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lemire / FastPFor

The FastPFOR C++ library: Fast integer compression
Apache License 2.0
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an equivalent code with x15 times less lines #31

Closed romange closed 7 years ago

romange commented 7 years ago

Checked with gcc -03 -S that the code generated is absolutely the same.

clang-3.8 returns differences in the assembler but from what i saw it's mostly different registers and slightly changed order. The number of instructions is the same in both versions.

lemire commented 7 years ago

If only I had some C++ points to award!

romange commented 7 years ago

Thanks! The main challenge was to divide the unrolled code into generic invariants. I did it by making each step to fully handle one integer - input for packing and output for unpacking. Once this was done the rest was easy. The template part is relatively simple thanks to c++11. Also, I used some plain "if" constructs where I could knowing that compiler's constant propagation would eliminate them completely.