[cortex-m] Add scalar c++ op for dequantize_per_tensor

Pull Request resolved: #10267

Only buck build for now, CMake is next.
No MVE, scalar only.
Strictly the dtypes we care about update arg_meta to reflect that.

Differential Revision: [D73164576](https://our.internmc.facebook.com/intern/diff/D73164576/)
ghstack-source-id: 278779290

Author: digantdesai

backends/cortex_m/ops/op_dequantize_per_tensor.cpp

@@ -0,0 +1,149 @@
+/*
+ * 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/runtime/kernel/kernel_includes.h>
+#include <cinttypes>
+
+// Check for Helium/MVE support
+#if defined(__ARM_FEATURE_MVE) && (__ARM_FEATURE_MVE & 1)
+#include <arm_mve.h>
+#define HAS_HELIUM_SIMD 1
+#endif
+
+namespace cortex_m {
+namespace native {
+
+using Tensor = executorch::aten::Tensor;
+using ScalarType = executorch::aten::ScalarType;
+using KernelRuntimeContext = torch::executor::KernelRuntimeContext;
+
+namespace {
+
+/**
+ * Asserts that the parameters are valid for float to int8 quantization.
+ */
+void check_dequantize_args(
+    const Tensor& input,
+    int64_t quant_min,
+    int64_t quant_max,
+    ScalarType dtype,
+    Tensor& out) {
+  // Ensure input is char type
+  ET_CHECK_MSG(
+      input.scalar_type() == ScalarType::Char,
+      "input.scalar_type() %" PRId8 " is not char type",
+      static_cast<int8_t>(input.scalar_type()));
+
+  // Check output dtype is float
+  ET_CHECK_MSG(
+      out.scalar_type() == ScalarType::Float,
+      "out.scalar_type() %" PRId8 " is not float",
+      static_cast<int8_t>(out.scalar_type()));
+
+  // Check dtype is int8 (Char)
+  ET_CHECK_MSG(
+      dtype == ScalarType::Char,
+      "dtype %" PRId8 " is not int8 (Char)",
+      static_cast<int8_t>(dtype));
+
+  // Validate quant_min and quant_max for int8
+  int32_t quant_min_lower_bound = std::numeric_limits<int8_t>::min();
+  int32_t quant_max_upper_bound = std::numeric_limits<int8_t>::max();
+
+  ET_CHECK_MSG(
+      quant_min >= quant_min_lower_bound,
+      "quant_min out of bound for int8, expected quant_min_lower_bound: %" PRId32
+      " actual quant_min: %" PRId64,
+      quant_min_lower_bound,
+      quant_min);
+
+  ET_CHECK_MSG(
+      quant_max <= quant_max_upper_bound,
+      "quant_max out of bound for int8, expected quant_max_upper_bound: %" PRId32
+      " actual quant_max: %" PRId64,
+      quant_max_upper_bound,
+      quant_max);
+}
+
+/**
+ * Scalar implementation of quantization for a single value.
+ */
+template <typename K, typename T>
+T dequantize_val(
+    float scale,
+    int32_t zero_point,
+    K value,
+    int64_t quant_min,
+    int64_t quant_max) {
+  (void)quant_min;
+  (void)quant_max;
+  return static_cast<T>((static_cast<int32_t>(value) - zero_point) * scale);
+}
+
+} // namespace
+
+Tensor& dequantize_per_tensor_out(
+    KernelRuntimeContext& context,
+    const Tensor& input,
+    double scale,
+    int64_t zero_point,
+    int64_t quant_min,
+    int64_t quant_max,
+    ScalarType dtype,
+    Tensor& out) {
+  // Ignore context for now
+  (void)context;
+
+  // Resize output tensor to match input dimensions
+  torch::executor::Error err = resize_tensor(out, input.sizes());
+  ET_CHECK_MSG(
+      err == torch::executor::Error::Ok,
+      "Failed to resize out Tensor in dequantize_per_tensor_out");
+
+  // Validate input parameters
+  check_dequantize_args(input, quant_min, quant_max, dtype, out);
+
+  // Pre-compute inverse scale for better performance
+  int32_t zp = static_cast<int32_t>(zero_point);
+  int32_t qmin = static_cast<int32_t>(quant_min);
+  int32_t qmax = static_cast<int32_t>(quant_max);
+
+  // Get pointers to input and output data
+  const int8_t* input_data = input.const_data_ptr<int8_t>();
+  float* out_data = out.mutable_data_ptr<float>();
+  const size_t numel = input.numel();
+
+#if defined(HAS_HELIUM_SIMD)
+// Helium MVE implementation for float32 to int8 quantization
+#Error "Implement MVE version!"
+#else
+  // Scalar implementation for float32 to int8 quantization
+  for (size_t i = 0; i < numel; i++) {
+    out_data[i] =
+        dequantize_val<int8_t, float>(scale, zp, input_data[i], qmin, qmax);
+  }
+#endif
+
+  return out;
+}
+
+Tensor& dequantize_per_tensor_out(
+    const Tensor& input,
+    double scale,
+    int64_t zero_point,
+    int64_t quant_min,
+    int64_t quant_max,
+    ScalarType dtype,
+    Tensor& out) {
+  KernelRuntimeContext context;
+  return dequantize_per_tensor_out(
+      context, input, scale, zero_point, quant_min, quant_max, dtype, out);
+}
+
+} // namespace native
+} // namespace cortex_m

backends/cortex_m/ops/operators.yaml

@@ -9,3 +9,9 @@
   kernels:
     - arg_meta: null
       kernel_name: cortex_m::quantize_per_tensor_out
+
+- func: cortex_m::dequantize_per_tensor.out(Tensor input, float scale, int zero_point, int quant_min, int quant_max, ScalarType dtype, *, Tensor(a!) out) -> Tensor(a!)
+  variants: function
+  kernels:
+    - arg_meta: null
+      kernel_name: cortex_m::dequantize_per_tensor_out

backends/cortex_m/ops/targets.bzl

@@ -24,6 +24,7 @@ def define_operator_target(name: str):
 
 OPERATORS = [
     "quantize_per_tensor",
+    "dequantize_per_tensor",
 ]
 
 def define_common_targets():

backends/cortex_m/test/op_dequantize_per_tensor_test.cpp

@@ -0,0 +1,55 @@
+/*
+ * 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/cortex_m/ops/NativeFunctions.h> // Declares the operator
+#include <executorch/runtime/core/exec_aten/exec_aten.h>
+#include <executorch/runtime/core/exec_aten/testing_util/tensor_factory.h>
+#include <executorch/runtime/core/exec_aten/testing_util/tensor_util.h>
+#include <executorch/runtime/kernel/kernel_includes.h>
+#include <gtest/gtest.h>
+
+using executorch::aten::ScalarType;
+using executorch::aten::Tensor;
+using executorch::runtime::KernelRuntimeContext;
+using torch::executor::testing::TensorFactory;
+
+// Test op
+using cortex_m::native::dequantize_per_tensor_out;
+
+void test_dtype() {
+  TensorFactory<ScalarType::Char> tf;
+
+  Tensor input = tf.full({3, 5}, 4);
+  double scale = 0.5;
+
+  int64_t zero_point = 108;
+  int64_t quant_min = -128;
+  int64_t quant_max = 127;
+
+  TensorFactory<ScalarType::Float> tfo;
+  Tensor out = tfo.zeros({3, 5});
+  // (4 - 108) * 0.5 = -52
+  Tensor expected = tfo.full({3, 5}, -52.0);
+
+  KernelRuntimeContext ctx;
+  dequantize_per_tensor_out(
+      ctx,
+      input,
+      scale,
+      zero_point,
+      quant_min,
+      quant_max,
+      ScalarType::Char,
+      out);
+
+  EXPECT_TENSOR_EQ(out, expected);
+}
+
+TEST(OpDequantizeOutTest, AllDtypesSupported) {
+  test_dtype();
+}

backends/cortex_m/test/targets.bzl

@@ -8,6 +8,7 @@ load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "runtime")
 
 OPERATORS = [
     "quantize_per_tensor",
+    "dequantize_per_tensor",
 ]
 
 def define_operator_test_target(op):