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Drivers/CMSIS/NN/Source/SoftmaxFunctions/arm_softmax_s16.c

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+/*
+ * Copyright (C) 2022 Arm Limited or its affiliates.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the License); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an AS IS BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/* ----------------------------------------------------------------------
+ * Project:      CMSIS NN Library
+ * Title:        arm_softmax_s16.c
+ * Description:  S16 softmax function
+ *
+ * $Date:        9 March 2022
+ * $Revision:    V.1.0.0
+ *
+ * Target Processor:  Cortex-M cores
+ *
+ * -------------------------------------------------------------------- */
+
+#include "arm_nnfunctions.h"
+#include "arm_nnsupportfunctions.h"
+
+/**
+ * @addtogroup Softmax
+ * @{
+ */
+
+arm_status arm_softmax_s16(const int16_t *input,
+                           const int32_t num_rows,
+                           const int32_t row_size,
+                           const int32_t mult,
+                           const int32_t shift,
+                           const cmsis_nn_softmax_lut_s16 *softmax_params,
+                           int16_t *output)
+{
+    int32_t col = 0;
+    int32_t row_idx;
+
+    if (softmax_params->exp_lut == NULL || softmax_params->one_by_one_lut == NULL)
+    {
+        return ARM_MATH_ARGUMENT_ERROR;
+    }
+
+    for (row_idx = 0; row_idx < num_rows; ++row_idx)
+    {
+        // Find the maximum value in order to ensure numerical stability
+        int16_t max = *input;
+        for (col = 1; col < row_size; ++col)
+        {
+            max = MAX(max, input[col]);
+        }
+
+        int32_t diff = 0;
+        int32_t sum = 0;
+        int16_t *cached_exp_results = output;
+
+        for (col = 0; col < row_size; ++col)
+        {
+            diff = input[col] - max;
+            const int32_t scaled_diff = arm_nn_requantize(diff, mult, shift);
+            const int32_t symmetric_scaled_diff = scaled_diff + NN_Q15_MAX;
+            const int16_t saturated_symmetric_scaled_diff = MIN(MAX(symmetric_scaled_diff, NN_Q15_MIN), NN_Q15_MAX);
+
+            // Lookup from exp table and cache result for next step
+            const int16_t index = (256 + (saturated_symmetric_scaled_diff >> 7));
+            const int16_t offset = saturated_symmetric_scaled_diff & 0x7f;
+            const int16_t base = softmax_params->exp_lut[index];
+            const int16_t slope = softmax_params->exp_lut[index + 1] - softmax_params->exp_lut[index];
+            const int16_t delta = (slope * offset + 64) >> 7;
+            const int16_t result = (base + delta);
+            cached_exp_results[col] = result;
+
+            sum += cached_exp_results[col];
+        }
+
+        const int32_t headroom = __CLZ(sum);
+
+        // Compute the reciprocal 1/sum
+        const int32_t shifted_sum = (((sum) << (headroom - 1)) + (1 << 13)) >> 14;
+
+        // Since LUT computes 1/(1 + x), compute x = (sum - 1) => -65536
+        // Since LUT expects a symmetrical input, recenter from [UINT16_MIN, UINT16_MAX] to [INT16_MIN, INT16_MAX] =>
+        // -32768 ==> So in total -65536 -32768 => -98304
+        const int16_t symmetric_shifted_sum = shifted_sum - 98304;
+
+        // Lookup from one by one table
+        const int16_t index = (256 + (symmetric_shifted_sum >> 7));
+        const int16_t offset = symmetric_shifted_sum & 0x7f;
+        const int16_t base = softmax_params->one_by_one_lut[index];
+        const int16_t slope = softmax_params->one_by_one_lut[index + 1] - softmax_params->one_by_one_lut[index];
+        const int16_t delta = (slope * offset + 64) >> 7;
+        const int16_t one_by_one_result = (base + delta);
+
+        for (col = 0; col < row_size; ++col)
+        {
+            const int16_t right_shift = 30 - headroom;
+            int32_t result = (cached_exp_results[col] * one_by_one_result) >> right_shift;
+            result = (result + 1) >> 1; // Last shift position and insert round
+            output[col] = (int16_t)result;
+        }
+
+        output += row_size;
+        input += row_size;
+    }
+
+    return ARM_MATH_SUCCESS;
+}
+
+/**
+ * @} end of Softmax group
+ */