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

[Florian Hahn via llvm-commits]

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@@ -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 ||
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fhahn wrote:

Hmm, I am wondering if the treatment of the legacy cost model makes more sense here. We could easily match it in the VPlan cost model I think, we know how many source instructions we had (# of members in the group) and could just multiply this by the forced cost?

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