[llvm] [VPlan] Compute cost of some VPWidenIntOrFpInductions in VPlan (PR #202232)

[David Sherwood via llvm-commits]

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@@ -2838,6 +2838,28 @@ bool VPWidenIntOrFpInductionRecipe::isCanonical() const {
          getScalarType() == getRegion()->getCanonicalIVType();
 }
 
+InstructionCost
+VPWidenIntOrFpInductionRecipe::computeCost(ElementCount VF,
+                                           VPCostContext &Ctx) const {
+  // A widened induction generates a vector phi and increments it by the
+  // splatted step each iteration.
+  // TODO: Also handle truncated inductions.
+  assert(!getTruncInst() &&
+         "truncated inductions should be costed by the legacy model");
+  const InductionDescriptor &ID = getInductionDescriptor();
+
+  InstructionCost Cost = Ctx.TTI.getCFInstrCost(Instruction::PHI, Ctx.CostKind);
+
+  Type *StepTy = getScalarType();
+  unsigned IncOpc = ID.getKind() == InductionDescriptor::IK_IntInduction
+                        ? Instruction::Add
+                        : ID.getInductionOpcode();
+  assert(IncOpc != Instruction::BinaryOpsEnd &&
+         "induction must have a valid increment opcode");
+  return Cost + Ctx.TTI.getArithmeticInstrCost(IncOpc, toVectorTy(StepTy, VF),
----------------
david-arm wrote:

It might be worth adding at least one cost test for non integer inductions, i.e. to test the case where we use `ID.getInductionOpcode()` as the increment opcode. In theory, the induction opcode could Sub, FAdd, FSub, etc., although in practice it's probably just FAdd.

https://github.com/llvm/llvm-project/pull/202232