[llvm] [LV] Use SCEV to compute final value of complex induction variables (PR #195059)

[Mel Chen via llvm-commits]

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@@ -1157,8 +1157,44 @@ optimizeLatchExitInductionUser(VPlan &Plan, VPValue *Op,
   return nullptr;
 }
 
+static VPValue *optimizeLatchExitIVUserViaSCEV(VPlan &Plan, VPValue *Op,
+                                               PredicatedScalarEvolution &PSE,
+                                               VPValue *ResumeTC,
+                                               const Loop *L) {
+  VPValue *Incoming;
+  if (!match(Op, m_ExtractLastLaneOfLastPart(m_VPValue(Incoming))))
+    return nullptr;
+
+  const SCEV *IncomingSCEV = vputils::getSCEVExprForVPValue(Incoming, PSE, L);
+  const SCEV *Start, *Step;
+  if (!match(IncomingSCEV, m_scev_AffineAddRec(m_SCEV(Start), m_SCEV(Step),
+                                               m_SpecificLoop(L))))
+    return nullptr;
+
+  auto *ExtractR = cast<VPInstruction>(Op);
+  DebugLoc DL = ExtractR->getDebugLoc();
+  VPBuilder Builder(ExtractR);
+  VPSCEVExpander Expander(Builder, *PSE.getSE(), DL);
+  VPValue *StartVPV = Expander.tryToExpand(Start);
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Mel-Chen wrote:

The SCEV expansion occurs in the middle.block rather than inside the vector loop, so it won't cause any verifier failures.

https://github.com/llvm/llvm-project/pull/195059