[llvm] [VPlan] Don't use the legacy cost model for loop conditions (PR #156864)

[Florian Hahn via llvm-commits]

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@@ -6988,6 +6948,35 @@ static bool planContainsAdditionalSimplifications(VPlan &Plan,
     });
   });
 }
+
+static bool planContainsDifferentCompares(VPlan &Plan, VPCostContext &CostCtx,
+                                          Loop *TheLoop, ElementCount VF) {
+  // Count how many compare instructions there are in the legacy cost model.
+  unsigned NumLegacyCompares = 0;
+  for (BasicBlock *BB : TheLoop->blocks()) {
+    for (auto &I : *BB) {
+      if (isa<CmpInst>(I)) {
+        NumLegacyCompares += 1;
+      }
+    }
+  }
+
+  // Count how many compare instructions there are in the VPlan.
+  unsigned NumVPlanCompares = 0;
+  auto Iter = vp_depth_first_deep(Plan.getVectorLoopRegion()->getEntry());
+  for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(Iter)) {
----------------
fhahn wrote:

Thanks for checking!

 It looks like there's still something related to the changes that effectively disables the assertion altogether. I tried the patch with something like the diff below which should cause lots of crashes, but it seems there aren't?

```
diff --git a/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp b/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
index 7c9302860a3b..4d821ca0954c 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
@@ -2270,6 +2270,7 @@ InstructionCost VPWidenRecipe::computeCost(ElementCount VF,
   case Instruction::ExtractValue:
   case Instruction::ICmp:
   case Instruction::FCmp:
+    return 100;
     return getCostForRecipeWithOpcode(getOpcode(), VF, Ctx);
   default:
     llvm_unreachable("Unsupported opcode for instruction");
```

https://github.com/llvm/llvm-project/pull/156864