[llvm] [LV] Don't trigger legacy/vplan assert when forcing costs (PR #156870)

[David Sherwood via llvm-commits]

================
@@ -7047,7 +7047,13 @@ VectorizationFactor LoopVectorizationPlanner::computeBestVF() {
   // Verify that the VPlan-based and legacy cost models agree, except for VPlans
   // with early exits and plans with additional VPlan simplifications. The
   // legacy cost model doesn't properly model costs for such loops.
+  // NOTE: If the user has forced a target instruction cost this assert is very
+  // likely to trigger because the VPlan recipes don't map 1:1 with the scalar
+  // instructions that the legacy cost model is based on. One example of this is
+  // for interleave groups - VPlan will use the forced cost for the whole group,
+  // whereas the legacy cost model will use it for each load.
   assert((BestFactor.Width == LegacyVF.Width || BestPlan.hasEarlyExit() ||
+          ForceTargetInstructionCost.getNumOccurrences() > 0 ||
----------------
david-arm wrote:

OK I can see that point, but I had the impression the assert is something that we should be killing off soon anyway given that we're using vplan for most costs now. I guess it sounds like in all likelihood the assert is here to stay for quite a long time.

https://github.com/llvm/llvm-project/pull/156870