Index: mlir/lib/Dialect/Arith/IR/ArithOps.cpp
===================================================================
--- mlir/lib/Dialect/Arith/IR/ArithOps.cpp
+++ mlir/lib/Dialect/Arith/IR/ArithOps.cpp
@@ -1308,7 +1308,7 @@
   // A little hack because we go through int. Otherwise, the size of the
   // constant might need to change.
   if (auto value = operands[0].dyn_cast_or_null<IntegerAttr>())
-    return IntegerAttr::get(getType(), value.getUInt());
+    return IntegerAttr::get(getType(), value.getValue().getZExtValue());
 
   return {};
 }
Index: mlir/test/Dialect/Arith/canonicalize.mlir
===================================================================
--- mlir/test/Dialect/Arith/canonicalize.mlir
+++ mlir/test/Dialect/Arith/canonicalize.mlir
@@ -317,6 +317,24 @@
   return %idx : index
 }
 
+// CHECK-LABEL: @indexCastFold
+//       CHECK:   %[[res:.*]] = arith.constant -2 : index
+//       CHECK:   return %[[res]]
+func.func @indexCastFold(%arg0: i8) -> index {
+  %c-2 = arith.constant -2 : i8
+  %idx = arith.index_cast %c-2 : i8 to index
+  return %idx : index
+}
+
+// CHECK-LABEL: @indexCastUIFold
+//       CHECK:   %[[res:.*]] = arith.constant 254 : index
+//       CHECK:   return %[[res]]
+func.func @indexCastUIFold(%arg0: i8) -> index {
+  %c-2 = arith.constant -2 : i8
+  %idx = arith.index_castui %c-2 : i8 to index
+  return %idx : index
+}
+
 // CHECK-LABEL: @signExtendConstant
 //       CHECK:   %[[cres:.+]] = arith.constant -2 : i16
 //       CHECK:   return %[[cres]]