diff --git a/support/tools/TableGen/FileParser.y b/support/tools/TableGen/FileParser.y
index 38efb83fa86..ff3ec40484c 100644
--- a/support/tools/TableGen/FileParser.y
+++ b/support/tools/TableGen/FileParser.y
@@ -89,14 +89,21 @@ static void setValue(const std::string &ValName,
 
     BitsInit *NewVal = new BitsInit(CurVal->getNumBits());
 
-    for (unsigned i = 0, e = CurVal->getNumBits(); i != e; ++i)
-      NewVal->setBit(i, CurVal->getBit(i));
-
-    // Loop over bits, assigning values as appopriate...
+    // Loop over bits, assigning values as appropriate...
     for (unsigned i = 0, e = BitList->size(); i != e; ++i) {
       unsigned Bit = (*BitList)[i];
+      if (NewVal->getBit(i)) {
+        err() << "Cannot set bit #" << i << " of value '" << ValName
+              << "' more than once!\n";
+        abort();
+      }
       NewVal->setBit(Bit, BInit->getBit(i));
     }
+
+    for (unsigned i = 0, e = CurVal->getNumBits(); i != e; ++i)
+      if (NewVal->getBit(i) == 0)
+        NewVal->setBit(i, CurVal->getBit(i));
+
     V = NewVal;
   }
 
diff --git a/support/tools/TableGen/Record.cpp b/support/tools/TableGen/Record.cpp
index 5931cb8db26..d48d987a85a 100644
--- a/support/tools/TableGen/Record.cpp
+++ b/support/tools/TableGen/Record.cpp
@@ -211,12 +211,14 @@ Init *BitsInit::resolveReferences(Record &R) {
 
   for (unsigned i = 0, e = Bits.size(); i != e; ++i) {
     Init *B;
-    New->setBit(i, getBit(i));
+    Init *CurBit = getBit(i);
+
     do {
-      B = New->getBit(i);
-      New->setBit(i, B->resolveReferences(R));
-      Changed |= B != New->getBit(i);
-    } while (B != New->getBit(i));
+      B = CurBit;
+      CurBit = CurBit->resolveReferences(R);
+      Changed |= B != CurBit;
+    } while (B != CurBit);
+    New->setBit(i, CurBit);
   }
 
   if (Changed)
diff --git a/support/tools/TableGen/Record.h b/support/tools/TableGen/Record.h
index 8c551400a36..4f955b57d93 100644
--- a/support/tools/TableGen/Record.h
+++ b/support/tools/TableGen/Record.h
@@ -245,6 +245,7 @@ public:
   }
   void setBit(unsigned Bit, Init *V) {
     assert(Bit < Bits.size() && "Bit index out of range!");
+    assert(Bits[Bit] == 0 && "Bit already set!");
     Bits[Bit] = V;
   }
 
diff --git a/utils/TableGen/FileParser.y b/utils/TableGen/FileParser.y
index 38efb83fa86..ff3ec40484c 100644
--- a/utils/TableGen/FileParser.y
+++ b/utils/TableGen/FileParser.y
@@ -89,14 +89,21 @@ static void setValue(const std::string &ValName,
 
     BitsInit *NewVal = new BitsInit(CurVal->getNumBits());
 
-    for (unsigned i = 0, e = CurVal->getNumBits(); i != e; ++i)
-      NewVal->setBit(i, CurVal->getBit(i));
-
-    // Loop over bits, assigning values as appopriate...
+    // Loop over bits, assigning values as appropriate...
     for (unsigned i = 0, e = BitList->size(); i != e; ++i) {
       unsigned Bit = (*BitList)[i];
+      if (NewVal->getBit(i)) {
+        err() << "Cannot set bit #" << i << " of value '" << ValName
+              << "' more than once!\n";
+        abort();
+      }
       NewVal->setBit(Bit, BInit->getBit(i));
     }
+
+    for (unsigned i = 0, e = CurVal->getNumBits(); i != e; ++i)
+      if (NewVal->getBit(i) == 0)
+        NewVal->setBit(i, CurVal->getBit(i));
+
     V = NewVal;
   }
 
diff --git a/utils/TableGen/Record.cpp b/utils/TableGen/Record.cpp
index 5931cb8db26..d48d987a85a 100644
--- a/utils/TableGen/Record.cpp
+++ b/utils/TableGen/Record.cpp
@@ -211,12 +211,14 @@ Init *BitsInit::resolveReferences(Record &R) {
 
   for (unsigned i = 0, e = Bits.size(); i != e; ++i) {
     Init *B;
-    New->setBit(i, getBit(i));
+    Init *CurBit = getBit(i);
+
     do {
-      B = New->getBit(i);
-      New->setBit(i, B->resolveReferences(R));
-      Changed |= B != New->getBit(i);
-    } while (B != New->getBit(i));
+      B = CurBit;
+      CurBit = CurBit->resolveReferences(R);
+      Changed |= B != CurBit;
+    } while (B != CurBit);
+    New->setBit(i, CurBit);
   }
 
   if (Changed)
diff --git a/utils/TableGen/Record.h b/utils/TableGen/Record.h
index 8c551400a36..4f955b57d93 100644
--- a/utils/TableGen/Record.h
+++ b/utils/TableGen/Record.h
@@ -245,6 +245,7 @@ public:
   }
   void setBit(unsigned Bit, Init *V) {
     assert(Bit < Bits.size() && "Bit index out of range!");
+    assert(Bits[Bit] == 0 && "Bit already set!");
     Bits[Bit] = V;
   }