c10/core/SymInt.h

#pragma once

#include <c10/macros/Macros.h>
#include <c10/util/Exception.h>

namespace c10 {

class SymbolicIntNode;

// `SymInt` is a C++ wrapper class around int64_t data_ which  and is used to
// represent concrete dimension values.
//
// `SymInt` is also a data type in Pytorch that can be used in function schemas
// to enable tracing.
//
// `SymInt` is introduced to enable tracing arithmetic
// operations on symbolic integers (e.g. sizes). Tracing symbolic sizes will
// allow LTC and AOTAutograd representing dynamic shapes in expression graphs
// faithfully without baking in concrete dimension values.
//
// To trace the operations, SymInt will overload arithmetic operators (e.g. +,
// -, *) and will provide overloads taking SymInt for commonly used math
// functions.
//
// SymInt will be extenteded to represent a union structure Union[int64_t,
// SymbolicIntNode*] which will be implemented as a single packed int64_t field
// named data_.
//
// data_ can be either a plain int64_t or (1 << 63 | `index`). `index` points to
// SymbolicIntNode* that will be responsible for constructing an IR node for
// a traced operation to represent it in LTC or Fx graphs.
class C10_API SymInt {
 public:
  SymInt(int64_t d) : data_(d){};

  int64_t expect_int() const {
    TORCH_CHECK(!is_symbolic());
    return data_;
  }

  bool is_symbolic() const {
    return static_cast<uint64_t>(SYM_TAG_MASK) &
        static_cast<uint64_t>(this->data_);
  }

  bool operator==(const SymInt& p2) const {
    return data_ == p2.data_;
  }

  SymInt operator+(SymInt sci) const {
    TORCH_CHECK(
        !this->is_symbolic() && !sci.is_symbolic(),
        "Symbolic Add isn't supported yet");
    return data_ + sci.data_;
  }

  std::shared_ptr<SymbolicIntNode> toSymbolicIntNode();
  static c10::SymInt toSymInt(std::shared_ptr<SymbolicIntNode> sin);

  // This is needed for interoperability with IValue
  int64_t data() const {
    return data_;
  }

 private:
  const static int64_t SYM_TAG_MASK = 1LL << 63;
  int64_t data_;
};

C10_API std::ostream& operator<<(std::ostream& os, SymInt s);
} // namespace c10