Stream: beginners

Topic: U32 endiannes conversion?

Matthieu Pizenberg (May 24 2024 at 14:07):

What’s the simplest and/or most efficient way to convert a U32 (big endian by default) into its little endian equivalent?

Matthieu Pizenberg (May 24 2024 at 14:20):

For now, I’ve written the following:

littleendian : U32 -> U32
littleendian = \b ->
    b3 = Num.shiftRightZfBy b 24
    b2 = Num.shiftRightZfBy (Num.shiftLeftBy b 8) 24
    b1 = Num.shiftRightZfBy (Num.shiftLeftBy b 16) 24
    b0 = Num.shiftRightZfBy (Num.shiftLeftBy b 24) 24
    Num.shiftLeftBy b0 24 + Num.shiftLeftBy b1 16 + Num.shiftLeftBy b2 8 + b3

Brendan Hansknecht (May 24 2024 at 14:29):

Yeah, bit shifting is the way. Personally, I prefer bitwise or to addition, but would need to play around with exact assembly to find the fastest way (and tight benchmarks).

Brendan Hansknecht (May 24 2024 at 14:30):

Also, I would expect bitwise and with 0xFF to be more likely to optimize correctly than shifting back and forth. But again, just a guess and llvm is really smart.

Matthieu Pizenberg (May 24 2024 at 14:32):

Thanks, yeah the performance part is just out of curiosity. I’m implementing salsa20 for the learning experience and not for production ready code.

Last updated: Jul 06 2025 at 12:14 UTC