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/*
* Copyright (c) 2017 ARM Limited.
*
* SPDX-License-Identifier: MIT
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to
* deal in the Software without restriction, including without limitation the
* rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
* sell copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in all
* copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
* SOFTWARE.
*/
#pragma once
#include "arm_compute/core/NEON/kernels/winograd/tensor.hpp"
namespace winograd {
/* Transform an input tensor into the Winograd domain.
*/
template <typename T>
struct Winograd2x2_3x3GemmInput {
static void execute(
const T *inptr,
const Tensor4DShape& input_shape,
const PaddingType padding_type,
const int tile_M,
const int tile_N,
T *outptr_base,
const int matrix_stride,
const int matrix_batch_stride,
const int matrix_row_stride
);
static size_t bytes_read(const Tensor4DShape &input_shape,
const Tensor4DShape &output_shape) {
const int tile_rows = iceildiv(output_shape.n_rows, 2);
const int tile_cols = iceildiv(output_shape.n_cols, 2);
return input_shape.n_batches * tile_rows * (16 + 8*(tile_cols - 1)) * input_shape.n_channels * sizeof(T);
}
static int flops_performed(const Tensor4DShape &input_shape,
const Tensor4DShape &output_shape) {
const int tile_rows = iceildiv(output_shape.n_rows, 2);
const int tile_cols = iceildiv(output_shape.n_cols, 2);
return input_shape.n_batches * tile_rows * (32 + 24*(tile_cols - 1)) * input_shape.n_channels;
}
static size_t bytes_written(const Tensor4DShape &input_shape,
const Tensor4DShape &output_shape) {
const int tile_rows = iceildiv(output_shape.n_rows, 2);
const int tile_cols = iceildiv(output_shape.n_cols, 2);
const int M = input_shape.n_batches * tile_rows * tile_cols;
return 16 * M * input_shape.n_channels * sizeof(T);
}
protected:
template <const PaddingType padding, const int pad_bottom, const int pad_right>
static void process_tile_tensor(
const int tile_M, // Number of rows of tiles
const int tile_N, // Number of columns of tiles
int n_channels, // Number of input channels
const T* const input, // Base input pointer (appropriate to batch and channel)
const int input_row_stride, // Stride between rows of the input
const int input_col_stride, // Stride between columns of the input
T* const matrix, // 1st output matrix (appropriate to batch and channel)
const int matrix_stride, // Stride between matrices
const int matrix_row_stride // Stride between rows of the output matrix
);
template <const int pad_top, const int pad_left,
const int pad_bottom, const int pad_right,
const int proc_channels>
static void process_tile_row(
const int tile_N, // Number of tiles in the row
const T* const input, // Base input pointer (appropriate to batch, channel and row)
const int input_row_stride, // Stride between rows of the input
const int input_col_stride, // Stride between columns of the input
T* const matrix, // 1st output matrix (appropriate to batch, channel and row)
const int matrix_stride, // Stride between matrices
const int matrix_row_stride // Stride between rows of the output matrix
);
};
template <typename T>
struct Winograd2x2_3x3GemmInputChannelwise {
static void execute(
const T *inptr,
const Tensor4DShape& input_shape,
const PaddingType padding_type,
const int tile_M,
const int tile_N,
T *outptr_base,
const int matrix_stride,
const int matrix_batch_stride,
const int matrix_row_stride
);
static size_t bytes_read(const Tensor4DShape &input_shape,
const Tensor4DShape &output_shape) {
// We read as many bytes as we write
return bytes_written(input_shape, output_shape);
}
static int flops_performed(const Tensor4DShape &input_shape,
const Tensor4DShape &output_shape) {
const int tile_rows = iceildiv(output_shape.n_rows, 2);
const int tile_cols = iceildiv(output_shape.n_cols, 2);
return input_shape.n_batches * tile_rows * 32 * tile_cols * input_shape.n_channels;
}
static size_t bytes_written(const Tensor4DShape &input_shape,
const Tensor4DShape &output_shape) {
return winograd::Winograd2x2_3x3GemmInput<T>::bytes_written(input_shape, output_shape);
}
protected:
typedef void (*tilefunc)(int, const T*, int, int, T*, int);
template <const int pad_top,
const int pad_left,
const int pad_bottom,
const int pad_right>
static void process_tile(
int n_channels, // Number of channels in the tile
const T* const input_base,
const int input_row_stride,
const int input_col_stride,
T* const matrix_base,
const int matrix_stride
);
private:
template <const int pad_top,
const int pad_left,
const int pad_bottom,
const int pad_right,
const int proc_channels>
static void _process_tile(
int &n_channels, const T* &inptr,
const int input_row_stride, const int input_col_stride,
T* &outptr, const int matrix_stride
);
};
}
/*****************************************************************************/
// Include specialised implementations here
#include "input_2x2_3x3/a64_float.hpp"
#include "input_2x2_3x3/a64_float_channelwise.hpp"
/*****************************************************************************/
/*****************************************************************************/
template <typename T>
void winograd::Winograd2x2_3x3GemmInput<T>::execute(
const T *inptr_base,
const Tensor4DShape& input_shape,
const PaddingType padding_type,
const int tile_M,
const int tile_N,
T *outptr_base,
const int matrix_stride,
const int matrix_batch_stride,
const int matrix_row_stride
) {
// Select an appropriate matrix processing method for the shape and padding
// of the input tensor.
typedef void (*tensorfunc)(int, int, int, const T*, int, int, T*, int, int);
const auto process_tensor = [&padding_type, &input_shape] () -> tensorfunc {
if (padding_type == PADDING_VALID) {
const int pad_bottom = input_shape.n_rows % 2;
const int pad_right = input_shape.n_cols % 2;
if (pad_bottom == 0 && pad_right == 0) {
return process_tile_tensor<PADDING_VALID, 0, 0>;
} else if (pad_bottom == 0 && pad_right == 1) {
return process_tile_tensor<PADDING_VALID, 0, 1>;
} else if (pad_bottom == 1 && pad_right == 0) {
return process_tile_tensor<PADDING_VALID, 1, 0>;
} else if (pad_bottom == 1 && pad_right == 1) {
return process_tile_tensor<PADDING_VALID, 1, 1>;
}
} else { // PADDING_SAME
const int pad_bottom = 1 + input_shape.n_rows % 2;
const int pad_right = 1 + input_shape.n_cols % 2;
if (pad_bottom == 1 && pad_right == 1) {
return process_tile_tensor<PADDING_SAME, 1, 1>;
} else if (pad_bottom == 1 && pad_right == 2) {
return process_tile_tensor<PADDING_SAME, 1, 2>;
} else if (pad_bottom == 2 && pad_right == 1) {
return process_tile_tensor<PADDING_SAME, 2, 1>;
} else if (pad_bottom == 2 && pad_right == 2) {
return process_tile_tensor<PADDING_SAME, 2, 2>;
}
}
printf("%s::%u Uncovered case.\n", __FILE__, __LINE__);
exit(-1);
return NULL; // No function found
} ();
// Compute strides
const int input_row_stride = input_shape.n_cols * input_shape.n_channels;
const int input_col_stride = input_shape.n_channels;
// Process each batch of the tensor in turn.
for (int batch = 0; batch < input_shape.n_batches; batch++) {
// Work out pointers
const T *inptr = inptr_base + (batch * input_shape.n_rows *
input_shape.n_cols * input_shape.n_channels);
T *outptr = outptr_base + batch * matrix_batch_stride;
// Delegate doing the actual work
process_tensor(
tile_M, tile_N, input_shape.n_channels,
inptr, input_row_stride, input_col_stride,
outptr, matrix_stride, matrix_row_stride
);
}
}
/*****************************************************************************/
template <typename T>
template <const PaddingType padding, const int pad_bottom, const int pad_right>
void winograd::Winograd2x2_3x3GemmInput<T>::process_tile_tensor(
const int tile_M, // Number of rows of tiles
const int tile_N, // Number of columns of tiles
int n_channels, // Number of input channels
const T* const input, // Base input pointer (appropriate to batch and channel)
const int input_row_stride, // Stride between rows of the input
const int input_col_stride, // Stride between columns of the input
T* const matrix, // 1st output matrix (appropriate to batch and channel)
const int matrix_stride, // Stride between matrices
const int matrix_row_stride // Stride between rows of the output matrix
) {
// Base row processing functions
typedef void (*rowfunc)(int, const T*, int, int, T*, int, int);
const rowfunc process_top_row[3] = {
(padding == PADDING_VALID)
? process_tile_row<0, 0, 0, pad_right, 1>
: process_tile_row<1, 1, 0, pad_right, 1>,
(padding == PADDING_VALID)
? process_tile_row<0, 0, 0, pad_right, 2>
: process_tile_row<1, 1, 0, pad_right, 2>,
(padding == PADDING_VALID)
? process_tile_row<0, 0, 0, pad_right, 4>
: process_tile_row<1, 1, 0, pad_right, 4>,
};
const rowfunc process_middle_row[3] = {
(padding == PADDING_VALID)
? process_tile_row<0, 0, 0, pad_right, 1>
: process_tile_row<0, 1, 0, pad_right, 1>,
(padding == PADDING_VALID)
? process_tile_row<0, 0, 0, pad_right, 2>
: process_tile_row<0, 1, 0, pad_right, 2>,
(padding == PADDING_VALID)
? process_tile_row<0, 0, 0, pad_right, 4>
: process_tile_row<0, 1, 0, pad_right, 4>,
};
const rowfunc process_bottom_row[3] = {
(padding == PADDING_VALID)
? process_tile_row<0, 0, pad_bottom, pad_right, 1>
: process_tile_row<0, 1, pad_bottom, pad_right, 1>,
(padding == PADDING_VALID)
? process_tile_row<0, 0, pad_bottom, pad_right, 2>
: process_tile_row<0, 1, pad_bottom, pad_right, 2>,
(padding == PADDING_VALID)
? process_tile_row<0, 0, pad_bottom, pad_right, 4>
: process_tile_row<0, 1, pad_bottom, pad_right, 4>,
};
// Method to get an input pointer for the given tile row
const auto get_inptr = [&input, &input_row_stride] (const int tile_i) {
if (padding == PADDING_VALID) {
return input + 2 * tile_i * input_row_stride;
} else {
return input + (2 * tile_i - (tile_i ? 1 : 0)) * input_row_stride;
}
};
// Wrapper to process a row of tiles, covering all channels.
const auto process_row =
[tile_N, input_row_stride, input_col_stride, matrix_stride, matrix_row_stride, n_channels]
(const rowfunc f[3], const T *inptr, T *outptr) {
int rem_channels = n_channels;
// While there remain channels to process continue to process the
// row.
for (; rem_channels >= 4; rem_channels -= 4, inptr += 4, outptr += 4) {
f[2](tile_N, inptr, input_row_stride, input_col_stride, outptr, matrix_stride, matrix_row_stride);
}
for (; rem_channels >= 2; rem_channels -= 2, inptr += 2, outptr += 2) {
f[1](tile_N, inptr, input_row_stride, input_col_stride, outptr, matrix_stride, matrix_row_stride);
}
if (rem_channels) {
f[0](tile_N, inptr, input_row_stride, input_col_stride, outptr, matrix_stride, matrix_row_stride);
}
};
// Process all rows of tiles in the tensor
for (int tile_i = 0; tile_i < tile_M; tile_i++) {
T* const m_row = matrix + tile_i * tile_N * matrix_row_stride;
const T *row_inptr = get_inptr(tile_i);
if (tile_i == 0) {
// Top row of the input
process_row(process_top_row, row_inptr, m_row);
} else if (tile_i == tile_M - 1) {
// Bottom row of the input
process_row(process_bottom_row, row_inptr, m_row);
} else {
// Any other row of the input
process_row(process_middle_row, row_inptr, m_row);
}
}
}
/*****************************************************************************/
template <typename T>
template <const int pad_top, const int pad_left,
const int pad_bottom, const int pad_right,
const int proc_channels>
void winograd::Winograd2x2_3x3GemmInput<T>::process_tile_row(
const int tile_N, // Number of tiles in the row
const T* const input, // Base input pointer (appropriate to batch, channel and row)
const int input_row_stride, // Stride between rows of the input
const int input_col_stride, // Stride between columns of the input
T* const matrix, // 1st output matrix (appropriate to batch, channel and row)
const int matrix_stride, // Stride between matrices
const int matrix_row_stride // Stride between rows of the output matrix
) {
// Construct copies of the pointers
const T *inptr = input;
T *outptr = matrix;
// Storage for the tensors x, X.T x, and X.T x X.
T x[4][4][proc_channels], XTx[4][4][proc_channels], XTxX[4][4][proc_channels];
// For every tile in the row
for (int tile_j = 0; tile_j < tile_N; tile_j++) {
// Determine the padding for the tile
const int tile_pad_left = (tile_j == 0) ? pad_left : 0;
const int tile_pad_right = (tile_j == tile_N - 1) ? pad_right : 0;
// Load tile values. If this is the first tile in the row then we must load
// all values, otherwise we can just load the final two columns of the input.
for (int i = 0; i < 4; i++) {
for (int j = ((tile_j == 0) ? 0 : 2); j < 4; j++) {
// Fill with padding if required
if (i < pad_top || 4 - pad_bottom <= i ||
j < tile_pad_left || 4 - tile_pad_right <= j) {
for (int c = 0; c < proc_channels; c++) {
x[i][j][c] = static_cast<T>(0); // Padding
}
} else {
// Load values, note that the initial padding offsets the pointer we
// were provided.
for (int c = 0; c < proc_channels; c++) {
const int row_offset = (i - pad_top) * input_row_stride;
const int col_offset = (j - tile_pad_left) * input_col_stride;
x[i][j][c] = inptr[row_offset + col_offset + c];
}
}
}
}
// Compute the matrix X.T x. Note, can elide operations depending on the
// padding. Furthermore, if this isn't the left-most tile we can skip half
// of the operations by copying results from the previous version of X.T x.
// This latter optimisation can be simplified by unrolling the outermost
// loop by two and by renaming the registers containing XTx.
if (tile_j == 0) {
for (int j = 0; j < 4; j++) {
for (int c = 0; c < proc_channels; c++) {
XTx[0][j][c] = x[0][j][c] - x[2][j][c];
XTx[1][j][c] = x[1][j][c] + x[2][j][c];
XTx[2][j][c] = -x[1][j][c] + x[2][j][c];
XTx[3][j][c] = x[1][j][c] - x[3][j][c];
}
}
} else {
for (int j = 0; j < 2; j++) {
for (int c = 0; c < proc_channels; c++) {
XTx[0][j][c] = XTx[0][j + 2][c];
XTx[1][j][c] = XTx[1][j + 2][c];
XTx[2][j][c] = XTx[2][j + 2][c];
XTx[3][j][c] = XTx[3][j + 2][c];
}
}
for (int j = 2; j < 4; j++) {
for (int c = 0; c < proc_channels; c++) {
XTx[0][j][c] = x[0][j][c] - x[2][j][c];
XTx[1][j][c] = x[1][j][c] + x[2][j][c];
XTx[2][j][c] = -x[1][j][c] + x[2][j][c];
XTx[3][j][c] = x[1][j][c] - x[3][j][c];
}
}
}
// Compute the matrix X.T x X. Note, can elide operations based on the
// padding.
for (int i = 0; i < 4; i++) {
for (int c = 0; c < proc_channels; c++) {
XTxX[i][0][c] = XTx[i][0][c] - XTx[i][2][c];
XTxX[i][1][c] = XTx[i][1][c] + XTx[i][2][c];
XTxX[i][2][c] = -XTx[i][1][c] + XTx[i][2][c];
XTxX[i][3][c] = XTx[i][1][c] - XTx[i][3][c];
}
}
// Store the output matrix (X.T x X)
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 4; j++) {
// Get a pointer to the relevant output matrix
T *mptr = outptr + (i*4 + j)*matrix_stride;
// Write out the channels
for (int c = 0; c < proc_channels; c++) {
mptr[c] = XTxX[i][j][c];
}
}
}
// Update the pointers
inptr += input_col_stride * ((tile_j == 0 && pad_left) ? 1 : 2);
outptr += matrix_row_stride;
}
}
/*****************************************************************************/
template <typename T>
void winograd::Winograd2x2_3x3GemmInputChannelwise<T>::execute(
const T *inptr,
const Tensor4DShape& input_shape,
const PaddingType padding_type,
const int tile_M,
const int tile_N,
T *outptr_base,
const int matrix_stride,
const int matrix_batch_stride,
const int matrix_row_stride
) {
const int n_channels = input_shape.n_channels;
const int input_col_stride = n_channels;
const int input_row_stride = input_shape.n_cols * input_col_stride;
// Determine the padding and hence select appropriate methods for each tile.
tilefunc fs[3][3];
if (padding_type == PADDING_VALID) {
constexpr int pad_top = 0;
constexpr int pad_left = 0;
const int pad_right = input_shape.n_cols % 2 == 0;
fs[0][0] = process_tile<pad_top, pad_left, 0, 0>;
fs[0][1] = process_tile<pad_top, 0, 0, 0>;
fs[0][2] = (pad_right) ? process_tile<pad_top, 0, 0, 0> : process_tile<pad_top, 0, 0, 1>;
fs[1][0] = process_tile<0, pad_left, 0, 0>;
fs[1][1] = process_tile<0, 0, 0, 0>;
fs[1][2] = (pad_right) ? process_tile<0, 0, 0, 0> : process_tile<0, 0, 0, 1>;
if (input_shape.n_rows % 2 == 0) {
constexpr int pad_bottom = 0;
fs[2][0] = process_tile<0, pad_left, pad_bottom, 0>;
fs[2][1] = process_tile<0, 0, pad_bottom, 0>;
fs[2][2] = (pad_right) ? process_tile<0, 0, pad_bottom, 0> : process_tile<0, 0, pad_bottom, 1>;
} else {
constexpr int pad_bottom = 1;
fs[2][0] = process_tile<0, pad_left, pad_bottom, 0>;
fs[2][1] = process_tile<0, 0, pad_bottom, 0>;
fs[2][2] = (pad_right) ? process_tile<0, 0, pad_bottom, 0> : process_tile<0, 0, pad_bottom, 1>;
}
} else {
constexpr int pad_top = 1;
constexpr int pad_left = 1;
const int pad_right = input_shape.n_cols % 2 == 0;
fs[0][0] = process_tile<pad_top, pad_left, 0, 0>;
fs[0][1] = process_tile<pad_top, 0, 0, 0>;
fs[0][2] = (pad_right) ? process_tile<pad_top, 0, 0, 1> : process_tile<pad_top, 0, 0, 2>;
fs[1][0] = process_tile<0, pad_left, 0, 0>;
fs[1][1] = process_tile<0, 0, 0, 0>;
fs[1][2] = (pad_right) ? process_tile<0, 0, 0, 1> : process_tile<0, 0, 0, 2>;
if (input_shape.n_rows % 2 == 0) {
constexpr int pad_bottom = 1;
fs[2][0] = process_tile<0, pad_left, pad_bottom, 0>;
fs[2][1] = process_tile<0, 0, pad_bottom, 0>;
fs[2][2] = (pad_right) ? process_tile<0, 0, pad_bottom, 1> : process_tile<0, 0, pad_bottom, 2>;
} else {
constexpr int pad_bottom = 2;
fs[2][0] = process_tile<0, pad_left, pad_bottom, 0>;
fs[2][1] = process_tile<0, 0, pad_bottom, 0>;
fs[2][2] = (pad_right) ? process_tile<0, 0, pad_bottom, 1> : process_tile<0, 0, pad_bottom, 2>;
}
}
// Process each tile in turn
for (int batch = 0; batch < input_shape.n_batches; batch++) {
const T* const input_base_batch = inptr + batch*input_shape.n_rows*input_shape.n_cols*n_channels;
for (int tile_i = 0; tile_i < tile_M; tile_i++) {
const int row_offset = (tile_i == 0) ? 0 : ((padding_type == PADDING_VALID) ? 0 : 1);
const T* const input_base_row = input_base_batch + (2*tile_i - row_offset)*input_shape.n_cols*n_channels;
// Select the set of functions for the row
const int fs_i = (tile_i == 0) ? 0 : ((tile_i < tile_M - 1) ? 1 : 2);
for (int tile_j = 0; tile_j < tile_N; tile_j++) {
// Select the function for the column
const int fs_j = (tile_j == 0) ? 0 : ((tile_j < tile_N - 1) ? 1 : 2);
const auto f = fs[fs_i][fs_j];
// Get pointers into the input and outputs
const int col_offset = (tile_j == 0) ? 0 : ((padding_type == PADDING_VALID) ? 0 : 1);
const T* const input_base_col = input_base_row + (2*tile_j - col_offset)*n_channels;
T* const matrix_base = outptr_base + batch*matrix_batch_stride + (tile_i*tile_N + tile_j)*matrix_row_stride;
f(n_channels, input_base_col, input_row_stride, input_col_stride,
matrix_base, matrix_stride);
}
}
}
}
template <typename T>
template <const int pad_top,
const int pad_left,
const int pad_bottom,
const int pad_right>
void winograd::Winograd2x2_3x3GemmInputChannelwise<T>::process_tile(
int n_channels, // Number of channels in the tile
const T* const input_base,
const int input_row_stride,
const int input_col_stride,
T* const matrix_base,
const int matrix_stride
) {
// Copy pointers
const T *inptr = input_base;
T *outptr = matrix_base;
// Process channels (modifies inptr, outptr and n_channels)
_process_tile<pad_top, pad_left, pad_bottom, pad_right, 4>(
n_channels, inptr, input_row_stride, input_col_stride,
outptr, matrix_stride
);
_process_tile<pad_top, pad_left, pad_bottom, pad_right, 2>(
n_channels, inptr, input_row_stride, input_col_stride,
outptr, matrix_stride
);
_process_tile<pad_top, pad_left, pad_bottom, pad_right, 1>(
n_channels, inptr, input_row_stride, input_col_stride,
outptr, matrix_stride
);
}
template <typename T>
template <const int pad_top,
const int pad_left,
const int pad_bottom,
const int pad_right,
const int proc_channels>
void winograd::Winograd2x2_3x3GemmInputChannelwise<T>::_process_tile(
int &n_channels,
const T* &inptr, const int input_row_stride, const int input_col_stride,
T* &outptr, const int matrix_stride
) {
// We use 4 pointers to point at matrices 0, 4, 8 and 12 and use three
// offsets to access the intermediate matrices.
T* outptrs[4] = {
outptr,
outptr + matrix_stride * 4,
outptr + matrix_stride * 8,
outptr + matrix_stride * 12
};
// The matrix X; zeroed to account for padding.
T x[4][4];
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 4; j++) {
x[i][j] = 0;
}
}
// The matrices X.T x and U
T XTx[4][4], U[4][4];
// Now progress through each channel
for (; n_channels >= proc_channels; n_channels -= proc_channels) {
for (int n = 0; n < proc_channels; n++) {
// Load the matrix X
for (int cell_i = pad_top, i = 0; cell_i < 4 - pad_bottom; cell_i++, i++) {
for (int cell_j = pad_left, j = 0; cell_j < 4 - pad_right; cell_j++, j++) {
x[cell_i][cell_j] = inptr[i*input_row_stride + j*input_col_stride];
}
}
inptr++;
// Compute the matrix X.T
for (int j = 0; j < 4; j++) {
XTx[0][j] = x[0][j] - x[2][j];
XTx[1][j] = x[1][j] + x[2][j];
XTx[2][j] = x[2][j] - x[1][j];
XTx[3][j] = x[1][j] - x[3][j];
}
// Hence compute the matrix U
for (int i = 0; i < 4; i++) {
U[i][0] = XTx[i][0] - XTx[i][2];
U[i][1] = XTx[i][1] + XTx[i][2];
U[i][2] = XTx[i][2] - XTx[i][1];
U[i][3] = XTx[i][1] - XTx[i][3];
}
// Store the matrix U
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 4; j++) {
outptrs[i][j * matrix_stride] = U[i][j];
}
outptrs[i]++;
}
}
}
// Update the output pointer for future calls
outptr = outptrs[0];
}