b4a99f1cd6
If we have a build_vector such as [i64 0, i64 3, i64 1, i64 2], we instead lower this as vsext([i8 0, i8 3, i8 1, i8 2]). For vectors with 4 or fewer elements, the resulting narrow vector can be generated via scalar materialization. For shuffles which get lowered to vrgathers, constant build_vectors of small constants are idiomatic. As such, this change covers all shuffles with an output type of 4 or less. I deliberately started narrow here. I think it makes sense to expand this to longer vectors, but we need a more robust profit model on the recursive expansion. It's questionable if we want to do the zsext if we're going to generate a constant pool load for the narrower type anyways. One possibility for future exploration is to allow the narrower VT to be less than 8 bits. We can't use vsext for that, but we could use something analogous to our widening interleave lowering with some extra shifts and ands.
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