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LibVideo/VP9: Make inter-prediction fast path accumulators 32-bit

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Some occasional cases could cause the accumulator to overflow and have
an incorrect result. It would be nice to use a smaller accumulator, but
it seems not to be correct. :^(

We now cast to i16 to allow 128-bit vectorization to make use of one
whole register instead of having to split the loop into multiple.

This results in about a 5% reduction in performance in my testing.
This commit is contained in:
Zaggy1024 2023-04-25 20:58:14 -05:00 • committed by Andreas Kling
parent 3a5d20d82b
commit 2ec043c4db
1 changed files with 11 additions and 7 deletions
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18
Userland/Libraries/LibVideo/VP9/Decoder.cpp
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@ -959,9 +959,13 @@ DecoderErrorOr<void> Decoder::predict_inter_block(u8 plane, BlockContext const&
auto const bit_depth = block_context.frame_context.color_config.bit_depth;
auto const* reference_start = reference_frame_buffer.data() + reference_block_y * reference_frame_width + reference_block_x;
// FIXME: We are using 16-bit accumulators for speed in these loops, but when accumulating for a high bit-depth video, they will overflow.
// Instead of hardcoding them, the Decoder class should have the bit depth as a template parameter, and the accumulators can select
// a size based on whether the bit depth > 8.
// FIXME: We are using 16-bit products to vectorize the filter loops, but when filtering in a high bit-depth video, they will truncate.
// Instead of hardcoding them, we should have the bit depth as a template parameter, and the accumulators can select a size based
// on whether the bit depth > 8.
// Note that we only get a benefit from this on the default CPU target. If we enable AVX2 here, we may want to specialize the
// function for the CPU target and remove the cast to i16 so that it doesn't have to truncate on AVX2, where it can do the full
// unrolled 32-bit product loops in one vector.
if (unscaled_x && unscaled_y && bit_depth == 8) {
if (copy_x && copy_y) {
// We can memcpy here to avoid doing any real work.
@ -983,10 +987,10 @@ DecoderErrorOr<void> Decoder::predict_inter_block(u8 plane, BlockContext const&
for (auto row = 0u; row < height; row++) {
for (auto column = 0u; column < width; column++) {
i16 accumulated_samples = 0;
i32 accumulated_samples = 0;
for (auto t = 0; t < 8; t++) {
auto sample = source[t];
accumulated_samples += subpel_filters[filter][subpixel_x][t] * sample;
accumulated_samples += static_cast<i16>(subpel_filters[filter][subpixel_x][t] * sample);
}
*destination = clip_1(bit_depth, rounded_right_shift(accumulated_samples, 7));
@ -1008,10 +1012,10 @@ DecoderErrorOr<void> Decoder::predict_inter_block(u8 plane, BlockContext const&
for (auto row = 0u; row < height; row++) {
for (auto column = 0u; column < width; column++) {
auto const* scan_column = source;
i16 accumulated_samples = 0;
i32 accumulated_samples = 0;
for (auto t = 0; t < 8; t++) {
auto sample = *scan_column;
accumulated_samples += subpel_filters[filter][subpixel_y][t] * sample;
accumulated_samples += static_cast<i16>(subpel_filters[filter][subpixel_y][t] * sample);
scan_column += source_stride;
}
*destination = clip_1(bit_depth, rounded_right_shift(accumulated_samples, 7));