LibVideo/VP9: Clamp reference frame prediction coords outside loops

Moving the clamping of the coordinates of the reference frame samples
as well as some bounds checks outside of the loop reduces the branches
needed in the `predict_inter_block()` significantly.

This results in a whopping ~41% improvement in decode performance
of an inter-prediction-heavy YouTube video (~35.4s -> ~20.8s).

1 files changed, 12 insertions, 5 deletions

diff --git a/Userland/Libraries/LibVideo/VP9/Decoder.cpp b/Userland/Libraries/LibVideo/VP9/Decoder.cpp
index 70f098742d..7c740b1425 100644
--- a/Userland/Libraries/LibVideo/VP9/Decoder.cpp
+++ b/Userland/Libraries/LibVideo/VP9/Decoder.cpp
@@ -939,15 +939,19 @@ DecoderErrorOr<void> Decoder::predict_inter_block(u8 plane, BlockContext const&
         return intermediate_buffer[row * width + column];
     };
 
+    // Check our reference frame bounds before starting the loop.
+    reference_frame_buffer_at(scaled_bottom, scaled_right);
+
     for (auto row = 0u; row < intermediate_height; row++) {
+        auto clamped_row = static_cast<size_t>(clip_3(0, scaled_bottom, (offset_scaled_block_y >> 4) + static_cast<i32>(row) - 3));
+        u16 const* scan_line = &reference_frame_buffer_at(clamped_row, 0);
+
         for (auto column = 0u; column < width; column++) {
             auto samples_start = offset_scaled_block_x + static_cast<i32>(scaled_step_x * column);
 
             i32 accumulated_samples = 0;
             for (auto t = 0u; t < 8u; t++) {
-                auto sample = reference_frame_buffer_at(
-                    clip_3(0, scaled_bottom, (offset_scaled_block_y >> 4) + static_cast<i32>(row) - 3),
-                    clip_3(0, scaled_right, (samples_start >> 4) + static_cast<i32>(t) - 3));
+                auto sample = scan_line[clip_3(0, scaled_right, (samples_start >> 4) + static_cast<i32>(t) - 3)];
                 accumulated_samples += subpel_filters[block_context.interpolation_filter][samples_start & 15][t] * sample;
             }
             intermediate_buffer_at(row, column) = clip_1(block_context.frame_context.color_config.bit_depth, rounded_right_shift(accumulated_samples, 7));
@@ -957,11 +961,14 @@ 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 samples_start = (offset_scaled_block_y & 15) + static_cast<i32>(scaled_step_y * row);
+            auto const* scan_column = &intermediate_buffer_at(samples_start >> 4, column);
+            auto const* subpel_filters_for_samples = subpel_filters[block_context.interpolation_filter][samples_start & 15];
 
             i32 accumulated_samples = 0;
             for (auto t = 0u; t < 8u; t++) {
-                auto sample = intermediate_buffer_at((samples_start >> 4) + t, column);
-                accumulated_samples += subpel_filters[block_context.interpolation_filter][samples_start & 15][t] * sample;
+                auto sample = *scan_column;
+                accumulated_samples += subpel_filters_for_samples[t] * sample;
+                scan_column += width;
             }
             block_buffer_at(row, column) = clip_1(block_context.frame_context.color_config.bit_depth, rounded_right_shift(accumulated_samples, 7));
         }