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sccn / liblsl

C++ lsl library for multi-modal time-synched data transmission over the local network
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Move sample copying/conversion to sample.cpp, use template overloading #132

Closed tstenner closed 2 years ago

tstenner commented 3 years ago

A sample has a char* array to store the values, but the size and type isn't known at compile time. The values are iterated over in a cast-happy C++98 for-loop: for (std::string *p = (std::string *)&data_, *e = p + num_channels_; p < e; ++p). Also, sample data can be copied in / out in formats different from the native sample format.

In order to simplify the code, this PR adds a helper class and helper methods, so for (std::string *p = (std::string *)&data_, *e = p + num_channels_; p < e; ++p) { *p = …; } becomes for (auto &val : samplevals<std::string>(*this)) { val = …;}. It also adds two methods: conv_from(T* src) copies the values (converting types if necessary) into the sample data, conv_into(T* dst) copies the sample data into dst (again, converting the types if necessary). The optimal copying / conversion strategy is found via template overloading.

Even though the implementation is fully in sample.cpp instead of sample.h, the throughput in a benchmark improved by ~3%.

tstenner commented 2 years ago

Some real-world data: with the changes above, the compiler optimized/inlined so much a sampling profiler couldn't detect any of the calls between stream_outlet_impl::enqueue and the AVX accelerated memmove instructions.