[llvm] [LV][VPlan] Implement VPlan-based cost for exit condition. (PR #125640)

[David Sherwood via llvm-commits]

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@@ -823,6 +823,29 @@ bool VPInstruction::onlyFirstPartUsed(const VPValue *Op) const {
   llvm_unreachable("switch should return");
 }
 
+InstructionCost VPInstruction::computeCost(ElementCount VF,
+                                           VPCostContext &Ctx) const {
+  Type *ValTy = Ctx.Types.inferScalarType(getOperand(0));
+
+  switch (getOpcode()) {
+  case VPInstruction::BranchOnCount: {
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david-arm wrote:

Are we guaranteed to only ever generate `BranchOnCount` here? I thought various vplan transforms took place that could change the terminator. For example, we might need `BranchOnCond` here for completeness.

I think `addVPLaneMaskPhiAndUpdateExitBranch` shows an example where `BranchOnCond` should not be free.

https://github.com/llvm/llvm-project/pull/125640