Proposal: Add RISC-V Optimization for XXH_mult32to64_add64 with Inline Assembly

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Hi @Cyan4973,

I’ve been working on compiling xxHash as part of CouchDB on a RISC-V machine (64-bit, riscv64-unknown-linux-gnu target). While reviewing `xxhash.h`, I noticed there’s no specific configuration or optimization for RISC-V, unlike the AArch64 case which uses `umaddl` for `XXH_mult32to64_add64`. Given RISC-V’s increasing relevance, I thought it might benefit from a tailored optimization.

To address this, I added a RISC-V-specific version of `XXH_mult32to64_add64` using inline assembly with `slli` and `srli` to mask inputs to 32 bits, followed by `mul` and `add`. Here’s the implementation I’ve tested:

```c
#elif defined(__riscv) && (__riscv_xlen == 64)
XXH_FORCE_INLINE xxh_u64
XXH_mult32to64_add64(xxh_u64 lhs, xxh_u64 rhs, xxh_u64 acc)
{
xxh_u64 ret;
__asm__ (
"slli t0, %1, 32 \n\t"
"srli t0, t0, 32 \n\t"
"slli t1, %2, 32 \n\t"
"srli t1, t1, 32 \n\t"
"mul %0, t0, t1 \n\t"
"add %0, %0, %3"
: "=r" (ret)
: "r" (lhs), "r" (rhs), "r" (acc)
: "t0", "t1"
);
return ret;
}
```

I built this within CouchDB on a RISC-V QEMU environment, disassembled the resulting `exxhash.so`, and confirmed the assembly matches my intent (e.g., `slli t0, a0, 32; srli t0, t0, 32; ...` appearing in `XXH3_hashLong_64b_default`).

Before I commit this change and create a pull request, I wanted to get your thoughts:

Do you think this RISC-V optimization aligns with xxHash’s goals?
Are there specific tests or benchmarks you’d recommend I run to validate it further?
Any suggestions for refining the implementation (e.g., alternative instructions or constraints)?
I’d be happy to provide more details or adjust based on your feedback. Thanks for your time and for maintaining this great library!

Best regards,
Jay

The proposed change belongs in xxhash.h alongside the AArch64 implementation of XXH_mult32to64_add64. Start by reviewing that existing case and the generated XXH3_hashLong_64b_default code, then build the riscv64-unknown-linux-gnu target in the CouchDB/QEMU setup. Done means the implementation is validated for correctness and its generated exxhash.so assembly and performance are compared.

Written by the indexing model from the issue text.