[llvm] [WebAssembly] Mask undef shuffle lanes (PR #149084)

[Thomas Lively via llvm-commits]

tlively wrote:

> As you may have noticed, I've found WebAssembly shuffles to be a real pain! I would really like to see a revision to the spec so that these undef lanes/bytes can be explicitly encoded :)

If we did add such an extension to the shuffle instruction, we would still have to specify what value ends up in the lanes of the result. Would it be portable and fast if we specified that the "undef" lanes all end up containing zeros, for instance?

https://github.com/llvm/llvm-project/pull/149084