diff --git a/include/llvm/CodeGen/MachineRegisterInfo.h b/include/llvm/CodeGen/MachineRegisterInfo.h
index 5eeaa287473..db9352ff887 100644
--- a/include/llvm/CodeGen/MachineRegisterInfo.h
+++ b/include/llvm/CodeGen/MachineRegisterInfo.h
@@ -215,13 +215,15 @@ public:
   void setRegClass(unsigned Reg, const TargetRegisterClass *RC);
 
   /// constrainRegClass - Constrain the register class of the specified virtual
-  /// register to be a common subclass of RC and the current register class.
-  /// Return the new register class, or NULL if no such class exists.
+  /// register to be a common subclass of RC and the current register class,
+  /// but only if the new class has at least MinNumRegs registers.  Return the
+  /// new register class, or NULL if no such class exists.
   /// This should only be used when the constraint is known to be trivial, like
   /// GR32 -> GR32_NOSP. Beware of increasing register pressure.
   ///
   const TargetRegisterClass *constrainRegClass(unsigned Reg,
-                                               const TargetRegisterClass *RC);
+                                               const TargetRegisterClass *RC,
+                                               unsigned MinNumRegs = 0);
 
   /// recomputeRegClass - Try to find a legal super-class of Reg's register
   /// class that still satisfies the constraints from the instructions using
diff --git a/lib/CodeGen/MachineRegisterInfo.cpp b/lib/CodeGen/MachineRegisterInfo.cpp
index d7953896be1..d513f29e7c1 100644
--- a/lib/CodeGen/MachineRegisterInfo.cpp
+++ b/lib/CodeGen/MachineRegisterInfo.cpp
@@ -49,15 +49,17 @@ MachineRegisterInfo::setRegClass(unsigned Reg, const TargetRegisterClass *RC) {
 
 const TargetRegisterClass *
 MachineRegisterInfo::constrainRegClass(unsigned Reg,
-                                       const TargetRegisterClass *RC) {
+                                       const TargetRegisterClass *RC,
+                                       unsigned MinNumRegs) {
   const TargetRegisterClass *OldRC = getRegClass(Reg);
   if (OldRC == RC)
     return RC;
   const TargetRegisterClass *NewRC = getCommonSubClass(OldRC, RC);
-  if (!NewRC)
+  if (!NewRC || NewRC == OldRC)
+    return NewRC;
+  if (NewRC->getNumRegs() < MinNumRegs)
     return 0;
-  if (NewRC != OldRC)
-    setRegClass(Reg, NewRC);
+  setRegClass(Reg, NewRC);
   return NewRC;
 }