kernels/portable/cpu/op_softmax.cpp

/*
 * 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 <cmath>

#include <executorch/kernels/portable/cpu/util/activation_ops_util.h>
#include <executorch/kernels/portable/cpu/util/functional_util.h>
#include <executorch/kernels/portable/cpu/util/reduce_util.h>
#include <executorch/runtime/kernel/kernel_includes.h>

namespace torch {
namespace executor {
namespace native {

using Tensor = exec_aten::Tensor;

Tensor& softmax_out(
    RuntimeContext& ctx,
    const Tensor& in,
    int64_t dim,
    bool half_to_float,
    Tensor& out) {
  (void)ctx;

  ET_KERNEL_CHECK(
      ctx,
      check_softmax_args(in, dim, half_to_float, out),
      InvalidArgument,
      out);

  ET_KERNEL_CHECK(
      ctx, resize_tensor(out, in.sizes()) == Error::Ok, InvalidArgument, out);

  // Adjust for negative dim
  dim = dim < 0 ? dim + nonzero_dim(in) : dim;

  ET_SWITCH_FLOATH_TYPES(in.scalar_type(), ctx, "_softmax.out", CTYPE, [&]() {
    const CTYPE* const in_data = in.const_data_ptr<CTYPE>();
    CTYPE* const out_data = out.mutable_data_ptr<CTYPE>();

    apply_over_dim(
        [in_data, out_data](
            const size_t size, const size_t stride, const size_t base) {
          // calculate max in softmax dim. During softmax computation each
          // value is subtracted by the maximum in value before calling exp
          // to preserve numerical stability.
          const CTYPE max_in = apply_unary_reduce_fn(
              [](const CTYPE val_in, CTYPE val_accum) {
                return std::max(val_in, val_accum);
              },
              in_data + base,
              size,
              stride);

          const CTYPE temp_sum = apply_unary_map_reduce_fn<CTYPE, CTYPE>(
              [max_in](const CTYPE val_in) {
                return std::exp(val_in - max_in);
              },
              [](const CTYPE mapped_in, CTYPE val_accum) {
                return val_accum + mapped_in;
              },
              in_data + base,
              size,
              stride);

          apply_unary_map_fn(
              [max_in, temp_sum](const CTYPE val_in) {
                return std::exp(val_in - max_in) / temp_sum;
              },
              in_data + base,
              out_data + base,
              size,
              stride);
        },
        in,
        dim);
  });

  return out;
}

} // namespace native
} // namespace executor
} // namespace torch