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

[Elvis Wang 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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ElvisWang123 wrote:

Agree that `BranchOnCond` will generate `extractelement(vec, 0)` and  isn't free for all target. But after this change, the test still change because the extractelement is free under AArch64.

In `clamped_tc_8` the `BranchOnCount` is replaced by `get.active.lane.mask` + `not` + `BranchOnCond`. 
I've tried to implement  the  cost of `get.active.lane.mask` locally but it will impact lots of tests because the cost of this intrinsics is very expensive in AArch64 (2 * element count). And the `clapmed_tc_8` will not even vectorized.


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