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

[John Brawn 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)) {
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john-brawn-arm wrote:

> Could we still add the IR instructions to the set to skip?

I've made this change. Doing this we also need to detect when VPInstruction::computeCost will return zero for BranchOnCount, as otherwise the assertion fails in llvm/test/Transforms/LoopVectorize/RISCV/low-trip-count.ll.

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