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Improve decompression in readbig to avoid unnecessary memcpy (#36095)

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jasperzhu 2022-05-02 21:06:52 +08:00 committed by GitHub
parent 7a54a3c07b
commit f4b275c4ac
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1 changed files with 21 additions and 7 deletions
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28
src/Compression/CompressedReadBufferFromFile.cpp
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@ -117,6 +117,25 @@ size_t CompressedReadBufferFromFile::readBig(char * to, size_t n)
bytes_read += size_decompressed;
bytes += size_decompressed;
}
else if (nextimpl_working_buffer_offset > 0)
{
/// Need to skip some bytes in decompressed data (seek happened before readBig call).
size_compressed = new_size_compressed;
bytes += offset();
/// This is for clang static analyzer.
assert(size_decompressed + additional_size_at_the_end_of_buffer > 0);
memory.resize(size_decompressed + additional_size_at_the_end_of_buffer);
working_buffer = Buffer(memory.data(), &memory[size_decompressed]);
decompress(working_buffer, size_decompressed, size_compressed_without_checksum);
/// Read partial data from first block. Won't run here at second block.
/// Avoid to call nextImpl and unnecessary memcpy in read when the second block fits entirely to output buffer.
size_t size_partial = std::min((size_decompressed - nextimpl_working_buffer_offset),(n - bytes_read));
pos = working_buffer.begin() + nextimpl_working_buffer_offset;
nextimpl_working_buffer_offset = 0;
bytes_read += read(to + bytes_read, size_partial);
}
else
{
size_compressed = new_size_compressed;
@ -124,17 +143,12 @@ size_t CompressedReadBufferFromFile::readBig(char * to, size_t n)
/// This is for clang static analyzer.
assert(size_decompressed + additional_size_at_the_end_of_buffer > 0);
memory.resize(size_decompressed + additional_size_at_the_end_of_buffer);
working_buffer = Buffer(memory.data(), &memory[size_decompressed]);
decompress(working_buffer, size_decompressed, size_compressed_without_checksum);
/// Manually take nextimpl_working_buffer_offset into account, because we don't use
/// nextImpl in this method.
pos = working_buffer.begin() + nextimpl_working_buffer_offset;
nextimpl_working_buffer_offset = 0;
///Read partial data from last block.
pos = working_buffer.begin();
bytes_read += read(to + bytes_read, n - bytes_read);
break;
}