program.fbs

// Copyright (c) Meta Platforms, Inc. and affiliates.
// Copyright 2026 Arm Limited and/or its affiliates.

//
// See README.md before modifying this file.
//

include "scalar_type.fbs";

namespace executorch_flatbuffer;

// Identifier of a valid executor schema.
file_identifier "ET12";
// Extension of written files.
file_extension "pte";

// Table that contains the metadata about how
// to unflatten the flattened input/output from compiler
table ContainerMetadata {
  encoded_inp_str: string;
  encoded_out_str: string;
}

table Null {}

// Contains information relevant to the allocation of non-constant
// buffer data (e.g. from tensors).
// This refers to where the buffer needs to be placed in an existing
// memory and at what offset from its base address.
table AllocationDetails {
  memory_id: uint;  // ID of the memory where this data needs to be placed.

  // Offset in bytes relative to the start of the memory area indicated by
  // memory_id.
  //
  // Originally this field was a single 32-bit uint, but we need 64 bits for
  // larger models. To preserve backwards compatibility, the high bits are
  // managed in a separate 32-bit field. Users should combine the two fields
  // to get the full 64-bit offset.
  memory_offset_low: uint;  // Least significant 32 bits
  memory_offset_high: uint;  // Most significant 32 bits. Defaults to zero.
}

// Indicates the types of shape a Tensor may have, from the point
// of view of their dynamism.
enum TensorShapeDynamism : byte {
  // Static shape. Memory is allocated by the compiler.
  STATIC = 0,
  // Dynamic shape but with an upper bound.
  // Memory is allocated by the compiler.
  DYNAMIC_BOUND = 1,
  // Dynamic shape without upper bound.
  // Memory allocation is handled by the runtime.
  DYNAMIC_UNBOUND = 2,
}

// Indicates where a tensor is stored.
enum TensorDataLocation : byte {
  // Stored in a segment of the PTE file.
  SEGMENT = 0,
  // Stored outside of the PTE file.
  EXTERNAL = 1,
}

// Device type enum indicating where a tensor resides or should be allocated.
// Please keep this in sync with executorch/runtime/core/portable_type/device.h
enum DeviceType : byte {
  CPU = 0,
  CUDA = 1,
}

// Table to put additional information about tensors in that is not applicable
// to the vast majority of tensors in the vast majority of programs.
table ExtraTensorInfo {
  // [Optional] Specifies the SubsegmentOffsets in
  //  program.mutable_data_segments that specifies where the data is located in.
  //  If not present and the data is located in a segment, then the data is in
  //  index zero.
  mutable_data_segments_idx: uint64;

  // [Optional] The unique name of the tensor. e.g. 'mod.linear.weight'
  fully_qualified_name: string;

  // [Optional] Specifies where the tensor's data is stored.
  // - SEGMENT (default): Data is stored in a segment.
  // - EXTERNAL: Data is stored outside of the PTE file. fully_qualified_name
  //   must be non-empty, and is used as a key to find the tensor's external
  //   data. Tensor.data_buffer_idx is ignored.
  location: TensorDataLocation;

  // [Optional] The device type where this tensor resides or should be allocated.
  // Defaults to CPU for backward compatibility with existing PTE files.
  device_type: DeviceType = CPU;

  // [Optional] The device index for multi-device scenarios (e.g., cuda:0, cuda:1).
  // Defaults to 0 (the first device of the given type).
  device_index: byte = 0;
}

table Tensor {
  scalar_type: ScalarType;

  // Offset in scalar_type elements (e.g., multiples of 4 bytes for an int
  // scalar type) from the beginning of the tensor buffer to the beginning of
  // the actual data. Currently, the runtime only supports a value of zero.
  storage_offset: int;

  sizes: [int];

  // Specifies in what order the dimensions are laid out in memory (from outer
  // to inner).
  //
  // For example, given a rank 3 Tensor of size (3, 5, 2). If we name
  // dimensions: [row, column, batch], then a dim_order of:
  // - (2, 0, 1) represents a [batch, row, column] ordering where "column" is
  //   the innermost dimension, then comes "row", and the outermost dimension is
  //   "batch".
  // - (0, 2, 1) represents a [row, batch, column] ordering where "column" is
  //   the innermost dimension, then comes "batch", and the outermost dimension
  //   is "row".
  dim_order: [ubyte];

  // out of scope M1
  requires_grad: bool;

  // Overall, a Tensor is either constant or mutable. At method load time
  //  constant tensors receive a dataptr into the serialized program. Mutable
  //  tensors can either receive a pointer from the hierarchical allocator or a
  //  nullptr if they will receive a data pointer at execution time (inputs
  //  and control flow placeholders can be like this). Mutable tensors may or
  //  may not also have an initial value in the serialized program.
  //
  // In summary:
  //   data_buffer_idx > 0, allocation_info = Null: Tensor is a constant.
  //   data_buffer_idx = 0, allocation_info = Non Null: Tensor is mutable and
  //     will receive a dataptr at method load time.
  //   data_buffer_idx = 0, allocation_info = Null: Tensor is mutable and
  //     will receive a dataptr at input time or during execution.
  //   data_buffer_idx > 0, allocation_info = Non Null: Tensor is mutable and
  //     will receive a dataptr at method load time, and has an initial state.
  //
  // Tensor data is stored inline if program.constant_buffer is null. Otherwise
  //  it is in a segment. If this tensor's allocation_info is null then the
  //  tensor data location is specified by program.constant_segment. If the
  //  allocation_info is non_null then the data is somewhere in
  //  program.mutable_data_segments. If tensor_info is Null, then the data is
  //  in program.mutable_data_segments[0] otherwise if tensor_info is non-null
  //  then the mutable_data_segment index is specified by
  //  tensor_info.mutable_data_segments_index.
  data_buffer_idx: uint;

  // [Optional] preallocation details for non-constants (null otherwise).
  allocation_info: AllocationDetails;

  // May not be needed.
  layout: byte;

  // Determines the type of the tensor's shape, from the point of view of its
  // dynamic or not behavior, and consequently how the allocation of the
  // underlying memory is handled, and also how to interpret the sizes and
  // strides fields.
  // 1. dynamism == STATIC: sizes field represents the static shape of
  //    the tensor.
  // 2. dynamism == DYNAMIC_BOUND: sizes field represents the upper bound shape
  //    of the tensor. Each dimension of the tensor at runtime should never
  //    exceed the corresponding dimension of the upper bound shape.
  //
  // 3. dynamism == DYNAMIC_UNBOUND: the stored sizes field can be ignored since
  //    shape is fully dynamic.
  shape_dynamism: TensorShapeDynamism;

  // [Optional] Additional information about the Tensor that is not applicable
  // to most tensors.
  extra_tensor_info: ExtraTensorInfo;
}

table Int {
  int_val: long;
}

table Bool {
  bool_val: bool;
}

table Double {
  double_val: double;
}

table String {
  string_val: string;
}

table IntList {
  items: [long];
}

table DoubleList {
  items: [double];
}

table BoolList {
  items: [bool];
}

// Unlike primitive lists, tensor lists have mutable members and aliasing behavior when
// elements are added to them. To match this aliasing behavior, the runtime tensor list is
// serialized by serializing its elements into the ExecutionPlan.values array, and then
// serializing their corresponding indices into TensorList.items.
table TensorList {
  items: [int];  // EValue indices.
}

// Similar to TensorList except the indices can also point to None.
table OptionalTensorList {
  items: [int];
}

// Supported values in Executorch kernels, Enums are serialized as ints.
union KernelTypes {
  Null,
  Int,
  Bool,
  Double,
  Tensor,
  String,
  IntList,
  DoubleList,
  BoolList,
  TensorList,
  OptionalTensorList,
}

// Abstraction for program values. A subset of types supported in core pytorch kernels.
table EValue {
  val: KernelTypes;
}

table Operator {
  // Operator registry and lookup is uniquely identified by its name, and overload name.
  // TODO(larryliu): is there a more efficient way to represent this
  name: string;
  overload: string;
}

table KernelCall {
  // Index to the operators table in the program.
  op_index: int;

  // Indexes to the (values) required by the operation (in and out).
  args: [int];
}

// A memory-planned scratch buffer that a delegate declared when it was
// lowered. Described directly rather than as a Tensor value, because the
// delegate receives raw bytes.
table DelegateScratch {
  // Where the memory planner placed this buffer.
  allocation: AllocationDetails;

  // Size in bytes. AllocationDetails carries only a location.
  size: uint64;
}

table DelegateCall {
  // Index to the delegates table in the program.
  delegate_index: int;

  // Indexes to the (values) required by the delegates (in and out).
  args: [int];

  // Scratch buffers the delegate declared at AOT time, in declaration order.
  // Not part of `args`: the runtime resolves them and hands the backend raw
  // byte ranges through BackendExecutionContext, so they are never delegate
  // inputs or outputs. Absent in programs exported before this field existed.
  scratch: [DelegateScratch];
}

table MoveCall {
  // Index into the values table of the evalue we are moving from
  move_from: int;

  // Index into the values table of the evalue we are moving into
  move_to: int;
}

table JumpFalseCall {
  // Index into the values table of boolean that specifies whether or not to jump
  cond_value_index: int;

  // Value to set the executor program counter if the jump occurs
  destination_instruction: int;
}

table FreeCall {
  // Index into values table of the tensor whose underlying data blob is being freed
  value_index: int;
}

union InstructionArguments {
  KernelCall,
  DelegateCall,
  MoveCall,
  JumpFalseCall,
  FreeCall,
}

// Basic unit of execution
table Instruction {
  instr_args: InstructionArguments;
}

table Frame {
  // For storing the frame to print stacktraces
  filename: string;  // Name of the file in which the instruction exists
  lineno: int;       // Line number at which the instruction was called
  name: string;      // Name of the function the instruction was called from
  context: string;   // Source code of the instruction
}

table FrameList {
  // For storing the frames to print stacktraces
  items: [Frame];
}

// Indicates where a piece of data is stored.
enum DataLocation : byte {
  // Stored directly in the flatbuffer.
  INLINE = 0,
  // Stored in a segment.
  SEGMENT = 1,
}

// Indicates where the delegate data is stored
table BackendDelegateDataReference {
  // Indicates which list to index into:
  //     INLINE -> Program.backend_delegate_data
  //     SEGMENT -> Program.segments
  location: DataLocation;

  // The index into the list indicated by the location.
  index: uint;
}

table CompileSpec {
  // One compile spec. There are can be multiple specs for one method
  key: string; // like max_value
  value: [ubyte]; // like 4, or other types based on needs.
}

table BackendDelegate {
  // Used to resolve the delegate backend classes, for example, "TCE0", "TCE1", etc.
  // This string is also used in to_backend.
  id: string;

  // A binary blob (from a subgraph) as an output of preprocessing. Will be
  // provided to the backend code at init time. Can be very large, on the
  // order of 10-100MB.
  processed: BackendDelegateDataReference;

  // The compilation spec for the lowered module's forward function
  // Example: [CompileSpec["max_value", 4]]
  compile_specs: [CompileSpec];
}

// A sequence of blocking instructions to be executed in order. The
// abstraction is not currently leveraged, all current programs are 1 chain.
// We are leaving chains as part of the program definition for future use cases
// around graph level async where different threads will be represented as
// separate chains.
table Chain {
  // Indices of the values that are (non-static) inputs into this Chain.
  inputs: [int];

  // Indices of the values that are outputs out of this Chain.
  outputs: [int];

  // List of instructions to be executed in order.
  instructions: [Instruction];

  // Optional list of frames for each instruction.
  // The backend config must have 'emit_stacktrace' set to true to emit
  stacktrace: [FrameList];
}

table ExecutionPlan {

  // Name of a method on the nn.Module that was traced to create this program.
  name: string;

  // Type meta data for input/output to the execution plan
  container_meta_type: ContainerMetadata;

  // A list of all values used in this execution plan.
  values: [EValue];

  // Indices to the 'Evalues' that are inputs to this execution plan.
  // This list contains only the non-constant tensors (i.e. not part of
  // the saved program).
  inputs: [int];

  // Indices to the 'Evalues' that are outputs of this execution plan.
  // This signals a lifespan that goes beyond the execution.
  outputs: [int];

  // List of Chains of kernels.
  chains: [Chain];

  // Operators used in this execution plan
  operators: [Operator];

  // A list of delegates and each is a special instance of execution, the same level of chains.
  delegates: [BackendDelegate];

  // List of buffer sizes for non_constant memory allocations. (Think neural net activations)
  // A list instead of a single buffer to account for complex memory hierarchies.
  // TODO(jakeszwe, razy): How to reconcile this with the ability for the hierarchical memory allocator
  // to be id based instead of index based.
  // Runtime should use the len(constant_buffer) as the ground truth of the
  // constants memory buffer size, and ignore non_const_buffer_sizes[0].
  non_const_buffer_sizes: [int64];

  // [Optional] Per-buffer device mapping, parallel to non_const_buffer_sizes.
  // Each entry maps a non-constant buffer to the device where it should be
  // allocated. For CPU-only programs, this field is absent and all buffers
  // default to CPU, ensuring zero regression.
  non_const_buffer_device: [NonConstBufferDevice];

}

// Maps a non-constant buffer to the device where it should be allocated.
// When present as part of ExecutionPlan.non_const_buffer_device, each entry
// describes the device placement for the corresponding planned memory buffer.
// For CPU-only programs, this table is absent (all buffers default to CPU).
table NonConstBufferDevice {
  // Index into the non_const_buffer_sizes list.
  buffer_idx: int;

  // The device type for this buffer (CPU, CUDA, etc.).
  device_type: DeviceType = CPU;

  // The device index for multi-device scenarios (e.g., cuda:0, cuda:1).
  device_index: byte = 0;
}

// Constant tensor data stored directly in the flatbuffer.
// NOTE: If you change Buffer or BackendDelegateInlineData fields, update the
// custom packers in exir/_serialize/_flatbuffer_program.py
// (_pack_buffer and _pack_backend_delegate_inline_data);
// they override the flatc-generated packers.
table Buffer {
  // During serialization, this alignment may be rewritten to a larger value.
  // The magic "@executorch-tensor-alignment" comment tells EXIR which lines to
  // patch.
  storage: [ubyte] (force_align: 16);  // @executorch-tensor-alignment
}

// Delegate data stored directly in the flatbuffer. This is a different type
// than Buffer because tensors and delegates can have different alignment
// requirements.
// NOTE: If you change Buffer or BackendDelegateInlineData fields, update the
// custom packers in exir/_serialize/_flatbuffer_program.py
// (_pack_buffer and _pack_backend_delegate_inline_data);
// they override the flatc-generated packers.
table BackendDelegateInlineData {
  // During serialization, this alignment may be rewritten to a larger value.
  // The magic "@executorch-delegate-alignment" comment tells EXIR which lines
  // to patch.
  data: [ubyte] (force_align: 16);  // @executorch-delegate-alignment
}

// Describes a contiguous piece of data that lives outside of the flatbuffer data,
// typically appended afterwards in the file. The "extended header" in the file,
// when present, points to the segment base offset.
table DataSegment {
  // Segment offsets are relative to the segment base offset provided in
  // the extended file header. Segments will typically be aligned in a
  // way to make it possible to use mmap() to load them.
  offset: uint64;

  // The size in bytes of valid data starting at the offset. The segment
  // data may be followed by padding before the segment that follows it,
  // to make it easier to use mmap().
  size: uint64;
}

// Describes data offsets into a particular segment
table SubsegmentOffsets {
  // Index of the segment in Program.segments
  segment_index: uint;

  // Each element is an offset in bytes into the data of the segment pointed to
  // by segment_index. Offsets must be aligned to @executorch-tensor-alignment.
  offsets: [uint64];
}

// Attributes a name to data referenced by Program.segments. Used when data is
// referenced by multiple users, in cases where indices are not guaranteed to
// be consistent across the users.
table NamedData {
  // The unique id of the data blob.
  key: string;

  // Index of the segment in Program.segments.
  segment_index: uint32;
}

table Program {
  // Schema version.
  version: uint;

  // List of ExecutionPlans that make up the program. Each ExecutionPlan corresponds with a
  // different entry point into the model.
  execution_plan: [ExecutionPlan];

  // Tables of constant data, used for constant Values (e.g.data field of weight tensors).
  // Each constant is assigned an index into the table which are each individually aligned.
  // 0 index is reserved to be pointed to by non-constant Tensors.
  // If this field is non-empty, constant_segment.offsets must be empty.
  // DEPRECATED: After D61996249 on 2024-09-05, no new PTE files will use this field.
  constant_buffer: [Buffer];

  // List of delegate data. Pointed to by BackendDelegateDataReference.
  backend_delegate_data: [BackendDelegateInlineData];

  // List of data segments that follow the Program data in this file, sorted by
  // offset. Elements in this schema can refer to these segments by index.
  segments: [DataSegment];

  // Describes the offsets of each constant tensor, relative to the segment
  // offset. If constant_segment.offsets field is non-empty, constant_buffer
  // must be empty. constant_segment.offsets[0] is reserved to be pointed to by
  // non-constant Tensors.
  constant_segment: SubsegmentOffsets;

  // [Optional] Describes the offsets into various segments for each mutable
  // tensor. Only mutable tensors with a meaningful initial state are
  // serialized here (for example weights that will be trained on-device as
  // opposed to just layer activations). Separate from the constant_segment to
  // reduce peak memory usage by letting us read directly from the PTE file
  // into the mutable tensor, as opposed to loading the .pte data into
  // constant memory, copying it over, and then being unable to release the
  // constant segment. No two elements should point to the same segment.
  mutable_data_segments: [SubsegmentOffsets];

  // [Optional] List of blobs keyed by a unique name. Note that multiple
  // 'NamedData' entries could point to the same segment index. Stored in
  // segments attached to the PTE file.
  named_data: [NamedData];
}

root_type Program;