[llvm] [VPlan] Model initial header mask as region value. (PR #196199)

[Florian Hahn via llvm-commits]

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@@ -51,6 +51,10 @@ VPValue *vputils::getOrCreateVPValueForSCEVExpr(VPlan &Plan, const SCEV *Expr) {
 }
 
 bool vputils::isHeaderMask(const VPValue *V, const VPlan &Plan) {
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fhahn wrote:

Agreed that would be good. The main question/challenge is how to handle cost modeling. We recently started to cost the compares of the wide header mask, which gives more accurate costs. Expanding before costing makes it easier to compute the cost.

To compute the cost of the header mask going forward without expansion, we would need at least the style.

https://github.com/llvm/llvm-project/pull/196199