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LibDEFLATE: Optimize decompression

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Instead of calculating bit-by-bit crc32, we now calculate a lookup table
during compile time. The old crc32 calculation was taking almost 50% of
the decompression time.

Also handle multiple symbols at once without outputting to user. It is
much more efficient to output many bytes instead of the up to 258 that a
single symbol can decode to :^)
This commit is contained in:
Bananymous
2026-04-14 01:50:30 +03:00
parent d471bbf856
commit 3ebadc5c74
1 changed files with 33 additions and 17 deletions
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40
userspace/libraries/LibDEFLATE/Decompressor.cpp
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@@ -22,6 +22,26 @@ namespace LibDEFLATE
}; };
}; };
struct crc32_table_t
{
consteval crc32_table_t()
{
for (uint32_t i = 0; i < 256; i++)
{
uint32_t crc32 = i;
for (size_t j = 0; j < 8; j++) {
if (crc32 & 1)
crc32 = (crc32 >> 1) ^ 0xEDB88320;
else
crc32 >>= 1;
}
table[i] = crc32;
}
}
uint32_t table[256];
};
static constexpr crc32_table_t s_crc32_table;
BAN::ErrorOr<uint16_t> Decompressor::read_symbol(const HuffmanTree& tree) BAN::ErrorOr<uint16_t> Decompressor::read_symbol(const HuffmanTree& tree)
{ {
const uint8_t instant_bits = tree.instant_bits(); const uint8_t instant_bits = tree.instant_bits();
@@ -177,7 +197,7 @@ namespace LibDEFLATE
m_stream.skip_to_byte_boundary(); m_stream.skip_to_byte_boundary();
auto& gzip = m_stream_info.gzip; auto& gzip = m_stream_info.gzip;
gzip.crc32 = ~gzip.crc32; gzip.crc32 ^= 0xFFFFFFFF;
const uint32_t crc32 = const uint32_t crc32 =
static_cast<uint32_t>(TRY(m_stream.take_bits(16))) | static_cast<uint32_t>(TRY(m_stream.take_bits(16))) |
@@ -370,16 +390,7 @@ namespace LibDEFLATE
auto& gzip = m_stream_info.gzip; auto& gzip = m_stream_info.gzip;
gzip.isize += to_write; gzip.isize += to_write;
for (size_t i = 0; i < to_write; i++) for (size_t i = 0; i < to_write; i++)
{ gzip.crc32 = (gzip.crc32 >> 8) ^ s_crc32_table.table[(gzip.crc32 ^ output[i]) & 0xFF];
gzip.crc32 ^= output[i];
for (size_t j = 0; j < 8; j++) {
if (gzip.crc32 & 1)
gzip.crc32 = (gzip.crc32 >> 1) ^ 0xEDB88320;
else
gzip.crc32 >>= 1;
}
}
break; break;
} }
} }
@@ -515,7 +526,7 @@ namespace LibDEFLATE
bool need_more_input = false; bool need_more_input = false;
bool restore_saved_stream = false; bool restore_saved_stream = false;
const auto saved_stream = m_stream; auto saved_stream = m_stream;
switch (m_state) switch (m_state)
{ {
@@ -645,12 +656,17 @@ namespace LibDEFLATE
} }
case State::Symbol: case State::Symbol:
{ {
while (m_produced_bytes + 258 < total_window_size && m_state == State::Symbol)
{
saved_stream = m_stream;
if (auto ret = handle_symbol(); ret.is_error()) if (auto ret = handle_symbol(); ret.is_error())
{ {
if (ret.error().get_error_code() != ENOBUFS) if (ret.error().get_error_code() != ENOBUFS)
return ret.release_error(); return ret.release_error();
need_more_input = true; need_more_input = true;
restore_saved_stream = true; restore_saved_stream = true;
break;
}
} }
break; break;
} }