[llvm] [RISCV] Extract subregister if VLEN is known when lowering extract_subvector (PR #65392)

[Luke Lau via llvm-commits]

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@@ -8653,36 +8656,68 @@ SDValue RISCVTargetLowering::lowerEXTRACT_SUBVECTOR(SDValue Op,
     return DAG.getBitcast(Op.getValueType(), Slidedown);
   }
 
+  if (VecVT.isFixedLengthVector()) {
+    VecVT = getContainerForFixedLengthVector(VecVT);
+    Vec = convertToScalableVector(VecVT, Vec, DAG, Subtarget);
+  }
+
+  // The semantics of extract_subvector are that if the extracted subvector is
+  // scalable, then the index is scaled by vscale. So if we have a fixed length
+  // subvector, we need to factor that in before we decompose it to
+  // subregisters...
+  MVT ContainerSubVecVT = SubVecVT;
+  unsigned EffectiveIdx = OrigIdx;
+  unsigned Vscale = *KnownVLen / RISCV::RVVBitsPerBlock;
+  if (SubVecVT.isFixedLengthVector()) {
+    assert(KnownVLen);
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lukel97 wrote:

IIUC `*` on a null optional is just UB, you need to use `value()` to have an exception thrown. This part is a bit hairy though, I'm avoiding `value()` so I don't have to define `Vscale` twice. Any ideas on a better way to structure this?

https://github.com/llvm/llvm-project/pull/65392