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

[Michael Maitland via llvm-commits]

================
@@ -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);
+  if (!ES)
+    return UnableToLegalize;
+
+  if (!CastTy.isVector())
+    return UnableToLegalize;
+
+  if (TypeIdx != 0)
+    return UnableToLegalize;
+
+  Register Dst = ES->getReg(0);
+  Register Src = ES->getSrcVec();
+  uint64_t Idx = ES->getIndexImm();
+
+  MachineRegisterInfo &MRI = *MIRBuilder.getMRI();
+
+  LLT DstTy = MRI.getType(Dst);
+  LLT SrcTy = MRI.getType(Src);
+  ElementCount DstTyEC = DstTy.getElementCount();
+  ElementCount SrcTyEC = SrcTy.getElementCount();
+  auto DstTyMinElts = DstTyEC.getKnownMinValue();
+  auto SrcTyMinElts = SrcTyEC.getKnownMinValue();
+
+  if (DstTy == CastTy)
+    return Legalized;
+
+  auto AdjustAmt = CastTy.getScalarType().getSizeInBits();
+  if (DstTyMinElts < AdjustAmt || SrcTyMinElts < AdjustAmt)
----------------
michaelmaitland wrote:

In selectiondag:

```
if (VecVT.getVectorMinNumElements() >= 8 && SubVecVT.getVectorMinNumElements() >= 8) {
  assert(OrigIdx % 8 == 0 && "Invalid index");
  assert(VecVT.getVectorMinNumElements() % 8 == 0 && SubVecVT.getVectorMinNumElements() % 8 == 0 && "Unexpected mask vector lowering");
  ...
}
```

I was bringing this over. It's probably redundant in both cases?

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