[executorch][native] Add Scalar value type to the in-memory IR

backends/native/runtime/graph/Scalar.cpp

@@ -0,0 +1,37 @@
+// Copyright (c) Meta Platforms, Inc. and affiliates.
+// All rights reserved.
+//
+// This source code is licensed under the BSD-style license found in the
+// LICENSE file in the root directory of this source tree.
+
+#include <executorch/backends/native/runtime/graph/Scalar.h>
+
+#include <stdexcept>
+
+namespace ptn {
+
+int64_t Scalar::to_int() const {
+  const int64_t* v = std::get_if<int64_t>(&value_);
+  if (v == nullptr) {
+    throw std::runtime_error("Scalar::to_int: scalar is not an Int");
+  }
+  return *v;
+}
+
+double Scalar::to_double() const {
+  const double* v = std::get_if<double>(&value_);
+  if (v == nullptr) {
+    throw std::runtime_error("Scalar::to_double: scalar is not a Double");
+  }
+  return *v;
+}
+
+bool Scalar::to_bool() const {
+  const bool* v = std::get_if<bool>(&value_);
+  if (v == nullptr) {
+    throw std::runtime_error("Scalar::to_bool: scalar is not a Bool");
+  }
+  return *v;
+}
+
+} // namespace ptn

backends/native/runtime/graph/Scalar.h

@@ -0,0 +1,61 @@
+// Copyright (c) Meta Platforms, Inc. and affiliates.
+// All rights reserved.
+//
+// This source code is licensed under the BSD-style license found in the
+// LICENSE file in the root directory of this source tree.
+
+#pragma once
+
+#include <cstdint>
+#include <variant>
+
+namespace ptn {
+
+// A concrete scalar value, analogous to c10::Scalar: one alternative per
+// domain — integral, floating point, boolean — each stored at its widest type.
+// A narrower value is stored exactly, and to<T>() narrows it back on read.
+// There is no "none" state — the graph's Value owns that.
+class Scalar {
+ private:
+  std::variant<int64_t, double, bool> value_ = int64_t{0};
+
+ public:
+  constexpr Scalar() = default;
+  // Implicit by design (ergonomic: `Scalar s = 5;`). The int overload
+  // disambiguates `Scalar(5)` — without it int -> {int64_t,double,bool} is an
+  // ambiguous conversion.
+  // cppcheck-suppress-begin noExplicitConstructor
+  /* implicit */ constexpr Scalar(int v) : value_(static_cast<int64_t>(v)) {}
+  /* implicit */ constexpr Scalar(int64_t v) : value_(v) {}
+  /* implicit */ constexpr Scalar(double v) : value_(v) {}
+  /* implicit */ constexpr Scalar(bool v) : value_(v) {}
+  // cppcheck-suppress-end noExplicitConstructor
+  // Every pointer converts to bool, so without this `Scalar s = some_ptr;`
+  // would quietly yield a Bool.
+  template <typename T>
+  Scalar(T*) = delete;
+
+  constexpr bool is_int() const {
+    return std::holds_alternative<int64_t>(value_);
+  }
+  constexpr bool is_double() const {
+    return std::holds_alternative<double>(value_);
+  }
+  constexpr bool is_bool() const {
+    return std::holds_alternative<bool>(value_);
+  }
+
+  // Strict accessors: throw std::runtime_error unless that alternative is live.
+  int64_t to_int() const;
+  double to_double() const;
+  bool to_bool() const;
+
+  // Promoting read: static_cast whichever alternative is live to T, like
+  // c10::Scalar::to<T>().
+  template <typename T>
+  constexpr T to() const {
+    return std::visit([](auto v) { return static_cast<T>(v); }, value_);
+  }
+};
+
+} // namespace ptn

backends/native/runtime/graph/targets.bzl

@@ -17,6 +17,15 @@ def define_common_targets():
         visibility = ["//executorch/backends/native/..."],
     )
 
+    runtime.cxx_library(
+        name = "scalar",
+        srcs = ["Scalar.cpp"],
+        exported_headers = [
+            "Scalar.h",
+        ],
+        visibility = ["//executorch/backends/native/..."],
+    )
+
     runtime.cxx_library(
         name = "tensor_meta",
         srcs = ["TensorMeta.cpp"],
@@ -39,7 +48,9 @@ def define_common_targets():
             "utils/Print.h",
         ],
         exported_deps = [
+            ":scalar",
             ":tensor_meta",
         ],
+        deps = [":string_format"],
         visibility = ["//executorch/backends/native/..."],
     )

backends/native/runtime/graph/utils/Print.cpp

@@ -9,6 +9,8 @@
 #include <cstddef>
 #include <string>
 
+#include <executorch/backends/native/runtime/graph/StringFormat.h>
+
 namespace ptn {
 
 std::string to_string(const TensorMeta& meta) {
@@ -31,4 +33,14 @@ std::string to_string(const TensorMeta& meta) {
   return s;
 }
 
+std::string to_string(const Scalar& scalar) {
+  if (scalar.is_bool()) {
+    return scalar.to_bool() ? "true" : "false";
+  }
+  if (scalar.is_int()) {
+    return std::to_string(scalar.to_int());
+  }
+  return format_double(scalar.to_double());
+}
+
 } // namespace ptn

backends/native/runtime/graph/utils/Print.h

@@ -8,6 +8,7 @@
 
 #include <string>
 
+#include <executorch/backends/native/runtime/graph/Scalar.h>
 #include <executorch/backends/native/runtime/graph/TensorMeta.h>
 
 namespace ptn {
@@ -21,4 +22,7 @@ namespace ptn {
 // (unbounded).
 std::string to_string(const TensorMeta& meta);
 
+// The live alternative only: "true", "-3", "1.5e-08".
+std::string to_string(const Scalar& scalar);
+
 } // namespace ptn