chore: import upstream snapshot with attribution

csrc/cpu/activation_lut_bf16.cpp

@@ -0,0 +1,71 @@
+#include "cpu_types.hpp"
+
+#include <array>
+#include <cstdint>
+#include <mutex>
+#include <string>
+
+#include <ATen/ops/empty.h>
+#include <ATen/ops/gelu.h>
+#include <c10/util/BFloat16.h>
+
+constexpr uint32_t ActivationLutSize = 1u << 16;
+
+at::Tensor gelu_reference(const at::Tensor& x) { return at::gelu(x, "none"); }
+
+void maybe_init_activation_lut_bf16(
+    uint16_t* lut, std::once_flag& once,
+    at::Tensor (*activation)(const at::Tensor&)) {
+  std::call_once(once, [&]() {
+    auto lut_input =
+        at::empty({static_cast<int64_t>(ActivationLutSize)},
+                  at::TensorOptions().device(at::kCPU).dtype(at::kFloat));
+    auto* lut_input_ptr = lut_input.data_ptr<float>();
+#pragma omp parallel for
+    for (uint32_t i = 0; i < ActivationLutSize; ++i) {
+      lut_input_ptr[i] = c10::detail::f32_from_bits(static_cast<uint16_t>(i));
+    }
+
+    auto lut_output = activation(lut_input);
+    const auto* lut_output_ptr = lut_output.data_ptr<float>();
+#pragma omp parallel for
+    for (uint32_t i = 0; i < ActivationLutSize; ++i) {
+      lut[i] = c10::detail::round_to_nearest_even(lut_output_ptr[i]);
+    }
+  });
+}
+
+void activation_lut_bf16(torch::Tensor& out, torch::Tensor& input,
+                         const uint16_t* lut, const char* op_name) {
+  TORCH_CHECK(input.scalar_type() == at::kBFloat16, op_name,
+              ": input must be bfloat16");
+  TORCH_CHECK(out.scalar_type() == at::kBFloat16, op_name,
+              ": out must be bfloat16");
+  TORCH_CHECK(input.is_contiguous(), op_name, ": input must be contiguous");
+  TORCH_CHECK(out.is_contiguous(), op_name, ": out must be contiguous");
+
+  const auto* src =
+      reinterpret_cast<const uint16_t*>(input.data_ptr<at::BFloat16>());
+  auto* dst = reinterpret_cast<uint16_t*>(out.data_ptr<at::BFloat16>());
+  const int64_t n = input.numel();
+
+  CPU_KERNEL_GUARD_IN(activation_lut_bf16_impl)
+#pragma omp parallel for
+  for (int64_t i = 0; i < n; ++i) {
+    dst[i] = lut[src[i]];
+  }
+  CPU_KERNEL_GUARD_OUT(activation_lut_bf16_impl)
+}
+
+void activation_lut_bf16(torch::Tensor& out, torch::Tensor& input,
+                         const std::string& activation) {
+  if (activation == "gelu") {
+    static std::array<uint16_t, ActivationLutSize> lut{};
+    static std::once_flag once;
+    maybe_init_activation_lut_bf16(lut.data(), once, gelu_reference);
+    activation_lut_bf16(out, input, lut.data(), "gelu_lut");
+    return;
+  }
+
+  TORCH_CHECK(false, "Unsupported activation: ", activation);
+}