diff --git a/include/llvm/Target/TargetInstrInfo.h b/include/llvm/Target/TargetInstrInfo.h
index 07f614d..590fc1e 100644
--- a/include/llvm/Target/TargetInstrInfo.h
+++ b/include/llvm/Target/TargetInstrInfo.h
@@ -718,6 +718,74 @@
   ///
   virtual void setExecutionDomain(MachineInstr *MI, unsigned Domain) const {}
 
+
+  /// getPartialRegUpdateClearance - Returns the preferred minimum clearance
+  /// before an instruction with an unwanted partial register update.
+  ///
+  /// Some instructions only write part of a register, and implicitly need to
+  /// read the other parts of the register.  This may cause unwanted stalls
+  /// preventing otherwise unrelated instructions from executing in parallel in
+  /// an out-of-order CPU.
+  ///
+  /// For example, the x86 instruction cvtsi2ss writes its result to bits
+  /// [31:0] of the destination xmm register. Bits [127:32] are unaffected, so
+  /// the instruction needs to wait for the old value of the register to become
+  /// available:
+  ///
+  ///   addps %xmm1, %xmm0
+  ///   movaps %xmm0, (%rax)
+  ///   cvtsi2ss %rbx, %xmm0
+  ///
+  /// In the code above, the cvtsi2ss instruction needs to wait for the addps
+  /// instruction before it can issue, even though the high bits of %xmm0
+  /// probably aren't needed.
+  ///
+  /// This hook returns the preferred clearance before MI, measured in
+  /// instructions.  Other defs of MI's operand OpNum are avoided in the last N
+  /// instructions before MI.  It should only return a positive value for
+  /// unwanted dependencies.  If the old bits of the defined register have
+  /// useful values, or if MI is determined to otherwise read the dependency,
+  /// the hook should return 0.
+  ///
+  /// The unwanted dependency may be handled by:
+  ///
+  /// 1. Allocating the same register for an MI def and use.  That makes the
+  ///    unwanted dependency identical to a required dependency.
+  ///
+  /// 2. Allocating a register for the def that has no defs in the previous N
+  ///    instructions.
+  ///
+  /// 3. Calling breakPartialRegDependency() with the same arguments.  This
+  ///    allows the target to insert a dependency breaking instruction.
+  ///
+  virtual unsigned
+  getPartialRegUpdateClearance(const MachineInstr *MI, unsigned OpNum,
+                               const TargetRegisterInfo *TRI) const {
+    // The default implementation returns 0 for no partial register dependency.
+    return 0;
+  }
+
+  /// breakPartialRegDependency - Insert a dependency-breaking instruction
+  /// before MI to eliminate an unwanted dependency on OpNum.
+  ///
+  /// If it wasn't possible to avoid a def in the last N instructions before MI
+  /// (see getPartialRegUpdateClearance), this hook will be called to break the
+  /// unwanted dependency.
+  ///
+  /// On x86, an xorps instruction can be used as a dependency breaker:
+  ///
+  ///   addps %xmm1, %xmm0
+  ///   movaps %xmm0, (%rax)
+  ///   xorps %xmm0, %xmm0
+  ///   cvtsi2ss %rbx, %xmm0
+  ///
+  /// An <imp-kill> operand should be added to MI if an instruction was
+  /// inserted.  This ties the instructions together in the post-ra scheduler.
+  ///
+  virtual void
+  breakPartialRegDependency(MachineBasicBlock::iterator MI, unsigned OpNum,
+                            const TargetRegisterInfo *TRI) const {}
+
 private:
   int CallFrameSetupOpcode, CallFrameDestroyOpcode;
 };
diff --git a/lib/CodeGen/ExecutionDepsFix.cpp b/lib/CodeGen/ExecutionDepsFix.cpp
index d094411..050edce 100644
--- a/lib/CodeGen/ExecutionDepsFix.cpp
+++ b/lib/CodeGen/ExecutionDepsFix.cpp
@@ -471,11 +471,34 @@
     DEBUG(dbgs() << TRI->getName(RC->getRegister(rx)) << ":\t" << CurInstr
                  << '\t' << *MI);
 
+    // How many instructions since rx was last written?
+    unsigned Clearance = CurInstr - LiveRegs[rx].Def;
     LiveRegs[rx].Def = CurInstr;
 
     // Kill off domains redefined by generic instructions.
     if (Kill)
       kill(rx);
+
+    // Verify clearance before partial register updates.
+    unsigned Pref = TII->getPartialRegUpdateClearance(MI, i, TRI);
+    if (!Pref)
+      continue;
+    DEBUG(dbgs() << "Clearance: " << Clearance << ", want " << Pref);
+    if (Pref > Clearance) {
+      DEBUG(dbgs() << ": Break dependency.\n");
+      TII->breakPartialRegDependency(MI, i, TRI);
+      continue;
+    }
+
+    // The current clearance seems OK, but we may be ignoring a def from a
+    // back-edge.
+    if (!SeenUnknownBackEdge || Pref <= unsigned(CurInstr)) {
+      DEBUG(dbgs() << ": OK.\n");
+      continue;
+    }
+
+    // A def from an unprocessed back-edge may make us break this dependency.
+    DEBUG(dbgs() << ": Wait for back-edge to resolve.\n");
   }
 
   ++CurInstr;
@@ -663,6 +686,10 @@
   for (unsigned i = 0, e = Loops.size(); i != e; ++i) {
     MachineBasicBlock *MBB = Loops[i];
     enterBasicBlock(MBB);
+    for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E;
+        ++I)
+      if (!I->isDebugValue())
+        processDefs(I, false);
     leaveBasicBlock(MBB);
   }
 
diff --git a/lib/Target/X86/X86InstrInfo.cpp b/lib/Target/X86/X86InstrInfo.cpp
index d9ffd81..9428fff 100644
--- a/lib/Target/X86/X86InstrInfo.cpp
+++ b/lib/Target/X86/X86InstrInfo.cpp
@@ -2761,6 +2761,10 @@
 ///
 static bool hasPartialRegUpdate(unsigned Opcode) {
   switch (Opcode) {
+  case X86::CVTSI2SSrr:
+  case X86::CVTSI2SS64rr:
+  case X86::CVTSI2SDrr:
+  case X86::CVTSI2SD64rr:
   case X86::CVTSD2SSrr:
   case X86::Int_CVTSD2SSrr:
   case X86::CVTSS2SDrr:
@@ -2789,6 +2793,54 @@
   return false;
 }
 
+/// getPartialRegUpdateClearance - Inform the ExeDepsFix pass how many idle
+/// instructions we would like before a partial register update.
+unsigned X86InstrInfo::
+getPartialRegUpdateClearance(const MachineInstr *MI, unsigned OpNum,
+                             const TargetRegisterInfo *TRI) const {
+  if (OpNum != 0 || !hasPartialRegUpdate(MI->getOpcode()))
+    return 0;
+
+  // If MI is marked as reading Reg, the partial register update is wanted.
+  const MachineOperand &MO = MI->getOperand(0);
+  unsigned Reg = MO.getReg();
+  if (TargetRegisterInfo::isVirtualRegister(Reg)) {
+    if (MO.readsReg() || MI->readsVirtualRegister(Reg))
+      return 0;
+  } else {
+    if (MI->readsRegister(Reg, TRI))
+      return 0;
+  }
+
+  // If any of the preceding 16 instructions are reading Reg, insert a
+  // dependency breaking instruction.  The magic number is based on a few
+  // Nehalem experiments.
+  return 16;
+}
+
+void X86InstrInfo::
+breakPartialRegDependency(MachineBasicBlock::iterator MI, unsigned OpNum,
+                          const TargetRegisterInfo *TRI) const {
+  unsigned Reg = MI->getOperand(OpNum).getReg();
+  if (X86::VR128RegClass.contains(Reg)) {
+    // These instructions are all floating point domain, so xorps is the best
+    // choice.
+    bool HasAVX = TM.getSubtarget<X86Subtarget>().hasAVX();
+    unsigned Opc = HasAVX ? X86::VXORPSrr : X86::XORPSrr;
+    BuildMI(*MI->getParent(), MI, MI->getDebugLoc(), get(Opc), Reg)
+      .addReg(Reg, RegState::Undef).addReg(Reg, RegState::Undef);
+  } else if (X86::VR256RegClass.contains(Reg)) {
+    // Use vxorps to clear the full ymm register.
+    // It wants to read and write the xmm sub-register.
+    unsigned XReg = TRI->getSubReg(Reg, X86::sub_xmm);
+    BuildMI(*MI->getParent(), MI, MI->getDebugLoc(), get(X86::VXORPSrr), XReg)
+      .addReg(XReg, RegState::Undef).addReg(XReg, RegState::Undef)
+      .addReg(Reg, RegState::ImplicitDefine);
+  } else
+    return;
+  MI->addRegisterKilled(Reg, TRI, true);
+}
+
 MachineInstr* X86InstrInfo::foldMemoryOperandImpl(MachineFunction &MF,
                                                   MachineInstr *MI,
                                            const SmallVectorImpl<unsigned> &Ops,
diff --git a/lib/Target/X86/X86InstrInfo.h b/lib/Target/X86/X86InstrInfo.h
index 97009db..ee488d8 100644
--- a/lib/Target/X86/X86InstrInfo.h
+++ b/lib/Target/X86/X86InstrInfo.h
@@ -345,6 +345,11 @@
 
   void setExecutionDomain(MachineInstr *MI, unsigned Domain) const;
 
+  unsigned getPartialRegUpdateClearance(const MachineInstr *MI, unsigned OpNum,
+                                        const TargetRegisterInfo *TRI) const;
+  void breakPartialRegDependency(MachineBasicBlock::iterator MI, unsigned OpNum,
+                                 const TargetRegisterInfo *TRI) const;
+
   MachineInstr* foldMemoryOperandImpl(MachineFunction &MF,
                                       MachineInstr* MI,
                                       unsigned OpNum,
diff --git a/test/CodeGen/X86/sse-domains.ll b/test/CodeGen/X86/sse-domains.ll
index d26d322..3b66f4f 100644
--- a/test/CodeGen/X86/sse-domains.ll
+++ b/test/CodeGen/X86/sse-domains.ll
@@ -43,3 +43,44 @@
 while.end:
   ret void
 }
+
+; CHECK: f2
+;
+; This loop contains two cvtsi2ss instructions that update the same xmm
+; register.  Verify that the execution dependency fix pass breaks those
+; dependencies by inserting xorps instructions.
+;
+; If the register allocator chooses different registers for the two cvtsi2ss
+; instructions, they are still dependent on themselves.
+; CHECK: xorps [[XMM1:%xmm[0-9]+]]
+; CHECK: , [[XMM1]]
+; CHECK: cvtsi2ss %{{.*}}, [[XMM1]]
+; CHECK: xorps [[XMM2:%xmm[0-9]+]]
+; CHECK: , [[XMM2]]
+; CHECK: cvtsi2ss %{{.*}}, [[XMM2]]
+;
+define float @f2(i32 %m) nounwind uwtable readnone ssp {
+entry:
+  %tobool3 = icmp eq i32 %m, 0
+  br i1 %tobool3, label %for.end, label %for.body
+
+for.body:                                         ; preds = %entry, %for.body
+  %m.addr.07 = phi i32 [ %dec, %for.body ], [ %m, %entry ]
+  %s1.06 = phi float [ %add, %for.body ], [ 0.000000e+00, %entry ]
+  %s2.05 = phi float [ %add2, %for.body ], [ 0.000000e+00, %entry ]
+  %n.04 = phi i32 [ %inc, %for.body ], [ 1, %entry ]
+  %conv = sitofp i32 %n.04 to float
+  %add = fadd float %s1.06, %conv
+  %conv1 = sitofp i32 %m.addr.07 to float
+  %add2 = fadd float %s2.05, %conv1
+  %inc = add nsw i32 %n.04, 1
+  %dec = add nsw i32 %m.addr.07, -1
+  %tobool = icmp eq i32 %dec, 0
+  br i1 %tobool, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body, %entry
+  %s1.0.lcssa = phi float [ 0.000000e+00, %entry ], [ %add, %for.body ]
+  %s2.0.lcssa = phi float [ 0.000000e+00, %entry ], [ %add2, %for.body ]
+  %sub = fsub float %s1.0.lcssa, %s2.0.lcssa
+  ret float %sub
+}