Support ARM v7+

[dblalock]

At present, Bolt is only implemented for x86 machines with AVX2 instructions. Adding support for other architectures would entail reimplementing the code in bolt.hpp and adding #ifdefs to select the appropriate implementation for the target architecture.

[zinovya]

@dblalock, This issue still seems to be applicable. I want to compiled madness algorithm for RISC-V architecture and I think i will have the same problem there as AVX2 instructions does not exist there. I would be interested to contribute if you have some ideas how to resolve it.

[dblalock]

Hi @zinovya. RISC-V support would be really cool. I've personally never used it, but I think it's just a matter of porting all the mithral C++ functions. There's a decent amount of indirection regarding vector widths, strides, etc, so it shouldn't be terrible to port. The main subtleties will be making sure the right instructions get emitted--e.g., I found that my compiler refused to emit vpavgb, which required me to use inline asm to get decent performance.

[jan-wassenberg]

Hi, saw this via HN. Congrats, very impressive result @dblalock!
Our github.com/google/highway might be useful for porting - it provides cross-platform intrinsics which generally map closely to x86 intrinsics. For example, PSHUFB is TableLookupBytes (or TableLookupBytesOr0 if you care about the zeroing behavior as well).
We support Arm v7, RISC-V V, SSE4/AVX2/AVX-512, SVE etc. Happy to discuss if you're interested.

[dblalock]

Thanks for reaching out! I'm personally unlikely to do this in the foreseeable future since I'm not really adding new code anymore, but great to have this reference here!