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

[Hari Limaye via llvm-commits]

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@@ -975,11 +978,74 @@ 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());
+  if (!TC || !BestVF.isFixed())
+    return false;
+
+  // Calculate the widest type required for known TC, VF and UF.
+  auto ComputeBitWidth = [](APInt TC, uint64_t Align) {
+    auto AlignedTC =
+        Align * APIntOps::RoundingUDiv(TC, APInt(TC.getBitWidth(), Align),
+                                       APInt::Rounding::UP);
+    auto MaxVal = AlignedTC - 1;
+    return std::max<unsigned>(PowerOf2Ceil(MaxVal.getActiveBits()), 8);
+  };
+  unsigned NewBitWidth =
+      ComputeBitWidth(TC->getValue(), BestVF.getKnownMinValue() * BestUF);
+
+  LLVMContext &Ctx = Plan.getCanonicalIV()->getScalarType()->getContext();
+  auto *NewIVTy = IntegerType::get(Ctx, NewBitWidth);
+
+  bool MadeChange = false;
+
+  VPBasicBlock *HeaderVPBB = Plan.getVectorLoopRegion()->getEntryBasicBlock();
+  for (VPRecipeBase &Phi : HeaderVPBB->phis()) {
+    auto *WideIV = dyn_cast<VPWidenIntOrFpInductionRecipe>(&Phi);
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hazzlim wrote:

Done.

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