diff --git a/third_party/xla/xla/hlo/ir/hlo_sharding.cc b/third_party/xla/xla/hlo/ir/hlo_sharding.cc
index 2583332f7101273912de412612cf956aa6592a89..3b3d1f516b4c53e28963da012d104bc53e8be026 100644
--- a/third_party/xla/xla/hlo/ir/hlo_sharding.cc
+++ b/third_party/xla/xla/hlo/ir/hlo_sharding.cc
@@ -276,12 +276,12 @@ HloSharding HloSharding::Subgroup(
     absl::InlinedVector<int64_t, 6> transposed_shape = merged_shape;
     std::vector<OpSharding::Type> merged_types;
     static constexpr std::array<OpSharding::Type, OpSharding::Type_ARRAYSIZE>
-        kOrderedTypes = {OpSharding::MAXIMAL, OpSharding::TUPLE,
-                         OpSharding::OTHER, OpSharding::MANUAL,
-                         OpSharding::REPLICATED};
+        kOrderedTypes = {OpSharding::MAXIMAL,    OpSharding::TUPLE,
+                         OpSharding::OTHER,      OpSharding::MANUAL,
+                         OpSharding::REPLICATED, OpSharding::UNKNOWN};
     static_assert(kOrderedTypes[0] == 1 && kOrderedTypes[1] == 2 &&
                   kOrderedTypes[2] == 3 && kOrderedTypes[3] == 4 &&
-                  kOrderedTypes[4] == 0);
+                  kOrderedTypes[4] == 0 && kOrderedTypes[5] == 5);
     for (OpSharding::Type type : kOrderedTypes) {
       auto& dims = type_to_dims[type];
       if (dims.empty()) continue;
@@ -426,6 +426,15 @@ void HloSharding::Print(Printer* printer, bool include_metadata) const {
     printer->Append("}");
     return;
   }
+
+  if (unknown_) {
+    printer->Append("{unknown");
+    print_shard_group();
+    print_metadata();
+    printer->Append("}");
+    return;
+  }
+
   if (maximal_) {
     AppendCat(printer, "{maximal device=",
               static_cast<int64_t>(*tile_assignment_.array().begin()));
@@ -510,6 +519,7 @@ std::map<int64_t, int64_t> HloSharding::UsedDevices(int64_t* count) const {
 std::vector<int64_t> HloSharding::TileIndexForDevice(int64_t device) const {
   CHECK(!maximal_);
   CHECK(!IsManual());
+  CHECK(!IsUnknown());
   CHECK(!IsTuple());
   std::vector<int64_t> ret_index;
   tile_assignment_.Each([&](absl::Span<const int64_t> index, int64_t d) {
@@ -525,6 +535,7 @@ std::vector<int64_t> HloSharding::TileIndexForDevice(int64_t device) const {
 int64_t HloSharding::DeviceForTileIndex(absl::Span<const int64_t> index) const {
   CHECK(!replicated_);
   CHECK(!IsManual());
+  CHECK(!IsUnknown());
   CHECK(!IsTuple());
   if (maximal_) {
     return *tile_assignment_.array().begin();
@@ -545,6 +556,7 @@ std::vector<int64_t> HloSharding::TileOffsetForDevice(const Shape& shape,
                                                       int64_t device) const {
   CHECK(!IsTuple());
   CHECK(!IsManual());
+  CHECK(!IsUnknown());
 
   if (maximal_) {
     return std::vector<int64_t>(shape.dimensions_size(), 0);
@@ -563,6 +575,7 @@ std::vector<int64_t> HloSharding::TileLimitForDevice(const Shape& shape,
                                                      int64_t device) const {
   CHECK(!IsTuple());
   CHECK(!IsManual());
+  CHECK(!IsUnknown());
 
   if (maximal_) {
     return std::vector<int64_t>(shape.dimensions().begin(),
@@ -744,7 +757,7 @@ Status HloSharding::ValidateNonTuple(const Shape& shape,
                        tile_assignment_.iota_->num_elements() == *num_devices;
   }
 
-  if (IsTileMaximal() || IsManual()) {
+  if (IsTileMaximal() || IsManual() || IsUnknown()) {
     return OkStatus();
   }
 
@@ -798,6 +811,8 @@ Status HloSharding::ValidateNonTuple(const Shape& shape,
     return Replicate(metadata).SetShardGroupFromProto(proto);
   } else if (proto.type() == OpSharding::MANUAL) {
     return Manual(metadata).SetShardGroupFromProto(proto);
+  } else if (proto.type() == OpSharding::UNKNOWN) {
+    return Unknown(metadata).SetShardGroupFromProto(proto);
   } else if (proto.tile_assignment_devices().size() == 1) {
     return HloSharding(proto.tile_assignment_devices(0), metadata)
         .SetShardGroupFromProto(proto);
@@ -921,6 +936,9 @@ OpSharding HloSharding::ToProto() const {
   } else if (IsManual()) {
     result.set_type(OpSharding::MANUAL);
     result.clear_tile_assignment_dimensions();
+  } else if (IsUnknown()) {
+    result.set_type(OpSharding::UNKNOWN);
+    result.clear_tile_assignment_dimensions();
   } else {
     result.set_type(OpSharding::OTHER);
     result.set_replicate_on_last_tile_dim(ReplicateOnLastTileDim());
@@ -942,7 +960,7 @@ OpSharding HloSharding::ToProto() const {
 }
 
 Shape HloSharding::TileShape(const Shape& shape) const {
-  if (IsTileMaximal() || IsManual()) {
+  if (IsTileMaximal() || IsManual() || IsUnknown()) {
     return shape;
   }
   Shape result_shape = shape;
@@ -954,7 +972,7 @@ Shape HloSharding::TileShape(const Shape& shape) const {
 }
 
 Shape HloSharding::TileShape(const Shape& shape, int64_t device) const {
-  if (IsTileMaximal() || IsManual()) {
+  if (IsTileMaximal() || IsManual() || IsUnknown()) {
     return shape;
   }
 
@@ -977,6 +995,7 @@ int64_t HloSharding::TotalNumTiles() const {
     return 1;
   }
   CHECK(!IsManual());
+  CHECK(!IsUnknown());
   return Product(absl::Span<const int64_t>(tile_assignment_.dimensions()));
 }
 
@@ -985,6 +1004,7 @@ int64_t HloSharding::NumTiles() const {
     return 1;
   }
   CHECK(!IsManual());
+  CHECK(!IsUnknown());
   return Product(absl::Span<const int64_t>(tile_assignment_.dimensions())
                      .subspan(0, TiledDataRank()));
 }
diff --git a/third_party/xla/xla/hlo/ir/hlo_sharding.h b/third_party/xla/xla/hlo/ir/hlo_sharding.h
index 53fcd4343a9fd42ab55f6e63e88bb7bdf5498a50..e30d2acc1ba3a2c34f1dd8616ce6d24965d39e0b 100644
--- a/third_party/xla/xla/hlo/ir/hlo_sharding.h
+++ b/third_party/xla/xla/hlo/ir/hlo_sharding.h
@@ -45,12 +45,20 @@ class HloSharding {
   // Creates a trivial sharding that replicates a maximal tile across all
   // devices.
   static HloSharding Replicate(absl::Span<const OpMetadata> metadata = {}) {
-    return HloSharding(/*manual=*/false, /*replicated=*/true, metadata);
+    return HloSharding(/*manual=*/false, /*replicated=*/true, /*unknown=*/false,
+                       metadata);
   }
 
   // Creates a sharding that represents the op is manually partitioned.
   static HloSharding Manual(absl::Span<const OpMetadata> metadata = {}) {
-    return HloSharding(/*manual=*/true, /*replicated=*/false, metadata);
+    return HloSharding(/*manual=*/true, /*replicated=*/false, /*unknown=*/false,
+                       metadata);
+  }
+
+  // Creates a sharding that represents the op has a placeholder sharding.
+  static HloSharding Unknown(absl::Span<const OpMetadata> metadata = {}) {
+    return HloSharding(/*manual=*/false, /*replicated=*/false, /*unknown=*/true,
+                       metadata);
   }
 
   // Creates a sharding that emulates device placement; a tile shape equal to
@@ -189,6 +197,15 @@ class HloSharding {
                           [](const HloSharding& s) { return s.IsManual(); });
   }
 
+  // Returns whether the sharding represents a placeholder sharding.
+  bool IsUnknown() const {
+    if (!IsTuple()) {
+      return unknown_;
+    }
+    return absl::c_all_of(tuple_elements_,
+                          [](const HloSharding& s) { return s.IsUnknown(); });
+  }
+
   bool IsShardGroup() const {
     if (!IsTuple()) {
       return shard_group_.shard_group_id != -1 &&
@@ -226,7 +243,9 @@ class HloSharding {
 
   // Returns weather the sharding represents a tiled sharding where the mapping
   // between devices and tiles is represented through 'tile_assignment()'.
-  bool IsTiled() const { return !IsTileMaximal() && !IsManual(); }
+  bool IsTiled() const {
+    return !IsTileMaximal() && !IsManual() && !IsUnknown();
+  }
 
   // Returns if the sharding has partial replication and partial sharding. If
   // true, data is sharded according to other dimensions of tile_assignment(),
@@ -334,7 +353,7 @@ class HloSharding {
 
   bool operator==(const HloSharding& other) const {
     return replicated_ == other.replicated_ && maximal_ == other.maximal_ &&
-           manual_ == other.manual_ &&
+           manual_ == other.manual_ && unknown_ == other.unknown_ &&
            tile_assignment_ == other.tile_assignment_ &&
            tuple_elements_ == other.tuple_elements_ &&
            replicate_on_last_tile_dim_ == other.replicate_on_last_tile_dim_ &&
@@ -349,7 +368,7 @@ class HloSharding {
       return H::combine(std::move(h), sharding.tuple_elements_);
     }
     return H::combine(std::move(h), sharding.replicated_, sharding.manual_,
-                      sharding.tile_assignment_.array(),
+                      sharding.unknown_, sharding.tile_assignment_.array(),
                       sharding.replicate_on_last_tile_dim_,
                       sharding.shard_group_.ToString());
   }
@@ -393,7 +412,7 @@ class HloSharding {
   std::vector<OpMetadata>& metadata() { return metadata_; }
   const std::vector<OpMetadata>& metadata() const { return metadata_; }
 
-  // Returns the replication subgroiup dim, or -1 if it doesn't exist.
+  // Returns the replication subgroup dim, or -1 if it doesn't exist.
   int64_t SubgroupReplicationDim() const {
     auto it = absl::c_find(subgroup_types_, OpSharding::REPLICATED);
     if (it != subgroup_types_.end()) {
@@ -498,13 +517,14 @@ class HloSharding {
   const ShardGroup& GetShardGroup() const { return shard_group_; }
 
  private:
-  explicit HloSharding(bool manual, bool replicated,
+  explicit HloSharding(bool manual, bool replicated, bool unknown,
                        absl::Span<const OpMetadata> metadata)
       : metadata_(metadata.begin(), metadata.end()),
         replicated_(replicated),
         maximal_(replicated),
         tuple_(false),
         manual_(manual),
+        unknown_(unknown),
         replicate_on_last_tile_dim_(false) {}
   // device_id values:
   // -2: magic number to mean unassigned device, used by spatial partitioning
@@ -519,6 +539,7 @@ class HloSharding {
         maximal_(true),
         tuple_(false),
         manual_(false),
+        unknown_(false),
         replicate_on_last_tile_dim_(false) {}
   explicit HloSharding(TileAssignment tile_assignment,
                        bool replicate_on_last_tile_dim,
@@ -529,6 +550,7 @@ class HloSharding {
         maximal_(false),
         tuple_(false),
         manual_(false),
+        unknown_(false),
         replicate_on_last_tile_dim_(replicate_on_last_tile_dim) {}
   explicit HloSharding(TileAssignment tile_assignment,
                        absl::Span<const OpSharding::Type> subgroup_types,
@@ -540,6 +562,7 @@ class HloSharding {
         maximal_(false),
         tuple_(false),
         manual_(false),
+        unknown_(false),
         replicate_on_last_tile_dim_(false) {}
   explicit HloSharding(const std::vector<HloSharding>& tuple_shardings)
       : tuple_elements_(tuple_shardings),
@@ -547,6 +570,7 @@ class HloSharding {
         maximal_(false),
         tuple_(true),
         manual_(false),
+        unknown_(false),
         replicate_on_last_tile_dim_(false) {}
 
   // Test-only constructor for sharding format code coverage. Copies the
@@ -560,6 +584,7 @@ class HloSharding {
         maximal_(other.maximal_),
         tuple_(other.tuple_),
         manual_(other.manual_),
+        unknown_(other.unknown_),
         replicate_on_last_tile_dim_(other.replicate_on_last_tile_dim_) {
     CHECK(tile_assignment_ == other.tile_assignment_)
         << tile_assignment_.ToString() << " v.s. "
@@ -614,6 +639,7 @@ class HloSharding {
   bool maximal_;
   bool tuple_;
   bool manual_;
+  bool unknown_;
   // This flag is to support partial replication and partial sharding. If it is
   // true, tile_assignment_ will have an extra dimension in addition to the data
   // shape rank, and the added last dimension represents the subgroups of
diff --git a/third_party/xla/xla/python/xla_compiler.cc b/third_party/xla/xla/python/xla_compiler.cc
index 7b30a85451995b9874b8f2e0e455dbda29f0fc53..4c18c7ba72b4a5688616d2d113921b0b9e714c70 100644
--- a/third_party/xla/xla/python/xla_compiler.cc
+++ b/third_party/xla/xla/python/xla_compiler.cc
@@ -987,7 +987,8 @@ void BuildXlaCompilerSubmodule(py::module& m) {
       .value("MAXIMAL", OpSharding::MAXIMAL)
       .value("MANUAL", OpSharding::MANUAL)
       .value("TUPLE", OpSharding::TUPLE)
-      .value("OTHER", OpSharding::OTHER);
+      .value("OTHER", OpSharding::OTHER)
+      .value("UNKNOWN", OpSharding::UNKNOWN);
 
   py::enum_<OpSharding::ShardGroupType> op_sharding_shard_group_type(
       m, "OpSharding_ShardGroupType");
@@ -1068,12 +1069,14 @@ void BuildXlaCompilerSubmodule(py::module& m) {
           py::arg("subgroup_types") = absl::Span<const xla::OpSharding::Type>())
       .def_static("manual", [] { return HloSharding::Manual(); })
       .def_static("replicate", [] { return HloSharding::Replicate(); })
+      .def_static("unknown", [] { return HloSharding::Unknown(); })
       .def("__eq__", [](const xla::HloSharding& a,
                         const xla::HloSharding& b) { return a == b; })
       .def("__hash__",
            [](const xla::HloSharding& self) { return absl::HashOf(self); })
       .def("is_replicated", &xla::HloSharding::IsReplicated)
       .def("is_manual", &xla::HloSharding::IsManual)
+      .def("is_unknown", &xla::HloSharding::IsUnknown)
       .def("is_tiled", &xla::HloSharding::IsTiled)
       .def("tile", [](const xla::HloSharding& self,
                       xla::Shape shape) { return self.TileShape(shape); })
diff --git a/third_party/xla/xla/python/xla_extension/__init__.pyi b/third_party/xla/xla/python/xla_extension/__init__.pyi
index d5ec9dadd4849518c2b4b444dd223611e3f7a4ea..6ea7e598f43681fbe043643954a60f68a4ac9a1d 100644
--- a/third_party/xla/xla/python/xla_extension/__init__.pyi
+++ b/third_party/xla/xla/python/xla_extension/__init__.pyi
@@ -308,6 +308,7 @@ class OpSharding_Type(enum.IntEnum):
   TUPLE: int
   OTHER: int
   MANUAL: int
+  UNKNOWN: int
 
 class OpSharding_ShardGroupType(enum.IntEnum):
   AS: int
@@ -346,12 +347,15 @@ class HloSharding:
   def replicate() -> HloSharding: ...
   @staticmethod
   def manual() -> HloSharding: ...
+  @staticmethod
+  def unknown() -> HloSharding: ...
   def __eq__(self, other: HloSharding) -> bool: ...
   def __hash__(self) -> int: ...
   def __repr__(self) -> str: ...
   def tile(self, shape: Shape) -> Shape: ...
   def is_replicated(self) -> bool: ...
   def is_manual(self) -> bool: ...
+  def is_unknown(self) -> bool: ...
   def is_tiled(self) -> bool: ...
   def tuple_elements(self) -> List[HloSharding]: ...
   def num_devices(self) -> int: ...
diff --git a/third_party/xla/xla/service/cpu/tests/cpu_literal_caching_test.cc b/third_party/xla/xla/service/cpu/tests/cpu_literal_caching_test.cc
index cec5492a13cffc073eb0051f2f3cfffedc6f0c66..c010cc7cf79ab8b690cac0dba2740aa1a13e10b4 100644
--- a/third_party/xla/xla/service/cpu/tests/cpu_literal_caching_test.cc
+++ b/third_party/xla/xla/service/cpu/tests/cpu_literal_caching_test.cc
@@ -40,7 +40,7 @@ while_cond {
   arg_cond = f32[2,3,2] parameter(0)
   token0 = token[] after-all()
   infeed = (pred[], token[]) infeed(token0)
-  ROOT unknown = pred[] get-tuple-element((pred[], token[]) infeed), index=0
+  ROOT cond = pred[] get-tuple-element((pred[], token[]) infeed), index=0
 }
 
 ENTRY main {
@@ -89,7 +89,7 @@ while_cond {
   arg_cond = (f32[2,1]{1,0}, f32[1]{0}) parameter(0)
   token0 = token[] after-all()
   infeed = (pred[], token[]) infeed(token0)
-  ROOT unknown = pred[] get-tuple-element((pred[], token[]) infeed), index=0
+  ROOT cond = pred[] get-tuple-element((pred[], token[]) infeed), index=0
 }
 
 ENTRY main {
diff --git a/third_party/xla/xla/service/hlo_lexer.cc b/third_party/xla/xla/service/hlo_lexer.cc
index 3fdf7888599e0597bfc705ae5d958818bbe29c40..0e53e6f844e99f676e9d46bbb14a0cd963c67dea 100644
--- a/third_party/xla/xla/service/hlo_lexer.cc
+++ b/third_party/xla/xla/service/hlo_lexer.cc
@@ -328,6 +328,7 @@ TokKind HloLexer::LexIdentifier() {
   KEYWORD(last_tile_dim_replicate);
   KEYWORD(shard_as);
   KEYWORD(shard_like);
+  KEYWORD(unknown);
 
 #undef KEYWORD
 
@@ -612,6 +613,8 @@ std::string TokKindToString(TokKind kind) {
       return "kw_shard_as";
     case TokKind::kw_shard_like:
       return "kw_shard_like";
+    case TokKind::kw_unknown:
+      return "kw_unknown";
     case TokKind::kw_inf:
       return "kw_inf";
     case TokKind::kNegInf:
diff --git a/third_party/xla/xla/service/hlo_lexer.h b/third_party/xla/xla/service/hlo_lexer.h
index ac498a90868825a62eb17255dbcdaa3821255a1e..031ec1ae295330c7c6f25c6348e7a68ac31885d7 100644
--- a/third_party/xla/xla/service/hlo_lexer.h
+++ b/third_party/xla/xla/service/hlo_lexer.h
@@ -69,6 +69,7 @@ enum class TokKind {
   kw_last_tile_dim_replicate,
   kw_shard_as,
   kw_shard_like,
+  kw_unknown,
   kw_inf,
 
   kNegInf,  // -inf
diff --git a/third_party/xla/xla/service/hlo_parser.cc b/third_party/xla/xla/service/hlo_parser.cc
index 6c44438a6a93eeb456f0bca9675748e481658dc2..faae4f7aa0000c2e2b6bb76b5668cc693724abc6 100644
--- a/third_party/xla/xla/service/hlo_parser.cc
+++ b/third_party/xla/xla/service/hlo_parser.cc
@@ -3223,7 +3223,7 @@ bool HloParserImpl::ParseStatisticsViz(StatisticsViz* statistics_viz) {
   return ParseToken(TokKind::kRbrace, "expects '}' at the end of statistics");
 }
 
-// ::= '{' 'replicated'? 'manual'? 'maximal'? ('device=' int)? shape?
+// ::= '{' 'replicated'? 'manual'? 'maximal'? 'unknown'? ('device=' int)? shape?
 //         ('devices=' ('[' dims ']')* device_list)?
 //         (('shard_like' | 'shard_as') int)* '}'
 //         ('metadata=' metadata)*
@@ -3245,6 +3245,7 @@ bool HloParserImpl::ParseSingleSharding(OpSharding* sharding,
   bool maximal = false;
   bool replicated = false;
   bool manual = false;
+  bool unknown = false;
   bool last_tile_dim_replicate = false;
   bool last_tile_dims = false;
   bool shard_like = false;
@@ -3269,6 +3270,10 @@ bool HloParserImpl::ParseSingleSharding(OpSharding* sharding,
         manual = true;
         lexer_.Lex();
         break;
+      case TokKind::kw_unknown:
+        unknown = true;
+        lexer_.Lex();
+        break;
       case TokKind::kAttributeName: {
         if (lexer_.GetStrVal() == "device") {
           if (lexer_.Lex() != TokKind::kInt) {
@@ -3421,6 +3426,12 @@ bool HloParserImpl::ParseSingleSharding(OpSharding* sharding,
                    "manual shardings should not have any devices assigned");
     }
     sharding->set_type(OpSharding::MANUAL);
+  } else if (unknown) {
+    if (!devices.empty()) {
+      return Error(loc,
+                   "unknown shardings should not have any devices assigned");
+    }
+    sharding->set_type(OpSharding::UNKNOWN);
   } else {
     if (tile_assignment_dimensions.empty()) {
       return Error(
diff --git a/third_party/xla/xla/service/hlo_parser_test.cc b/third_party/xla/xla/service/hlo_parser_test.cc
index 9adf66000c51a9a2487189037fa10dc64c9bc57d..20220bd38034b2398481952a88762acf1737e96e 100644
--- a/third_party/xla/xla/service/hlo_parser_test.cc
+++ b/third_party/xla/xla/service/hlo_parser_test.cc
@@ -3625,6 +3625,13 @@ TEST_F(HloParserTest, ParseShardLike) {
             original);
 }
 
+TEST_F(HloParserTest, ParseUnknownSharding) {
+  const std::string original = "{unknown}";
+  TF_ASSERT_OK_AND_ASSIGN(HloSharding sharding, ParseSharding(original));
+  EXPECT_EQ(sharding.ToString(), original);
+  EXPECT_EQ(HloSharding::Unknown().ToString(), original);
+}
+
 TEST_F(HloParserTest, ParseFrontendAttributes) {
   const std::string original =
       R"({attr_a="test_a",attr_b="b",attr_c="s64",attr_d="a/b"})";
diff --git a/third_party/xla/xla/xla_data.proto b/third_party/xla/xla/xla_data.proto
index 3cd6b87322d4c8633c51d863a22877c23bb5aba0..46f4dba1279444a0b65a105f64228878df225a76 100644
--- a/third_party/xla/xla/xla_data.proto
+++ b/third_party/xla/xla/xla_data.proto
@@ -795,6 +795,9 @@ message OpSharding {
     // This op is manually sharded: the shapes are already partitioned and the
     // partitioner should not change this op.
     MANUAL = 4;
+    // This sharding is a placeholder sharding with lowest precedence, it can be
+    // overwriten by any other shardings.
+    UNKNOWN = 5;
   }
   Type type = 1;
   // The shape of the sharded tile.