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AK: Optimize BitStream refilling a bit further

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This tries to optimize the refill code by making it easier to digest for
the branch predictor. This includes not looping as much across function
calls and marking our EOF case to be unlikely.

Co-Authored-By: Lucas Chollet <lucas.chollet@free.fr>
This commit is contained in:
Tim Schumacher 2023-11-28 13:06:19 +01:00 committed by Andreas Kling
parent 197331c922
commit e9e89d7e4e
Notes: sideshowbarker 2024-07-17 09:49:48 +09:00 
Author: https://github.com/timschumi
Commit: https://github.com/SerenityOS/serenity/commit/e9e89d7e4e
Pull-request: https://github.com/SerenityOS/serenity/pull/21890
Issue: https://github.com/SerenityOS/serenity/issues/21869
Reviewed-by: https://github.com/IdanHo
Reviewed-by: https://github.com/LucasChollet
Reviewed-by: https://github.com/trflynn89
1 changed files with 20 additions and 19 deletions
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39
AK/BitStream.h
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@ -215,15 +215,8 @@ public:
template<Unsigned T = u64>
ErrorOr<T> peek_bits(size_t count)
{
while (count > m_bit_count) {
if (TRY(refill_buffer_from_stream()))
continue;
if (m_unsatisfiable_read_behavior == UnsatisfiableReadBehavior::Reject)
return Error::from_string_literal("Reached end-of-stream without collecting the required number of bits");
break;
}
if (count > m_bit_count)
TRY(refill_buffer_from_stream(count));
return m_bit_buffer & lsb_mask<T>(min(count, m_bit_count));
}
@ -254,21 +247,29 @@ public:
}
private:
ErrorOr<bool> refill_buffer_from_stream()
ErrorOr<void> refill_buffer_from_stream(size_t requested_bit_count)
{
if (m_stream->is_eof())
return false;
while (requested_bit_count > m_bit_count) [[likely]] {
if (m_stream->is_eof()) [[unlikely]] {
if (m_unsatisfiable_read_behavior == UnsatisfiableReadBehavior::FillWithZero) {
m_bit_count = requested_bit_count;
return {};
}
size_t bits_to_read = bit_buffer_size - m_bit_count;
size_t bytes_to_read = bits_to_read / bits_per_byte;
return Error::from_string_literal("Reached end-of-stream without collecting the required number of bits");
}
BufferType buffer = 0;
auto bytes = TRY(m_stream->read_some({ &buffer, bytes_to_read }));
size_t bits_to_read = bit_buffer_size - m_bit_count;
size_t bytes_to_read = bits_to_read / bits_per_byte;
m_bit_buffer |= (buffer << m_bit_count);
m_bit_count += bytes.size() * bits_per_byte;
BufferType buffer = 0;
auto bytes = TRY(m_stream->read_some({ &buffer, bytes_to_read }));
return true;
m_bit_buffer |= (buffer << m_bit_count);
m_bit_count += bytes.size() * bits_per_byte;
}
return {};
}
UnsatisfiableReadBehavior m_unsatisfiable_read_behavior;