Use c++ backend for lfilter when c++ extension is available (#1319)

torchaudio/csrc/lfilter.cpp

 #include <torch/script.h>
+#include <torch/torch.h>
 
 namespace {
 
@@ -62,6 +63,69 @@ void cpu_lfilter_core_loop(
       });
 }
 
+void lfilter_core_generic_loop(
+    const torch::Tensor& input_signal_windows,
+    const torch::Tensor& a_coeff_flipped,
+    torch::Tensor& padded_output_waveform) {
+  int64_t n_samples_input = input_signal_windows.size(1);
+  int64_t n_order = a_coeff_flipped.size(0);
+  for (int64_t i_sample = 0; i_sample < n_samples_input; i_sample++) {
+    auto windowed_output_signal = padded_output_waveform.index(
+        {torch::indexing::Slice(),
+         torch::indexing::Slice(i_sample, i_sample + n_order)});
+    auto o0 = input_signal_windows.index({torch::indexing::Slice(), i_sample})
+                  .addmv(windowed_output_signal, a_coeff_flipped, 1, -1);
+    padded_output_waveform.index_put_(
+        {torch::indexing::Slice(), i_sample + n_order - 1}, o0);
+  }
+}
+
+torch::Tensor lfilter_core(
+    const torch::Tensor& waveform,
+    const torch::Tensor& a_coeffs,
+    const torch::Tensor& b_coeffs) {
+  TORCH_CHECK(waveform.device() == a_coeffs.device());
+  TORCH_CHECK(b_coeffs.device() == a_coeffs.device());
+  TORCH_CHECK(a_coeffs.size(0) == b_coeffs.size(0));
+
+  TORCH_INTERNAL_ASSERT(waveform.sizes().size() == 2);
+
+  auto device = waveform.device();
+  int64_t n_order = a_coeffs.size(0);
+
+  TORCH_INTERNAL_ASSERT(n_order > 0);
+
+  namespace F = torch::nn::functional;
+
+  auto padded_waveform = F::pad(waveform, F::PadFuncOptions({n_order - 1, 0}));
+  auto padded_output_waveform = torch::zeros_like(padded_waveform);
+
+  auto a_coeff_flipped = a_coeffs.flip(0).contiguous();
+  auto b_coeff_flipped = b_coeffs.flip(0).contiguous();
+
+  auto input_signal_windows =
+      F::conv1d(
+          padded_waveform.unsqueeze(1), b_coeff_flipped.view({1, 1, n_order}))
+          .squeeze(1);
+
+  input_signal_windows.div_(a_coeffs[0]);
+  a_coeff_flipped.div_(a_coeffs[0]);
+
+  if (device.is_cpu()) {
+    cpu_lfilter_core_loop(
+        input_signal_windows, a_coeff_flipped, padded_output_waveform);
+  } else {
+    lfilter_core_generic_loop(
+        input_signal_windows, a_coeff_flipped, padded_output_waveform);
+  }
+
+  auto output = padded_output_waveform.index(
+      {torch::indexing::Slice(),
+       torch::indexing::Slice(n_order - 1, torch::indexing::None)});
+
+  return output;
+}
+
 } // namespace
 
 // Note: We want to avoid using "catch-all" kernel.
@@ -69,3 +133,12 @@ void cpu_lfilter_core_loop(
 TORCH_LIBRARY_FRAGMENT(torchaudio, m) {
   m.def("torchaudio::_lfilter_core_loop", &cpu_lfilter_core_loop);
 }
+
+TORCH_LIBRARY(torchaudio, m) {
+  m.def(
+      "torchaudio::_lfilter(Tensor waveform, Tensor a_coeffs, Tensor b_coeffs) -> Tensor");
+}
+
+TORCH_LIBRARY_IMPL(torchaudio, Math, m) {
+  m.impl("torchaudio::_lfilter", lfilter_core);
+}

torchaudio/functional/filtering.py

@@ -825,30 +825,11 @@ except RuntimeError as err:
     _lfilter_core_cpu_loop = _lfilter_core_generic_loop
 
 
-def lfilter(
+def _lfilter_core(
     waveform: Tensor,
     a_coeffs: Tensor,
     b_coeffs: Tensor,
-    clamp: bool = True,
 ) -> Tensor:
-    r"""Perform an IIR filter by evaluating difference equation.
-
-    Args:
-        waveform (Tensor): audio waveform of dimension of ``(..., time)``.  Must be normalized to -1 to 1.
-        a_coeffs (Tensor): denominator coefficients of difference equation of dimension of ``(n_order + 1)``.
-                                Lower delays coefficients are first, e.g. ``[a0, a1, a2, ...]``.
-                                Must be same size as b_coeffs (pad with 0's as necessary).
-        b_coeffs (Tensor): numerator coefficients of difference equation of dimension of ``(n_order + 1)``.
-                                 Lower delays coefficients are first, e.g. ``[b0, b1, b2, ...]``.
-                                 Must be same size as a_coeffs (pad with 0's as necessary).
-        clamp (bool, optional): If ``True``, clamp the output signal to be in the range [-1, 1] (Default: ``True``)
-
-    Returns:
-        Tensor: Waveform with dimension of ``(..., time)``.
-    """
-    # pack batch
-    shape = waveform.size()
-    waveform = waveform.reshape(-1, shape[-1])
 
     assert a_coeffs.size(0) == b_coeffs.size(0)
     assert len(waveform.size()) == 2
@@ -886,6 +867,41 @@ def lfilter(
         _lfilter_core_generic_loop(input_signal_windows, a_coeffs_flipped, padded_output_waveform)
 
     output = padded_output_waveform[:, n_order - 1:]
+    return output
+
+try:
+    _lfilter = torch.ops.torchaudio._lfilter
+except RuntimeError as err:
+    assert str(err) == 'No such operator torchaudio::_lfilter'
+    _lfilter = _lfilter_core
+
+
+def lfilter(
+    waveform: Tensor,
+    a_coeffs: Tensor,
+    b_coeffs: Tensor,
+    clamp: bool = True,
+) -> Tensor:
+    r"""Perform an IIR filter by evaluating difference equation.
+
+    Args:
+        waveform (Tensor): audio waveform of dimension of ``(..., time)``.  Must be normalized to -1 to 1.
+        a_coeffs (Tensor): denominator coefficients of difference equation of dimension of ``(n_order + 1)``.
+                                Lower delays coefficients are first, e.g. ``[a0, a1, a2, ...]``.
+                                Must be same size as b_coeffs (pad with 0's as necessary).
+        b_coeffs (Tensor): numerator coefficients of difference equation of dimension of ``(n_order + 1)``.
+                                 Lower delays coefficients are first, e.g. ``[b0, b1, b2, ...]``.
+                                 Must be same size as a_coeffs (pad with 0's as necessary).
+        clamp (bool, optional): If ``True``, clamp the output signal to be in the range [-1, 1] (Default: ``True``)
+
+    Returns:
+        Tensor: Waveform with dimension of ``(..., time)``.
+    """
+    # pack batch
+    shape = waveform.size()
+    waveform = waveform.reshape(-1, shape[-1])
+
+    output = _lfilter(waveform, a_coeffs, b_coeffs)
 
     if clamp:
         output = torch.clamp(output, min=-1.0, max=1.0)