Index: lib/IR/ConstantRange.cpp
===================================================================
--- lib/IR/ConstantRange.cpp
+++ lib/IR/ConstantRange.cpp
@@ -567,7 +567,6 @@
     return ConstantRange(DstTySize, /*isFullSet=*/true);
 
   APInt MaxValue = APInt::getLowBitsSet(getBitWidth(), DstTySize);
-  APInt MaxBitValue = APInt::getOneBitSet(getBitWidth(), DstTySize);
 
   APInt LowerDiv(Lower), UpperDiv(Upper);
   ConstantRange Union(DstTySize, /*isFullSet=*/false);
@@ -591,9 +590,10 @@
 
   // Chop off the most significant bits that are past the destination bitwidth.
   if (LowerDiv.uge(MaxValue)) {
-    APInt Div(getBitWidth(), 0);
-    APInt::udivrem(LowerDiv, MaxBitValue, Div, LowerDiv);
-    UpperDiv -= MaxBitValue * Div;
+    // Mask to just the signficant bits and subtract from LowerDiv/UpperDiv.
+    APInt Adjust = LowerDiv & ~MaxValue;
+    LowerDiv -= Adjust;
+    UpperDiv -= Adjust;
   }
 
   if (UpperDiv.ule(MaxValue))
@@ -601,7 +601,8 @@
                          UpperDiv.trunc(DstTySize)).unionWith(Union);
 
   // The truncated value wraps around. Check if we can do better than fullset.
-  UpperDiv -= MaxBitValue;
+  UpperDiv -= MaxValue;
+  UpperDiv -= 1;
   if (UpperDiv.ult(LowerDiv))
     return ConstantRange(LowerDiv.trunc(DstTySize),
                          UpperDiv.trunc(DstTySize)).unionWith(Union);