[VPlan] Don't reset canonical IV start value.

Instead of re-setting the start value of the canonical IV when
vectorizing the epilogue we can emit an Add VPInstruction to provide
canonical IV value, adjusted by the resume value from the main loop.

This is in preparation to make the canonical IV a VPValue defined by
loop regions. It ensures that the canonical IV always starts at 0.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -9590,7 +9590,10 @@ static SmallVector<Instruction *> preparePlanForEpilogueVectorLoop(
                 }) &&
          "the canonical IV should only be used by its increment or "
          "ScalarIVSteps when resetting the start value");
-  IV->setOperand(0, VPV);
+  VPBuilder Builder(Header, Header->getFirstNonPhi());
+  VPInstruction *Add = Builder.createNaryOp(Instruction::Add, {IV, VPV});
+  IV->replaceAllUsesWith(Add);
+  Add->setOperand(0, IV);
 
   DenseMap<Value *, Value *> ToFrozen;
   SmallVector<Instruction *> InstsToMove;

llvm/lib/Transforms/Vectorize/VPlanPatternMatch.h

@@ -431,6 +431,12 @@ m_c_Binary(const Op0_t &Op0, const Op1_t &Op1) {
   return AllRecipe_commutative_match<Opcode, Op0_t, Op1_t>(Op0, Op1);
 }
 
+template <typename Op0_t, typename Op1_t>
+inline AllRecipe_match<Instruction::Add, Op0_t, Op1_t> m_Add(const Op0_t &Op0,
+                                                             const Op1_t &Op1) {
+  return m_Binary<Instruction::Add, Op0_t, Op1_t>(Op0, Op1);
+}
+
 template <typename Op0_t, typename Op1_t>
 inline AllRecipe_commutative_match<Instruction::Add, Op0_t, Op1_t>
 m_c_Add(const Op0_t &Op0, const Op1_t &Op1) {

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -1234,6 +1234,17 @@ static void simplifyRecipe(VPRecipeBase &R, VPTypeAnalysis &TypeInfo) {
   if (!Plan->isUnrolled())
     return;
 
+  if (match(Def, m_Add(m_VPValue(X), m_VPValue(Y))) && Y->isLiveIn() &&
+      isa<VPPhi>(X)) {
+    auto *Phi = cast<VPPhi>(X);
+    if (Phi->getOperand(1) != Def && match(Phi->getOperand(0), m_ZeroInt()) &&
+        Phi->getNumUsers() == 1 && (*Phi->user_begin() == &R)) {
+      Phi->setOperand(0, Y);
+      Def->replaceAllUsesWith(Phi);
+      return;
+    }
+  }
+
   // VPVectorPointer for part 0 can be replaced by their start pointer.
   if (auto *VecPtr = dyn_cast<VPVectorPointerRecipe>(&R)) {
     if (VecPtr->isFirstPart()) {