[llvm] [RISCV] Decompose single source shuffles (without exact VLEN) (PR #126108)

[Philip Reames via llvm-commits]

preames wrote:

Well, the good news, I definitely found the bug.  I'm more than a bit embarrassed by this one, the code just doesn't do what the comments or the review said it did.  The code was using each source register in the source group, not the low register as it should have been.  This results in a completely wrong result, including all of the tests changed in the review.  I'm not sure how I screwed this up - I know I had a working version before splitting and rebasing - but yeah, I definitely screwed that up.  

I'm going to reopen the review as this seems beyond a reasonable fix and reland.  


https://github.com/llvm/llvm-project/pull/126108