[mlir][arith] Support bitcast with index type

Use kInternalStorageBitWidth as the bit width of index type.

Author: jacquesguan

mlir/lib/Dialect/Arith/IR/ArithOps.cpp

@@ -1747,7 +1747,18 @@ bool arith::BitcastOp::areCastCompatible(TypeRange inputs, TypeRange outputs) {
   if (!srcType || !dstType)
     return false;
 
-  return srcType.getIntOrFloatBitWidth() == dstType.getIntOrFloatBitWidth();
+  unsigned srcWidth, dstWidth;
+  if (auto indexTy = dyn_cast<IndexType>(srcType))
+    srcWidth = IndexType::kInternalStorageBitWidth;
+  else
+    srcWidth = srcType.getIntOrFloatBitWidth();
+
+  if (auto indexTy = dyn_cast<IndexType>(dstType))
+    dstWidth = IndexType::kInternalStorageBitWidth;
+  else
+    dstWidth = dstType.getIntOrFloatBitWidth();
+
+  return srcWidth == dstWidth;
 }
 
 OpFoldResult arith::BitcastOp::fold(FoldAdaptor adaptor) {

mlir/test/Dialect/Arith/ops.mlir

@@ -954,6 +954,12 @@ func.func @test_bitcast_scalable_vector1(%arg0 : vector<[8]xf32>) -> vector<[8]x
   return %0 : vector<[8]xi32>
 }
 
+// CHECK-LABEL: test_bitcast_index
+func.func @test_bitcast_index(%arg0 : i64) -> index {
+  %0 = arith.bitcast %arg0 : i64 to index
+  return %0 : index
+}
+
 // CHECK-LABEL: test_cmpi
 func.func @test_cmpi(%arg0 : i64, %arg1 : i64) -> i1 {
   %0 = arith.cmpi ne, %arg0, %arg1 : i64