[llvm] [LSR][term-fold] Adjust expansion budget based on trip count (PR #80304)

[Nikita Popov via llvm-commits]

================
@@ -6813,6 +6813,17 @@ canFoldTermCondOfLoop(Loop *L, ScalarEvolution &SE, DominatorTree &DT,
   if (!isAlmostDeadIV(ToFold, LoopLatch, TermCond))
     return std::nullopt;
 
+  // Inserting instructions in the preheader has a runtime cost, scale
+  // the allowed cost with the loops trip count as best we can.
+  const unsigned ExpansionBudget = [&]() {
+    if (unsigned SmallTC = SE.getSmallConstantMaxTripCount(L))
+      return std::min(2*SCEVCheapExpansionBudget, SmallTC);
+    if (std::optional<unsigned> SmallTC = getLoopEstimatedTripCount(L))
+      return std::min(2*SCEVCheapExpansionBudget, *SmallTC);
+    // Unknown trip count, assume long running by default.
+    return 2*SCEVCheapExpansionBudget;
----------------
nikic wrote:

```suggestion
    unsigned Budget = 2 * SCEVCheapExpansionBudget;
    if (unsigned SmallTC = SE.getSmallConstantMaxTripCount(L))
      return std::min(Budget, SmallTC);
    if (std::optional<unsigned> SmallTC = getLoopEstimatedTripCount(L))
      return std::min(Budget, *SmallTC);
    // Unknown trip count, assume long running by default.
    return Budget;
```

https://github.com/llvm/llvm-project/pull/80304