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

[Hari Limaye via llvm-commits]

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@@ -661,11 +663,65 @@ static void recursivelyDeleteDeadRecipes(VPValue *V) {
   }
 }
 
-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) {
+  auto *TC = dyn_cast_if_present<ConstantInt>(
+      Plan.getTripCount()->getUnderlyingValue());
+  if (!TC || !BestVF.isFixed())
+    return false;
+
+  auto TCVal = TC->getValue().tryZExtValue();
+  if (!TCVal)
+    return false;
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hazzlim wrote:

Sure - I've updated to do the majority of the calculations on APInt to avoid issues with bitwidth, and I've added a test with TC > 64 bits :) 

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