[llvm] [LV] Optimize VPWidenIntOrFpInductionRecipe for known TC (PR #118828)

[Florian Hahn via llvm-commits]

================
@@ -1009,11 +1012,83 @@ void VPlanTransforms::simplifyRecipes(VPlan &Plan, Type &CanonicalIVTy) {
   }
 }
 
-void VPlanTransforms::optimizeForVFAndUF(VPlan &Plan, ElementCount BestVF,
-                                         unsigned BestUF,
-                                         PredicatedScalarEvolution &PSE) {
-  assert(Plan.hasVF(BestVF) && "BestVF is not available in Plan");
-  assert(Plan.hasUF(BestUF) && "BestUF is not available in Plan");
+/// Optimize the width of vector induction variables in \p Plan based on a known
+/// constant Trip Count, \p BestVF and \p BestUF.
+static bool optimizeVectorInductionWidthForTCAndVFUF(VPlan &Plan,
+                                                     ElementCount BestVF,
+                                                     unsigned BestUF) {
+  // Only proceed if we have not completely removed the vector region.
+  if (!Plan.getVectorLoopRegion())
+    return false;
+
+  if (!Plan.getTripCount()->isLiveIn())
+    return false;
+  auto *TC = dyn_cast_if_present<ConstantInt>(
+      Plan.getTripCount()->getUnderlyingValue());
+  if (!TC || !BestVF.isFixed())
+    return false;
+
+  // Calculate the widest type required for known TC, VF and UF.
----------------
fhahn wrote:

```suggestion
  // Calculate the minimum power-of-2 bit width that can fit the known TC, VF and UF. Returns at least 8.
```

https://github.com/llvm/llvm-project/pull/118828