Convert between big-endian and little-endian on RISC-V

Question

What is the simplest way to work with big-endian values in RISC-V at the assembly language level? That is, how to load a big-endian value from memory into a register, work w

Answer 1

Note that while it's nice a pretty and convenient to have an instruction to do it, the __bswapsi2 function used in other answers will run at around 400 MB/s on a 1.5 GHz HiFive Unleashed, which is quite a lot faster than the gigE interface is ever going to moved data around.

Even on the HiFive1 running at the default 256 MHz it will do 60 MB/s and you've only got 16 KB of RAM and a bunch of GPIOs that you're not going to wiggle at more than a few MHz or maybe 10s of MHz.

I'm on the BitManipulation working group. The full GREV instruction needs a fair bit of hardware (something close to a multiplier) so small microcontrollers might never include it. However we're planing to use the same GREVI opcodes that give full word bit reversal and byte order reversal and implement them as simpler special-case instructions that don't need much circuitry and hopefully everyone will include them.