[VPlan] Add recipe to handle SCEV expansion (NFC).

[fhahn]

Summary

This can be used to explicitly model VPValues that depend on SCEV
expansion, like the step for inductions.

[fhahn]

Now that D115953 is almost ready, it would be great if you could take a look at this change, which is required by D115953. It just adds a VPExpandSCEVRecipe. It will only be used once D115953 lands.

[Ayal]

llvm/lib/Transforms/Vectorize/VPlan.cpp

1403

Set the single value per part? (Tests with UF>1 work?)
Assert it's not called per lane.

llvm/lib/Transforms/Vectorize/VPlan.h

1725

but the recipe is currently placed in the header; place it in the pre-header once the latter is modelled in VPlan as a VPBasicBlock.

[fhahn]

Thanks Ayal! Latest comments should be addressed and I am planning on landing this soon.

[Ayal]

llvm/lib/Transforms/Vectorize/VPlan.cpp

1411

Should this set the value for all Parts from 0 to UF-1, or is only Part 0 needed?

[fhahn]

llvm/lib/Transforms/Vectorize/VPlan.cpp

1411

It's not needed for the anticipated use case, but I updated the code to set it for all UFs.

llvm/lib/Transforms/Vectorize/VPlan.h

assert(is_contained(operands(), Op) &&
            "Op must be an operand of the recipe");
 
     // Widened, consecutive memory operations only demand the first lane of
     // their address.
     return Op == getAddr() && isConsecutive();
   }
 };
 
+/// Recipe to expand a SCEV expression.
+/// TODO: Currently the recipe always expands the expression in the loop
+/// pre-header, but the recipe is currently placed in the header; place it in
+/// the pre-header once the latter is modeled in VPlan as a VPBasicBlock.
+class VPExpandSCEVRecipe : public VPRecipeBase, public VPValue {
+  const SCEV *Expr;
+  ScalarEvolution &SE;
+
+public:
+  VPExpandSCEVRecipe(const SCEV *Expr, ScalarEvolution &SE)
+      : VPRecipeBase(VPExpandSCEVSC, {}), VPValue(nullptr, this), Expr(Expr),
+        SE(SE) {}
+
+  ~VPExpandSCEVRecipe() override = default;
+
+  /// Method to support type inquiry through isa, cast, and dyn_cast.
+  static inline bool classof(const VPDef *D) {
+    return D->getVPDefID() == VPExpandSCEVSC;
+  }
+
+  /// Generate a canonical vector induction variable of the vector loop, with
+  void execute(VPTransformState &State) override;
+
+#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
+  /// Print the recipe.
+  void print(raw_ostream &O, const Twine &Indent,
+             VPSlotTracker &SlotTracker) const override;
+#endif
+};
+
 /// Canonical scalar induction phi of the vector loop. Starting at the specified
 /// start value (either 0 or the resume value when vectorizing the epilogue
 /// loop). VPWidenCanonicalIVRecipe represents the vector version of the
 /// canonical induction variable.
 class VPCanonicalIVPHIRecipe : public VPHeaderPHIRecipe {
   DebugLoc DL;
 
 public:

llvm/lib/Transforms/Vectorize/VPlan.cpp

 #include "llvm/Support/Casting.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/GenericDomTreeConstruction.h"
 #include "llvm/Support/GraphWriter.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
 #include <cassert>
 #include <iterator>
 #include <string>
 #include <vector>
 
 using namespace llvm;
 extern cl::opt<bool> EnableVPlanNativePath;
 

 void VPCanonicalIVPHIRecipe::print(raw_ostream &O, const Twine &Indent,
                                    VPSlotTracker &SlotTracker) const {
   O << Indent << "EMIT ";
   printAsOperand(O, SlotTracker);
   O << " = CANONICAL-INDUCTION";
 }
 #endif
 
+void VPExpandSCEVRecipe::execute(VPTransformState &State) {
+  assert(!State.Instance && "cannot be used in per-lane");
+  const DataLayout &DL =
+      State.CFG.VectorPreHeader->getModule()->getDataLayout();
+  SCEVExpander Exp(SE, DL, "induction");
+  auto Res = Exp.expandCodeFor(Expr, Expr->getType(),
+                               State.CFG.VectorPreHeader->getTerminator());
+
+  State.set(this, Res, 0);
+}
+
+#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
+void VPExpandSCEVRecipe::print(raw_ostream &O, const Twine &Indent,
+                               VPSlotTracker &SlotTracker) const {
+  O << Indent << "EMIT ";
+  getVPSingleValue()->printAsOperand(O, SlotTracker);
+  O << " = EXPAND SCEV " << *Expr;
+}
+#endif
+
 void VPWidenCanonicalIVRecipe::execute(VPTransformState &State) {
   Value *CanonicalIV = State.get(getOperand(0), 0);
   Type *STy = CanonicalIV->getType();
   IRBuilder<> Builder(State.CFG.PrevBB->getTerminator());
   ElementCount VF = State.VF;
   Value *VStart = VF.isScalar()
                       ? CanonicalIV
                       : Builder.CreateVectorSplat(VF, CanonicalIV, "broadcast");

llvm/lib/Transforms/Vectorize/VPlanValue.h

 
 public:
   /// An enumeration for keeping track of the concrete subclass of VPRecipeBase
   /// that is actually instantiated. Values of this enumeration are kept in the
   /// SubclassID field of the VPRecipeBase objects. They are used for concrete
   /// type identification.
   using VPRecipeTy = enum {
     VPBranchOnMaskSC,
+    VPExpandSCEVSC,
     VPInstructionSC,
     VPInterleaveSC,
     VPReductionSC,
     VPReplicateSC,
     VPWidenCallSC,
     VPWidenCanonicalIVSC,
     VPWidenGEPSC,
     VPWidenMemoryInstructionSC,