MO.isValid(); ++MO)
           if (MO->isReg() && MO->isDef() && MO->getReg() == LI.reg)
             MO->setIsUndef(false);
-	// This value will reach instructions below, but we need to make sure
-	// the live range also reaches the instruction at Def.
-	EndPoints.push_back(Def);
+        // This value will reach instructions below, but we need to make sure
+        // the live range also reaches the instruction at Def.
+        EndPoints.push_back(Def);
       }
       DEBUG(dbgs() << "\t\tpruned " << PrintReg(Other.LI.reg) << " at " << Def
                    << ": " << Other.LI << '\n');
       break;
+    }
     case CR_Erase:
     case CR_Merge:
       if (isPrunedValue(i, Other)) {
@@ -1762,21 +1775,41 @@ void JoinVals::pruneValues(JoinVals &Other,
 void JoinVals::eraseInstrs(SmallPtrSet<MachineInstr*, 8> &ErasedInstrs,
                            SmallVectorImpl<unsigned> &ShrinkRegs) {
   for (unsigned i = 0, e = LI.getNumValNums(); i != e; ++i) {
-    if (Vals[i].Resolution != CR_Erase)
-      continue;
+    // Get the def location before markUnused() below invalidates it.
     SlotIndex Def = LI.getValNumInfo(i)->def;
-    MachineInstr *MI = Indexes->getInstructionFromIndex(Def);
-    assert(MI && "No instruction to erase");
-    if (MI->isCopy()) {
-      unsigned Reg = MI->getOperand(1).getReg();
-      if (TargetRegisterInfo::isVirtualRegister(Reg) &&
-          Reg != CP.getSrcReg() && Reg != CP.getDstReg())
-        ShrinkRegs.push_back(Reg);
+    switch (Vals[i].Resolution) {
+    case CR_Keep:
+      // If an IMPLICIT_DEF value is pruned, it doesn't serve a purpose any
+      // longer. The IMPLICIT_DEF instructions are only inserted by
+      // PHIElimination to guarantee that all PHI predecessors have a value.
+      if (!Vals[i].IsImplicitDef || !Vals[i].Pruned)
+        break;
+      // Remove value number i from LI. Note that this VNInfo is still present
+      // in NewVNInfo, so it will appear as an unused value number in the final
+      // joined interval.
+      LI.getValNumInfo(i)->markUnused();
+      LI.removeValNo(LI.getValNumInfo(i));
+      DEBUG(dbgs() << "\t\tremoved " << i << '@' << Def << ": " << LI << '\n');
+      // FALL THROUGH.
+
+    case CR_Erase: {
+      MachineInstr *MI = Indexes->getInstructionFromIndex(Def);
+      assert(MI && "No instruction to erase");
+      if (MI->isCopy()) {
+        unsigned Reg = MI->getOperand(1).getReg();
+        if (TargetRegisterInfo::isVirtualRegister(Reg) &&
+            Reg != CP.getSrcReg() && Reg != CP.getDstReg())
+          ShrinkRegs.push_back(Reg);
+      }
+      ErasedInstrs.insert(MI);
+      DEBUG(dbgs() << "\t\terased:\t" << Def << '\t' << *MI);
+      LIS->RemoveMachineInstrFromMaps(MI);
+      MI->eraseFromParent();
+      break;
+    }
+    default:
+      break;
     }
-    ErasedInstrs.insert(MI);
-    DEBUG(dbgs() << "\t\terased:\t" << Def << '\t' << *MI);
-    LIS->RemoveMachineInstrFromMaps(MI);
-    MI->eraseFromParent();
   }
 }
 
diff --git a/llvm/test/CodeGen/X86/crash.ll b/llvm/test/CodeGen/X86/crash.ll
index ae5804195c71..3eb7b37ee67b 100644
--- a/llvm/test/CodeGen/X86/crash.ll
+++ b/llvm/test/CodeGen/X86/crash.ll
@@ -477,3 +477,106 @@ for.inc:                                          ; preds = %for.cond
 }
 
 declare void @fn3(...)
+
+; Check coalescing of IMPLICIT_DEF instructions:
+;
+; %vreg1 = IMPLICIT_DEF
+; %vreg2 = MOV32r0
+;
+; When coalescing %vreg1 and %vreg2, the IMPLICIT_DEF instruction should be
+; erased along with its value number.
+;
+define void @rdar12474033() nounwind ssp {
+bb:
+  br i1 undef, label %bb21, label %bb1
+
+bb1:                                              ; preds = %bb
+  switch i32 undef, label %bb10 [
+    i32 4, label %bb2
+    i32 1, label %bb9
+    i32 5, label %bb3
+    i32 6, label %bb3
+    i32 2, label %bb9
+  ]
+
+bb2:                                              ; preds = %bb1
+  unreachable
+
+bb3:                                              ; preds = %bb1, %bb1
+  br i1 undef, label %bb4, label %bb5
+
+bb4:                                              ; preds = %bb3
+  unreachable
+
+bb5:                                              ; preds = %bb3
+  %tmp = load <4 x float>* undef, align 1
+  %tmp6 = bitcast <4 x float> %tmp to i128
+  %tmp7 = load <4 x float>* undef, align 1
+  %tmp8 = bitcast <4 x float> %tmp7 to i128
+  br label %bb10
+
+bb9:                                              ; preds = %bb1, %bb1
+  unreachable
+
+bb10:                                             ; preds = %bb5, %bb1
+  %tmp11 = phi i128 [ undef, %bb1 ], [ %tmp6, %bb5 ]
+  %tmp12 = phi i128 [ 0, %bb1 ], [ %tmp8, %bb5 ]
+  switch i32 undef, label %bb21 [
+    i32 2, label %bb18
+    i32 3, label %bb13
+    i32 5, label %bb16
+    i32 6, label %bb17
+    i32 1, label %bb18
+  ]
+
+bb13:                                             ; preds = %bb10
+  br i1 undef, label %bb15, label %bb14
+
+bb14:                                             ; preds = %bb13
+  br label %bb21
+
+bb15:                                             ; preds = %bb13
+  unreachable
+
+bb16:                                             ; preds = %bb10
+  unreachable
+
+bb17:                                             ; preds = %bb10
+  unreachable
+
+bb18:                                             ; preds = %bb10, %bb10
+  %tmp19 = bitcast i128 %tmp11 to <4 x float>
+  %tmp20 = bitcast i128 %tmp12 to <4 x float>
+  br label %bb21
+
+bb21:                                             ; preds = %bb18, %bb14, %bb10, %bb
+  %tmp22 = phi <4 x float> [ undef, %bb ], [ undef, %bb10 ], [ undef, %bb14 ], [ %tmp20, %bb18 ]
+  %tmp23 = phi <4 x float> [ undef, %bb ], [ undef, %bb10 ], [ undef, %bb14 ], [ %tmp19, %bb18 ]
+  store <4 x float> %tmp23, <4 x float>* undef, align 16
+  store <4 x float> %tmp22, <4 x float>* undef, align 16
+  switch i32 undef, label %bb29 [
+    i32 5, label %bb27
+    i32 1, label %bb24
+    i32 2, label %bb25
+    i32 14, label %bb28
+    i32 4, label %bb26
+  ]
+
+bb24:                                             ; preds = %bb21
+  unreachable
+
+bb25:                                             ; preds = %bb21
+  br label %bb29
+
+bb26:                                             ; preds = %bb21
+  br label %bb29
+
+bb27:                                             ; preds = %bb21
+  unreachable
+
+bb28:                                             ; preds = %bb21
+  br label %bb29
+
+bb29:                                             ; preds = %bb28, %bb26, %bb25, %bb21
+  unreachable
+}