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AK: Discard bits from LittleEndianInputBitStream as they are read

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Rather than tracking our position in the bit buffer, we can simply shift
away the bits that we read. This is mostly for simplicity, but also does
help performance a bit.

Using the "enwik8" file as a test (100MB uncompressed, commonly used in
benchmarks: https://www.mattmahoney.net/dc/enwik8.zip), compression time
decreases from:

    3.96s to 3.79s on Serenity (cold)
    1.08s to 1.04s on Serenity (warm)
    0.83s to 0.82s on Linux
This commit is contained in:
Timothy Flynn 2023-05-18 07:23:49 -04:00 committed by Tim Flynn
parent 0c53b02149
commit 70c977aa56
Notes: sideshowbarker 2024-07-17 16:23:55 +09:00 
Author: https://github.com/trflynn89
Commit: https://github.com/SerenityOS/serenity/commit/70c977aa56
Pull-request: https://github.com/SerenityOS/serenity/pull/18901
Reviewed-by: https://github.com/kleinesfilmroellchen ✅
1 changed files with 3 additions and 13 deletions
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16
AK/BitStream.h
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@ -138,17 +138,11 @@ protected:
return bits == 0 ? 0 : max >> (digits - bits);
}
ALWAYS_INLINE BufferType lsb_aligned_buffer() const
{
return m_bit_offset == bit_buffer_size ? 0 : m_bit_buffer >> m_bit_offset;
}
ALWAYS_INLINE bool is_aligned_to_byte_boundary() const { return m_bit_count % bits_per_byte == 0; }
MaybeOwned<Stream> m_stream;
BufferType m_bit_buffer { 0 };
u8 m_bit_offset { 0 };
u8 m_bit_count { 0 };
};
@ -217,7 +211,7 @@ public:
if (count > m_bit_count)
TRY(refill_buffer_from_stream());
return lsb_aligned_buffer() & lsb_mask<T>(min(count, m_bit_count));
return m_bit_buffer & lsb_mask<T>(min(count, m_bit_count));
}
ALWAYS_INLINE void discard_previously_peeked_bits(u8 count)
@ -226,7 +220,7 @@ public:
if (count > m_bit_count)
count = m_bit_count;
m_bit_offset += count;
m_bit_buffer >>= count;
m_bit_count -= count;
}
@ -236,9 +230,6 @@ public:
{
u8 remaining_bits = 0;
m_bit_buffer = lsb_aligned_buffer();
m_bit_offset = 0;
if (auto offset = m_bit_count % bits_per_byte; offset != 0) {
remaining_bits = m_bit_buffer & lsb_mask<u8>(offset);
discard_previously_peeked_bits(offset);
@ -256,9 +247,8 @@ private:
BufferType buffer = 0;
auto bytes = TRY(m_stream->read_some({ &buffer, bytes_to_read }));
m_bit_buffer = (buffer << m_bit_count) | lsb_aligned_buffer();
m_bit_buffer |= (buffer << m_bit_count);
m_bit_count += bytes.size() * bits_per_byte;
m_bit_offset = 0;
return {};
}