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Engineering a fixed-width bit-packed integer vector in Rust

Engineering a fixed-width bit-packed integer vector in Rust

2 comments

·September 24, 2025
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Const-me

The support for BMI1 instruction set extension is almost universal by now. The extension was introduced in AMD Jaguar and Intel Haswell, both launched in 2013 i.e. 12 years ago.

Instead of doing stuff like (word >> bit_offset) & self.mask, in C or C++ I usually write _bextr_u64, or when using modern C# Bmi1.X64.BitFieldExtract. Note however these intrinsics compile into 2 instructions not one, because the CPU instruction takes start/length arguments from lower/higher bytes of a single 16-bit number.

pjsg

I suspect that this unaligned read apporach doesn't work for a bit length of 59, 61, 62 and 63.

In the case of 63, reading out value at offset 1, requires two reads as it need 1 bit from the first byte, then 56 bits from the next 7 bytes and finally 6 bits from the 9th byte. Nine bytes cannot be loaded in a single u64 load.