[Executorch][quant] Optimize per channel dequantize

When using quantized kv cache, dequantization routine takes significantly long.
This diff just vectorizes dequant per channel for common case.

Differential Revision: [D63338858](https://our.internmc.facebook.com/intern/diff/D63338858/)

[ghstack-poisoned]

Author: kimishpatel

kernels/quantized/cpu/op_dequantize.cpp

@@ -22,6 +22,8 @@ namespace native {
 using Tensor = exec_aten::Tensor;
 using Scalar = exec_aten::Scalar;
 using ScalarType = exec_aten::ScalarType;
+using StridesType = exec_aten::StridesType;
+using SizesType = exec_aten::SizesType;
 
 namespace {
 
@@ -61,6 +63,163 @@ void check_dequantize_per_tensor_args(
       quant_max);
 }
 
+/**
+ * Useful to reduce a tensor `in` over a given dimension `dim` using the
+ * reduce function `fn`, which should have the following signature:
+ * void fn(const size_t size, const size_t stride, const size_t base_ix)
+ * where `size` and `stride` are the size and stride of the dimension being
+ * reduced and `base_ix` is the index of the first element of the reduction.
+ */
+template <typename Fn>
+void apply_over_unpacked_dim(
+    const Fn& fn,
+    const exec_aten::Tensor& in,
+    const int64_t& dim) {
+  if (in.numel() == 0) {
+    return;
+  }
+
+  ET_CHECK_MSG(in.dim() > 0, "Input tensor must have at least one dimension");
+  ET_CHECK_VALID_DIM(dim, in.dim());
+
+  const size_t d = ET_NORMALIZE_IX(dim, in.dim());
+  const size_t dim_size = in.size(d);
+  const size_t outer_size = getLeadingDims(in, d);
+  const size_t inner_size = getTrailingDims(in, d);
+  // Loop through all outer dimensions
+  for (size_t outer_idx = 0; outer_idx < outer_size; ++outer_idx) {
+    // Loop through dim
+    for (size_t unpacked_dim_idx = 0; unpacked_dim_idx < dim_size;
+         ++unpacked_dim_idx) {
+      fn(inner_size, outer_idx, unpacked_dim_idx);
+    }
+  }
+}
+
+void dequantize_optimized(
+    const int8_t* in,
+    const double scale,
+    const int64_t zero_point,
+    float* out,
+    int64_t quant_min,
+    int64_t quant_max,
+    size_t numel) {
+  ET_CHECK_MSG(
+      zero_point >= quant_min,
+      "zero_point must be %" PRId64 " <= quant_min %" PRId64,
+      zero_point,
+      quant_min);
+  ET_CHECK_MSG(
+      zero_point <= quant_max,
+      "zero_point must be %" PRId64 " >= quant_max %" PRId64,
+      zero_point,
+      quant_max);
+  size_t i = 0;
+#if defined(__aarch64__) || defined(__ARM_NEON)
+  int8x8_t zero_point_vec = vdup_n_s8(zero_point);
+  float32x4_t scales = vdupq_n_f32(static_cast<float>(scale));
+  constexpr int32_t kVecSize = 16;
+  const size_t num_vecs = numel / kVecSize;
+  const size_t rem = numel % kVecSize;
+  for (; i < numel; i += kVecSize) {
+    int8x16_t in_vec = vld1q_s8(in);
+    int16x8_t sub_vec_0_7 = vsubl_s8(vget_low_s8(in_vec), zero_point_vec);
+    int32x4_t sub_vec_0_3 = vmovl_s16(vget_low_s16(sub_vec_0_7));
+    int32x4_t sub_vec_4_7 = vmovl_s16(vget_high_s16(sub_vec_0_7));
+    float32x4_t out_vec_0_3 = vmulq_f32(vcvtq_f32_s32(sub_vec_0_3), scales);
+    float32x4_t out_vec_4_7 = vmulq_f32(vcvtq_f32_s32(sub_vec_4_7), scales);
+
+    int16x8_t sub_vec_8_15 = vsubl_s8(vget_high_s8(in_vec), zero_point_vec);
+    int32x4_t sub_vec_8_11 = vmovl_s16(vget_low_s16(sub_vec_8_15));
+    int32x4_t sub_vec_12_15 = vmovl_s16(vget_high_s16(sub_vec_8_15));
+    float32x4_t out_vec_8_11 = vmulq_f32(vcvtq_f32_s32(sub_vec_8_11), scales);
+    float32x4_t out_vec_12_15 = vmulq_f32(vcvtq_f32_s32(sub_vec_12_15), scales);
+    in += kVecSize;
+  }
+#endif
+  for (; i < numel; i++) {
+    out[i] = (in[i] - zero_point) * scale;
+  }
+}
+
+bool can_use_optimized_dequantize_per_channel(
+    const Tensor& in,
+    const ScalarType in_dtype,
+    exec_aten::optional<ScalarType>& out_dtype) {
+  if (!executorch::runtime::is_contiguous_dim_order(
+          in.dim_order().data(), in.dim()) ||
+      (in_dtype != ScalarType::Char) ||
+      (out_dtype.has_value() && out_dtype.value() != ScalarType::Float)) {
+    return false;
+  }
+  return true;
+}
+
+void dequantize_per_channel_optimized(
+    const Tensor& in,
+    const Tensor& scales,
+    const optional<Tensor>& opt_zero_points,
+    Tensor& out,
+    int64_t axis,
+    int64_t quant_min,
+    int64_t quant_max,
+    ScalarType in_dtype,
+    exec_aten::optional<ScalarType>& out_dtype) {
+  check_dequantize_per_tensor_args(
+      in, quant_min, quant_max, in_dtype, out_dtype, out);
+  ET_CHECK_MSG(
+      executorch::runtime::is_contiguous_dim_order(
+          in.dim_order().data(), in.dim()),
+      "in must be in contiguous dim order");
+  ET_CHECK_MSG(
+      in_dtype == ScalarType::Char,
+      "in.scalar_type() %" PRId8 " is not supported:",
+      static_cast<int8_t>(in.scalar_type()));
+  if (out_dtype.has_value()) {
+    ET_CHECK_MSG(
+        out_dtype.value() == ScalarType::Float,
+        "Only float output is supported");
+  }
+  const int8_t* in_data = in.const_data_ptr<int8_t>();
+  float* out_data = out.mutable_data_ptr<float>();
+  const int64_t* zero_points_data = nullptr;
+  if (opt_zero_points.has_value()) {
+    zero_points_data = opt_zero_points.value().const_data_ptr<int64_t>();
+  }
+  const double* scales_data = scales.const_data_ptr<double>();
+  const StridesType axis_stride = in.strides()[axis];
+  const StridesType outer_stride = in.size(axis) * axis_stride;
+  apply_over_unpacked_dim(
+      [in_data,
+       out_data,
+       scales_data,
+       zero_points_data,
+       axis_stride,
+       outer_stride,
+       quant_min,
+       quant_max](
+          SizesType numel, SizesType outer_idx, SizesType unpacked_dim_idx) {
+        const int8_t* in_data_local =
+            in_data + outer_idx * outer_stride + unpacked_dim_idx * axis_stride;
+        const double scale = scales_data[unpacked_dim_idx];
+        const int64_t zero_point = zero_points_data != nullptr
+            ? zero_points_data[unpacked_dim_idx]
+            : 0;
+        float* out_data_local = out_data + outer_idx * outer_stride +
+            unpacked_dim_idx * axis_stride;
+        dequantize_optimized(
+            in_data_local,
+            scale,
+            zero_point,
+            out_data_local,
+            quant_min,
+            quant_max,
+            numel);
+      },
+      in,
+      axis);
+}
+
 } // namespace
 
 /**
@@ -217,6 +376,20 @@ Tensor& dequantize_per_channel_out(
   check_dequantize_per_tensor_args(
       input, quant_min, quant_max, dtype, out_dtype, out);
 
+  if (can_use_optimized_dequantize_per_channel(input, dtype, out_dtype)) {
+    dequantize_per_channel_optimized(
+        input,
+        scale,
+        opt_zero_points,
+        out,
+        axis,
+        quant_min,
+        quant_max,
+        dtype,
+        out_dtype);
+    return out;
+  }
+
   // a list contains all dimensions except axis
   int64_t dims[kTensorDimensionLimit];
   for (int64_t i = 0; i < input.dim() - 1; i++) {