diff --git a/lib/CodeGen/ProcessImplicitDefs.cpp b/lib/CodeGen/ProcessImplicitDefs.cpp
index b1d8c976022..30a9776f422 100644
--- a/lib/CodeGen/ProcessImplicitDefs.cpp
+++ b/lib/CodeGen/ProcessImplicitDefs.cpp
@@ -50,10 +50,10 @@ ProcessImplicitDefs::CanTurnIntoImplicitDef(MachineInstr *MI,
                                             SmallSet<unsigned, 8> &ImpDefRegs) {
   switch(OpIdx) {
   case 1:
-    return MI->isCopy() && (MI->getOperand(0).getSubReg() == 0 ||
+    return MI->isCopy() && (!MI->getOperand(0).readsReg() ||
                             ImpDefRegs.count(MI->getOperand(0).getReg()));
   case 2:
-    return MI->isSubregToReg() && (MI->getOperand(0).getSubReg() == 0 ||
+    return MI->isSubregToReg() && (!MI->getOperand(0).readsReg() ||
                                   ImpDefRegs.count(MI->getOperand(0).getReg()));
   default: return false;
   }
@@ -66,7 +66,7 @@ static bool isUndefCopy(MachineInstr *MI, unsigned Reg,
     MachineOperand &MO1 = MI->getOperand(1);
     if (MO1.getReg() != Reg)
       return false;
-    if (!MO0.getSubReg() || ImpDefRegs.count(MO0.getReg()))
+    if (!MO0.readsReg() || ImpDefRegs.count(MO0.getReg()))
       return true;
     return false;
   }
@@ -105,7 +105,9 @@ bool ProcessImplicitDefs::runOnMachineFunction(MachineFunction &fn) {
       MachineInstr *MI = &*I;
       ++I;
       if (MI->isImplicitDef()) {
-        if (MI->getOperand(0).getSubReg())
+        ImpDefMIs.push_back(MI);
+        // Is this a sub-register read-modify-write?
+        if (MI->getOperand(0).readsReg())
           continue;
         unsigned Reg = MI->getOperand(0).getReg();
         ImpDefRegs.insert(Reg);
@@ -113,12 +115,11 @@ bool ProcessImplicitDefs::runOnMachineFunction(MachineFunction &fn) {
           for (const unsigned *SS = TRI->getSubRegisters(Reg); *SS; ++SS)
             ImpDefRegs.insert(*SS);
         }
-        ImpDefMIs.push_back(MI);
         continue;
       }
 
       // Eliminate %reg1032:sub<def> = COPY undef.
-      if (MI->isCopy() && MI->getOperand(0).getSubReg()) {
+      if (MI->isCopy() && MI->getOperand(0).readsReg()) {
         MachineOperand &MO = MI->getOperand(1);
         if (MO.isUndef() || ImpDefRegs.count(MO.getReg())) {
           if (MO.isKill()) {
@@ -140,7 +141,7 @@ bool ProcessImplicitDefs::runOnMachineFunction(MachineFunction &fn) {
       bool ChangedToImpDef = false;
       for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
         MachineOperand& MO = MI->getOperand(i);
-        if (!MO.isReg() || (MO.isDef() && !MO.getSubReg()) || MO.isUndef())
+        if (!MO.isReg() || !MO.readsReg())
           continue;
         unsigned Reg = MO.getReg();
         if (!Reg)
@@ -172,10 +173,10 @@ bool ProcessImplicitDefs::runOnMachineFunction(MachineFunction &fn) {
           continue;
         }
         if (MO.isKill() || MI->isRegTiedToDefOperand(i)) {
-          // Make sure other uses of 
+          // Make sure other reads of Reg are also marked <undef>.
           for (unsigned j = i+1; j != e; ++j) {
             MachineOperand &MOJ = MI->getOperand(j);
-            if (MOJ.isReg() && MOJ.isUse() && MOJ.getReg() == Reg)
+            if (MOJ.isReg() && MOJ.getReg() == Reg && MOJ.readsReg())
               MOJ.setIsUndef();
           }
           ImpDefRegs.erase(Reg);
diff --git a/test/CodeGen/ARM/2012-01-24-RegSequenceLiveRange.ll b/test/CodeGen/ARM/2012-01-24-RegSequenceLiveRange.ll
index faf65b60be8..872eca34ad1 100644
--- a/test/CodeGen/ARM/2012-01-24-RegSequenceLiveRange.ll
+++ b/test/CodeGen/ARM/2012-01-24-RegSequenceLiveRange.ll
@@ -1,5 +1,6 @@
 ; RUN: llc < %s -mcpu=cortex-a8 -verify-machineinstrs -verify-coalescing
 ; PR11841
+; PR11829
 target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:64:128-a0:0:64-n32-S64"
 target triple = "armv7-none-linux-eabi"
 
@@ -37,7 +38,30 @@ bb:
   ret void
 }
 
+define arm_aapcs_vfpcc void @foo2() nounwind uwtable {
+entry:
+  br i1 undef, label %for.end, label %cond.end295
+
+cond.end295:                                      ; preds = %entry
+  %shuffle.i39.i.i1035 = shufflevector <2 x i64> undef, <2 x i64> undef, <1 x i32> zeroinitializer
+  %shuffle.i38.i.i1036 = shufflevector <2 x i64> zeroinitializer, <2 x i64> undef, <1 x i32> zeroinitializer
+  %shuffle.i37.i.i1037 = shufflevector <1 x i64> %shuffle.i39.i.i1035, <1 x i64> %shuffle.i38.i.i1036, <2 x i32> <i32 0, i32 1>
+  %0 = bitcast <2 x i64> %shuffle.i37.i.i1037 to <4 x float>
+  %1 = bitcast <4 x float> undef to <2 x i64>
+  %shuffle.i36.i.i = shufflevector <2 x i64> %1, <2 x i64> undef, <1 x i32> zeroinitializer
+  %shuffle.i35.i.i = shufflevector <2 x i64> undef, <2 x i64> undef, <1 x i32> zeroinitializer
+  %shuffle.i34.i.i = shufflevector <1 x i64> %shuffle.i36.i.i, <1 x i64> %shuffle.i35.i.i, <2 x i32> <i32 0, i32 1>
+  %2 = bitcast <2 x i64> %shuffle.i34.i.i to <4 x float>
+  tail call void @llvm.arm.neon.vst1.v4f32(i8* undef, <4 x float> %0, i32 4) nounwind
+  tail call void @llvm.arm.neon.vst1.v4f32(i8* undef, <4 x float> %2, i32 4) nounwind
+  unreachable
+
+for.end:                                          ; preds = %entry
+  ret void
+}
+
 declare arm_aapcs_vfpcc void @bar(i8*, float, float, float)
+declare void @llvm.arm.neon.vst1.v4f32(i8*, <4 x float>, i32) nounwind
 
 !0 = metadata !{metadata !"omnipotent char", metadata !1}
 !1 = metadata !{metadata !"Simple C/C++ TBAA", null}