[llvm] [VPlan] Add ExtractLane VPInst to extract across multiple parts. (PR #148817)

[Florian Hahn via llvm-commits]

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@@ -860,6 +860,31 @@ Value *VPInstruction::generate(VPTransformState &State) {
       Res = Builder.CreateOr(Res, State.get(Op));
     return Builder.CreateOrReduce(Res);
   }
+  case VPInstruction::ExtractLane: {
+    Value *LaneToExtract = State.get(getOperand(0), true);
+    Type *IdxTy = State.TypeAnalysis.inferScalarType(getOperand(0));
+    Value *Res = nullptr;
+    Value *RuntimeVF = getRuntimeVF(State.Builder, IdxTy, State.VF);
+
+    for (unsigned Idx = 1; Idx != getNumOperands(); ++Idx) {
+      Value *VectorStart =
+          Builder.CreateMul(RuntimeVF, ConstantInt::get(IdxTy, Idx - 1));
+      Value *VectorIdx = Idx == 1
+                             ? LaneToExtract
+                             : Builder.CreateSub(LaneToExtract, VectorStart);
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fhahn wrote:

InstSimplifyFolder can't be used, the IR is in an invalid/incomplete state which can cause incorrect folding (.e.g blocks are unreachable, phis not fully formed)

https://github.com/llvm/llvm-project/pull/148817