[llvm] [RISCV] Increase default tail duplication threshold to 6 at -O3 (PR #98873)

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

> Sorry for the delay, I've been running SPEC CPU 2017 intrate with the changes but the results are a bit noisy, the standard deviation is about +-1%. There's been no noticeable regressions anyway (and no improvements either but I wouldn't expect any given that it's dual-issue)

Thanks a lot! I just got my Spacemit K1 laptops. :-)
I was thinking that maybe we should make tail dup size a CPU-specific value so that we can set a fine-tuned value to different CPUs, just like existing `MinimumJumpTableEntries`, etc.

https://github.com/llvm/llvm-project/pull/98873