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

[Mel Chen via llvm-commits]

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@@ -1148,8 +1148,43 @@ 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;
+
+  // TODO: Start value can be defined be a VPExpandSCEVRecipe after
+  // VPDerivedIVRecipe supports a general VPValue as the start value.
+  VPIRValue *StartIRV = vputils::getVPIRValueForSCEVExpr(Plan, Start);
+  if (!StartIRV)
+    return nullptr;
+
+  Type *StartTy = StartIRV->getType();
+  assert(StartTy->isIntOrPtrTy() && "The type must be SCEVable");
+  InductionDescriptor::InductionKind Kind =
+      StartTy->isPointerTy() ? InductionDescriptor::IK_PtrInduction
+                             : InductionDescriptor::IK_IntInduction;
+  VPValue *StepVPV = vputils::getOrCreateVPValueForSCEVExpr(Plan, Step);
+  VPBuilder Builder(cast<VPInstruction>(Op));
+  Type *TCTy = ResumeTC->getScalarType();
+  VPValue *ExitCount = Builder.createOverflowingOp(
+      Instruction::Sub, {ResumeTC, Plan.getConstantInt(TCTy, 1)},
+      {/*HasNUW=*/true, /*HasNSW=*/false}, DebugLoc::getUnknown());
+  return Builder.createDerivedIV(Kind, /*FPBinOp=*/nullptr, StartIRV, ExitCount,
----------------
Mel-Chen wrote:

I think separate patches would feel cleaner, but combining them is fine too.
c1b59e332eb10648a820b99370f5a6e93d7d777f

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