Add AdamW optimizer to extension/training (carries #18848 by @BryanBradfo)

extension/training/README.md

@@ -8,7 +8,7 @@ current state.
 ## Layout
 - `examples/` : Example end to end flows from model definition to optimizer.step()
 - `module/`: Utility class to provide an improved UX when using ExecuTorch for Training.
-- `optimizer/`: Cpp implementations of various optimizers, currently only SGD though Adam is planned.
+- `optimizer/`: C++ implementations of various optimizers, currently SGD and AdamW. Adam is planned.
 - `test/`: Tests that cover multiple subdirs.
 
 ## Technical Birds Eye view

extension/training/optimizer/adamw.cpp

@@ -0,0 +1,231 @@
+/*
+ * 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/extension/training/optimizer/adamw.h>
+
+#include <executorch/extension/tensor/tensor_ptr.h>
+#include <executorch/runtime/core/error.h>
+
+#include <cmath>
+#include <cstring>
+
+using executorch::aten::Tensor;
+using executorch::extension::make_tensor_ptr;
+using executorch::extension::TensorPtr;
+using ::executorch::runtime::Error;
+
+namespace executorch {
+namespace extension {
+namespace training {
+namespace optimizer {
+
+namespace {
+// out[i] = a[i] + alpha * b[i]
+void add_out_hack(
+    const Tensor& a,
+    const Tensor& b,
+    const double alpha,
+    Tensor& out) {
+  auto a_ptr = a.const_data_ptr<float>();
+  auto b_ptr = b.const_data_ptr<float>();
+  auto out_ptr = out.mutable_data_ptr<float>();
+  for (size_t i = 0; i < a.numel(); ++i) {
+    out_ptr[i] = a_ptr[i] + b_ptr[i] * alpha;
+  }
+}
+
+// out[i] = a[i] * alpha
+void mul_out_hack(const Tensor& a, const double alpha, Tensor& out) {
+  auto a_ptr = a.const_data_ptr<float>();
+  auto out_ptr = out.mutable_data_ptr<float>();
+  for (size_t i = 0; i < a.numel(); ++i) {
+    out_ptr[i] = a_ptr[i] * alpha;
+  }
+}
+
+// Fused second-moment update: v[i] = beta2 * v[i] + (1 - beta2) * g[i]^2.
+// Avoids materializing a separate g^2 tensor.
+void addcmul_sq_out_hack(
+    const Tensor& v,
+    const Tensor& g,
+    const double beta2,
+    Tensor& out) {
+  auto v_ptr = v.const_data_ptr<float>();
+  auto g_ptr = g.const_data_ptr<float>();
+  auto out_ptr = out.mutable_data_ptr<float>();
+  const double one_minus_beta2 = 1.0 - beta2;
+  for (size_t i = 0; i < v.numel(); ++i) {
+    const double gi = static_cast<double>(g_ptr[i]);
+    out_ptr[i] = static_cast<float>(
+        static_cast<double>(v_ptr[i]) * beta2 + one_minus_beta2 * gi * gi);
+  }
+}
+
+// Fused AdamW parameter update:
+//   p[i] -= lr * (m[i] / bias_correction1) /
+//           (sqrt(v[i] / bias_correction2) + eps)
+// Performed in double precision internally to limit accumulated FP error on
+// the division-by-sqrt path.
+void adamw_update_hack(
+    Tensor& p,
+    const Tensor& m,
+    const Tensor& v,
+    const double lr,
+    const double bias_correction1,
+    const double bias_correction2,
+    const double eps) {
+  auto p_ptr = p.mutable_data_ptr<float>();
+  auto m_ptr = m.const_data_ptr<float>();
+  auto v_ptr = v.const_data_ptr<float>();
+  const double inv_bc1 = 1.0 / bias_correction1;
+  const double inv_sqrt_bc2 = 1.0 / std::sqrt(bias_correction2);
+  for (size_t i = 0; i < p.numel(); ++i) {
+    const double m_hat = static_cast<double>(m_ptr[i]) * inv_bc1;
+    const double v_hat_sqrt =
+        std::sqrt(static_cast<double>(v_ptr[i])) * inv_sqrt_bc2;
+    p_ptr[i] = static_cast<float>(
+        static_cast<double>(p_ptr[i]) - lr * m_hat / (v_hat_sqrt + eps));
+  }
+}
+} // namespace
+
+bool AdamWParamGroup::has_options() const {
+  return options_ != nullptr;
+}
+
+AdamWOptions& AdamWParamGroup::options() {
+  return *options_.get();
+}
+
+const AdamWOptions& AdamWParamGroup::options() const {
+  return *options_.get();
+}
+
+void AdamWParamGroup::set_options(std::unique_ptr<AdamWOptions> options) {
+  options_ = std::move(options);
+}
+
+const std::map<std::string_view, executorch::aten::Tensor>&
+AdamWParamGroup::named_parameters() const {
+  return named_parameters_;
+}
+
+void AdamW::add_param_group(const AdamWParamGroup& param_group) {
+  AdamWParamGroup param_group_(param_group.named_parameters());
+  if (!param_group.has_options()) {
+    param_group_.set_options(defaults_.clone());
+  } else {
+    param_group_.set_options(param_group.options().clone());
+  }
+  param_groups_.emplace_back(std::move(param_group_));
+}
+
+Error AdamW::step(const std::map<std::string_view, executorch::aten::Tensor>&
+                      named_gradients) {
+  for (auto& group : param_groups_) {
+    auto& options = static_cast<AdamWOptions&>(group.options());
+    const double lr = options.lr();
+    const double beta1 = options.beta1();
+    const double beta2 = options.beta2();
+    const double eps = options.eps();
+    const double weight_decay = options.weight_decay();
+
+    for (auto param_iter = group.named_parameters().begin();
+         param_iter != group.named_parameters().end();
+         ++param_iter) {
+      const auto& named_gradient = named_gradients.find(param_iter->first);
+      if (named_gradient == named_gradients.end()) {
+        continue;
+      }
+      auto g = named_gradient->second;
+      auto p = param_iter->second;
+
+      // Look up or lazily allocate the per-parameter state (two moment
+      // buffers sized and shaped like the gradient, plus a step counter).
+      // Done before p is touched: the allocation below can fail, and a
+      // half-applied step that decayed p without advancing its moments would
+      // decay p twice if the caller retried.
+      auto param_state_it = state_.find(p.unsafeGetTensorImpl());
+      AdamWParamState* state_ptr = nullptr;
+      if (param_state_it == state_.end()) {
+        void* m_buf_ptr = malloc(g.nbytes());
+        void* v_buf_ptr = malloc(g.nbytes());
+        if (m_buf_ptr == nullptr || v_buf_ptr == nullptr) {
+          free(m_buf_ptr);
+          free(v_buf_ptr);
+          return Error::MemoryAllocationFailed;
+        }
+        std::memset(m_buf_ptr, 0, g.nbytes());
+        std::memset(v_buf_ptr, 0, g.nbytes());
+
+        std::vector<executorch::aten::SizesType> sizes(
+            g.sizes().begin(), g.sizes().end());
+        // The buffers come from malloc, so they live on the CPU regardless of
+        // where the gradient does.
+        const auto cpu =
+            executorch::aten::Device(executorch::aten::DeviceType::CPU);
+        auto m_ptr = make_tensor_ptr(
+            sizes,
+            m_buf_ptr,
+            g.scalar_type(),
+            cpu,
+            executorch::aten::TensorShapeDynamism::STATIC,
+            [](void* p) { free(p); });
+        auto v_ptr = make_tensor_ptr(
+            sizes,
+            v_buf_ptr,
+            g.scalar_type(),
+            cpu,
+            executorch::aten::TensorShapeDynamism::STATIC,
+            [](void* p) { free(p); });
+
+        auto state = std::make_unique<AdamWParamState>(
+            std::move(m_ptr), std::move(v_ptr));
+        state_ptr = state.get();
+        state_[p.unsafeGetTensorImpl()] = std::move(state);
+      } else {
+        state_ptr = param_state_it->second.get();
+      }
+
+      // Decoupled weight decay: p <- p - lr * weight_decay * p. Applied to
+      // the parameter directly, BEFORE the moment-based update, and NOT
+      // folded into the gradient. This is the defining property of AdamW
+      // (Loshchilov & Hutter, 2019).
+      if (weight_decay != 0.0) {
+        add_out_hack(p, p, -lr * weight_decay, p);
+      }
+
+      state_ptr->increment_step_count();
+      const int64_t step = state_ptr->step_count();
+
+      Tensor& exp_avg = state_ptr->exp_avg();
+      Tensor& exp_avg_sq = state_ptr->exp_avg_sq();
+
+      // First moment: m <- beta1 * m + (1 - beta1) * g
+      mul_out_hack(exp_avg, beta1, exp_avg);
+      add_out_hack(exp_avg, g, 1.0 - beta1, exp_avg);
+
+      // Second moment: v <- beta2 * v + (1 - beta2) * g^2
+      addcmul_sq_out_hack(exp_avg_sq, g, beta2, exp_avg_sq);
+
+      // Bias-corrected update.
+      const double bias_correction1 = 1.0 - std::pow(beta1, step);
+      const double bias_correction2 = 1.0 - std::pow(beta2, step);
+      adamw_update_hack(
+          p, exp_avg, exp_avg_sq, lr, bias_correction1, bias_correction2, eps);
+    }
+  }
+  return Error::Ok;
+}
+
+AdamW::~AdamW() = default;
+
+} // namespace optimizer
+} // namespace training
+} // namespace extension
+} // namespace executorch