Index: lib/Target/SystemZ/SystemZSubtarget.h
===================================================================
--- lib/Target/SystemZ/SystemZSubtarget.h
+++ lib/Target/SystemZ/SystemZSubtarget.h
@@ -102,6 +102,9 @@
   // Always enable the early if-conversion pass.
   bool enableEarlyIfConversion() const override { return true; }
 
+  // Enable tracking of subregister liveness in register allocator.
+  bool enableSubRegLiveness() const override;
+
   // Automatically generated by tblgen.
   void ParseSubtargetFeatures(StringRef CPU, StringRef FS);
 
Index: lib/Target/SystemZ/SystemZSubtarget.cpp
===================================================================
--- lib/Target/SystemZ/SystemZSubtarget.cpp
+++ lib/Target/SystemZ/SystemZSubtarget.cpp
@@ -19,6 +19,11 @@
 #define GET_SUBTARGETINFO_CTOR
 #include "SystemZGenSubtargetInfo.inc"
 
+static cl::opt<bool> UseSubRegLiveness(
+    "systemz-subreg-liveness",
+    cl::desc("Enable subregister liveness tracking for SystemZ (experimental)"),
+    cl::Hidden);
+
 // Pin the vtable to this file.
 void SystemZSubtarget::anchor() {}
 
@@ -54,6 +59,11 @@
       TargetTriple(TT), InstrInfo(initializeSubtargetDependencies(CPU, FS)),
       TLInfo(TM, *this), TSInfo(), FrameLowering() {}
 
+
+bool SystemZSubtarget::enableSubRegLiveness() const {
+  return UseSubRegLiveness;
+}
+
 bool SystemZSubtarget::isPC32DBLSymbol(const GlobalValue *GV,
                                        CodeModel::Model CM) const {
   // PC32DBL accesses require the low bit to be clear.  Note that a zero