Rename SoftArgMax operator to SoftMax

- Match naming in TensorFlow, PyTorch, and ONNX
- Avoid confusion with occasionally proposed SoftArgMax operator with different
  semantics y[i] := exp(x[i]) * i / sum(exp(x[j]))

PiperOrigin-RevId: 291812880
diff --git a/bench/softmax.cc b/bench/softmax.cc
new file mode 100644
index 0000000..c0eb362
--- /dev/null
+++ b/bench/softmax.cc
@@ -0,0 +1,104 @@
+// Copyright (c) Facebook, Inc. and its 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 <algorithm>
+#include <cmath>
+#include <functional>
+#include <random>
+#include <vector>
+
+#include <xnnpack.h>
+
+#include <benchmark/benchmark.h>
+#include "bench/utils.h"
+
+
+static void softmax_q8(benchmark::State& state) {
+  const size_t batch_size = static_cast<size_t>(state.range(0));
+  const size_t channels = static_cast<size_t>(state.range(1));
+
+  std::random_device random_device;
+  auto rng = std::mt19937(random_device());
+  auto u8rng = std::bind(std::uniform_int_distribution<uint8_t>(), rng);
+
+  std::vector<uint8_t> input(batch_size * channels);
+  std::vector<uint8_t> output(batch_size * channels);
+  std::generate(input.begin(), input.end(), std::ref(u8rng));
+  std::fill(output.begin(), output.end(), 0xA5);
+
+  xnn_status status = xnn_initialize(nullptr /* allocator */);
+  if (status != xnn_status_success) {
+    state.SkipWithError("failed to initialize XNNPACK");
+    return;
+  }
+
+  xnn_operator_t softmax_op = nullptr;
+  status = xnn_create_softmax_nc_q8(
+    channels, channels /* input stride */, channels /* output stride */,
+    1.0f /* input scale */,
+    0 /* output zero point */, 1.0f / 256.0f /* output scale */,
+    0 /* flags */, &softmax_op);
+  if (status != xnn_status_success || softmax_op == nullptr) {
+    state.SkipWithError("failed to create SoftMax operator");
+    return;
+  }
+
+  status = xnn_setup_softmax_nc_q8(
+    softmax_op,
+    batch_size,
+    input.data(), output.data(),
+    nullptr /* thread pool */);
+  if (status != xnn_status_success) {
+    state.SkipWithError("failed to setup SoftMax operator");
+    return;
+  }
+
+  for (auto _ : state) {
+    status = xnn_run_operator(softmax_op, nullptr /* thread pool */);
+    if (status != xnn_status_success) {
+      state.SkipWithError("failed to run SoftMax operator");
+      return;
+    }
+  }
+
+  status = xnn_delete_operator(softmax_op);
+  if (status != xnn_status_success) {
+    state.SkipWithError("failed to delete SoftMax operator");
+    return;
+  }
+
+  state.counters["Freq"] = benchmark::utils::GetCurrentCpuFrequency();
+
+  const size_t elements_per_iteration = batch_size * channels;
+  state.counters["elements"] =
+    benchmark::Counter(uint64_t(state.iterations()) * elements_per_iteration, benchmark::Counter::kIsRate);
+
+  const size_t bytes_per_iteration = 2 * elements_per_iteration * sizeof(uint8_t);
+  state.counters["bytes"] =
+    benchmark::Counter(uint64_t(state.iterations()) * bytes_per_iteration, benchmark::Counter::kIsRate);
+}
+
+static void CharacteristicArguments(benchmark::internal::Benchmark* b)
+{
+  b->ArgNames({"N", "C"});
+
+  // CIFAR-10
+  b->Args({1, 10});
+  // CIFAR-100 */
+  b->Args({1, 100});
+  // ImageNet-1K
+  b->Args({1, 1000});
+  // ImageNet-1K+1
+  b->Args({1, 1001});
+  // ImageNet-22K
+  b->Args({1, 21841});
+}
+
+BENCHMARK(softmax_q8)->Apply(CharacteristicArguments)->UseRealTime();
+
+#ifndef XNNPACK_BENCHMARK_NO_MAIN
+BENCHMARK_MAIN();
+#endif