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LibVideo/VP9: Make a lookup table for bit reversals

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Bit reversals are used very often in intra-predicted frames. Turning
these into a constexpr lookup table reduces the branching needed for
block transforms significantly. This reduces the times spent decoding
an intra-heavy 1080p video by about 9% (~14.3s -> ~12.9s).
This commit is contained in:
Zaggy1024 2023-04-16 10:03:21 -05:00 • committed by Tim Flynn
parent f6764beead
commit 6e6cc1ddb2
2 changed files with 21 additions and 10 deletions
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4
Userland/Libraries/LibVideo/VP9/Decoder.cpp
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@ -1400,7 +1400,7 @@ inline DecoderErrorOr<void> Decoder::inverse_discrete_cosine_transform_array_per
// 1.2. T[ i ] is set equal to copyT[ brev( n, i ) ] for i = 0..((1<<n) - 1).
for (auto i = 0u; i < block_size; i++)
data[i] = data_copy[brev(log2_of_block_size, i)];
data[i] = data_copy[brev<log2_of_block_size>(i)];
return {};
}
@ -1432,7 +1432,7 @@ inline DecoderErrorOr<void> Decoder::inverse_discrete_cosine_transform(Span<Inte
// 2.6 Invoke B( n1+i, n0-1-i, 32-brev( 5, n1+i), 0 ) for i = 0..(n2-1).
for (auto i = 0u; i < quarter_block_size; i++) {
auto index = half_block_size + i;
butterfly_rotation_in_place(data, index, block_size - 1 - i, 32 - brev(5, index), false);
butterfly_rotation_in_place(data, index, block_size - 1 - i, 32 - brev<5>(index), false);
}
// 2.7 If n is greater than or equal to 3: