[llvm] [RISCV][DAG][TLI] Avoid scalarizing length decreasing shuffles (PR #115532)

[Matt Arsenault via llvm-commits]

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@@ -4181,11 +4181,26 @@ void SelectionDAGBuilder::visitShuffleVector(const User &I) {
     return;
   }
 
-  // We can't use either concat vectors or extract subvectors so fall back to
+  // If the target prefers, emit a padded shuffle vector at the
+  // source operand width, then extract the original destination
+  // type.
+  if (!TLI.shouldScalarizeLengthDecreasingShuffle()) {
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arsenm wrote:

> . IR supports mixed length shuffles; DAG does not.

Then it should. There is a complexity cost to structural deviations in the second IR 


https://github.com/llvm/llvm-project/pull/115532