[llvm] [RISCV][GISEL] Legalize G_EXTRACT_SUBVECTOR (PR #109426)

[Craig Topper via llvm-commits]

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@@ -3666,6 +3666,63 @@ LegalizerHelper::bitcastConcatVector(MachineInstr &MI, unsigned TypeIdx,
   return Legalized;
 }
 
+/// This attempts to bitcast G_EXTRACT_SUBVECTOR to CastTy.
+///
+///  <vscale x 8 x i1> = G_EXTRACT_SUBVECTOR <vscale x 16 x i1>, N
+///
+/// ===>
+///
+///  <vscale x 2 x i1> = G_BITCAST <vscale x 16 x i1>
+///  <vscale x 1 x i8> = G_EXTRACT_SUBVECTOR <vscale x 2 x i1>, N / 8
+///  <vscale x 8 x i1> = G_BITCAST <vscale x 1 x i8>
+LegalizerHelper::LegalizeResult
+LegalizerHelper::bitcastExtractSubvector(MachineInstr &MI, unsigned TypeIdx,
+                                         LLT CastTy) {
+  auto ES = dyn_cast<GExtractSubvector>(&MI);
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topperc wrote:

If its not possible to hit we should just use `cast`. If it is possible, then it should have a test. Otherwise it's an untested path.

https://github.com/llvm/llvm-project/pull/109426