[llvm] [VPlan] Compute induction end values in VPlan. (PR #112145)

[via llvm-commits]

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@@ -10206,6 +10187,57 @@ LoopVectorizePass::LoopVectorizePass(LoopVectorizeOptions Opts)
       VectorizeOnlyWhenForced(Opts.VectorizeOnlyWhenForced ||
                               !EnableLoopVectorization) {}
 
+/// Prepare \p MainPlan for vectorizing the main vector loop during epilogue
+/// vectorization. Remove ResumePhis from \p MainPlan for inductions that
+/// don't have a corresponding wide induction in \p EpiPlan.
+static void preparePlanForMainVectorLoop(VPlan &MainPlan, VPlan &EpiPlan) {
+  // Collect PHI nodes of widened phis in the VPlan for the epilogue. Those
+  // will need their resume-values computed in the main vector loop. Others
+  // can be removed from the main VPlan.
+  SmallPtrSet<PHINode *, 2> EpiWidenedPhis;
+  for (VPRecipeBase &R :
+       EpiPlan.getVectorLoopRegion()->getEntryBasicBlock()->phis()) {
+    if (isa<VPCanonicalIVPHIRecipe>(&R))
+      continue;
+    EpiWidenedPhis.insert(
+        cast<PHINode>(R.getVPSingleValue()->getUnderlyingValue()));
+  }
+  for (VPRecipeBase &R : make_early_inc_range(
+           *cast<VPIRBasicBlock>(MainPlan.getScalarHeader()))) {
+    auto *VPIRInst = cast<VPIRInstruction>(&R);
+    auto *IRI = dyn_cast<PHINode>(&VPIRInst->getInstruction());
+    if (!IRI)
+      break;
+    if (EpiWidenedPhis.contains(IRI))
+      continue;
+    // There is no corresponding wide induction in the epilogue plan that would
+    // need a resume value. Set the operand in VPIRInst to zero, so ResumePhi
+    // can be removed. The resume values for the scalar loop will be created
+    // during execution of EpiPlan.
+    VPRecipeBase *ResumePhi = VPIRInst->getOperand(0)->getDefiningRecipe();
+    VPIRInst->setOperand(
+        0, MainPlan.getOrAddLiveIn(Constant::getNullValue(IRI->getType())));
+    ResumePhi->eraseFromParent();
+  }
+
+  using namespace VPlanPatternMatch;
+  VPBasicBlock *MainScalarPH = MainPlan.getScalarPreheader();
+  VPValue *VectorTC = &MainPlan.getVectorTripCount();
+  // If there is no suitable resume value for the canonical induction in the
+  // scalar (which will become vector) epilogue loop, create it.
+  if (none_of(*MainScalarPH, [VectorTC](VPRecipeBase &R) {
+        return match(&R, m_VPInstruction<VPInstruction::ResumePhi>(
+                             m_Specific(VectorTC), m_SpecificInt(0)));
+      })) {
+    VPBuilder ScalarPHBuilder(MainScalarPH, MainScalarPH->begin());
+    ScalarPHBuilder.createNaryOp(
+        VPInstruction::ResumePhi,
+        {VectorTC, MainPlan.getOrAddLiveIn(ConstantInt::get(
+                       MainPlan.getCanonicalIV()->getScalarType(), 0))},
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ayalz wrote:

Can `MainPlan.getCanonicalIV()->getStartValue()` be reused instead of being recreated?

https://github.com/llvm/llvm-project/pull/112145