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

[Hari Limaye via llvm-commits]

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@@ -975,11 +978,78 @@ 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;
+
+  auto *TC = dyn_cast_if_present<ConstantInt>(
+      Plan.getTripCount()->getUnderlyingValue());
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hazzlim wrote:

So if I refactor to use `getLiveInIRValue()`, we get quite a few lit test crashes where TripCount is not a live-in, so I'm not sure this is guaranteed?

I've added a check above to make sure that the TC is a live-in, so that we are not calling `getUnderlyingValue()` on non live-ins.

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