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

[John Brawn via llvm-commits]

================
@@ -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)) {
----------------
john-brawn-arm wrote:

Sorry, it's still not clear what you're asking here. Do you mean: planContainsDifferentCompares should return false for plans that contain ActiveLaneMask terminated exiting blocks, so that the assert in computeBestVF that calls planContainsDifferentCompares will do the check BestFactor.Width == LegacyVF.Width? If so then possibly we could, though I don't think it would make a difference as I haven't found an example where planContainsAdditionalSimplifications doesn't also return true (which it will do because the cmp in the legacy cost model doesn't correspond to anything in the vplan).

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