[LV] Rename blockNeedsPredication to blockNeedsPredicationForAnyReason.

[sdesmalen]

Summary

The interface is a convenience function to ask if a block requires
predication when widening, but it's important that there are two
separate concepts to consider:
(A) The block was predicated in the original loop.
(B) The block was unpredicated in the original loop, but requires

predication because of tail folding.

In the case of (B) we know that at least one lane of the vector will
be executed, which means we can implementing a load from a uniform address
with a scalar load + splat (D112552). In the case of predication because
of (A), we cannot do this, because the scalar load itself requires
predication.

The name 'blockNeedsPredication' does not make the distinction between
(A) and (B), hence the reason to rename it.

[david-arm]

LGTM!

[fhahn]

Having a dedicated function to check whether a block needs predication for any reason still seems convenient IMO from looking at the updates, if the only place we need to make the distinction for now is isPredicateInst. Perhaps it would be better to change the name to blockNeedsPredicationForAnyReason or something like that so there's a clearer distinction between blockNeedsPredication in Legal?

With a proper doc-comment this would make the code slightly more explicit and it would also be easier to find all places that check for A and B. It would also be easier to update all occurrences, if the condition for A and B needs adjusting.

[sdesmalen]

Changed patch to rename blockNeedsPredication -> blockNeedsPredicationForAnyReason.

[david-arm]

LGTM!

[fhahn]

Thanks for the update, LG!

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

public:
   /// If a non-zero VF has been calculated, we check if I will be scalarized
   /// predication for that VF.
   bool isScalarWithPredication(Instruction *I) const;
 
   // Returns true if \p I is an instruction that will be predicated either
   // through scalar predication or masked load/store or masked gather/scatter.
   // Superset of instructions that return true for isScalarWithPredication.
   bool isPredicatedInst(Instruction *I) {
-    if (!blockNeedsPredication(I->getParent()))
+    if (!blockNeedsPredicationForAnyReason(I->getParent()))
       return false;
     // Loads and stores that need some form of masked operation are predicated
     // instructions.
     if (isa<LoadInst>(I) || isa<StoreInst>(I))
       return Legal->isMaskRequired(I);
     return isScalarWithPredication(I);
   }
 
public:
   /// loop hint annotation.
   bool isScalarEpilogueAllowed() const {
     return ScalarEpilogueStatus == CM_ScalarEpilogueAllowed;
   }
 
   /// Returns true if all loop blocks should be masked to fold tail loop.
   bool foldTailByMasking() const { return FoldTailByMasking; }
 
-  bool blockNeedsPredication(BasicBlock *BB) const {
+  /// Returns true if the instructions in this block requires predication
+  /// for any reason, e.g. because tail folding now requires a predicate
+  /// or because the block in the original loop was predicated.
+  bool blockNeedsPredicationForAnyReason(BasicBlock *BB) const {
     return foldTailByMasking() || Legal->blockNeedsPredication(BB);
   }
 
   /// A SmallMapVector to store the InLoop reduction op chains, mapping phi
   /// nodes to the chain of instructions representing the reductions. Uses a
   /// MapVector to ensure deterministic iteration order.
   using ReductionChainMap =
       SmallMapVector<PHINode *, SmallVector<Instruction *, 4>, 4>;
for (auto &Induction : Legal->getInductionVars()) {
     LLVM_DEBUG(dbgs() << "LV: Found scalar instruction: " << *IndUpdate
                       << "\n");
   }
 
   Scalars[VF].insert(Worklist.begin(), Worklist.end());
 }
 
 bool LoopVectorizationCostModel::isScalarWithPredication(Instruction *I) const {
-  if (!blockNeedsPredication(I->getParent()))
+  if (!blockNeedsPredicationForAnyReason(I->getParent()))
     return false;
   switch(I->getOpcode()) {
   default:
     break;
   case Instruction::Load:
   case Instruction::Store: {
     if (!Legal->isMaskRequired(I))
       return false;
if (hasIrregularType(ScalarTy, DL))
     return false;
 
   // Check if masking is required.
   // A Group may need masking for one of two reasons: it resides in a block that
   // needs predication, or it was decided to use masking to deal with gaps
   // (either a gap at the end of a load-access that may result in a speculative
   // load, or any gaps in a store-access).
   bool PredicatedAccessRequiresMasking =
-      blockNeedsPredication(I->getParent()) && Legal->isMaskRequired(I);
+      blockNeedsPredicationForAnyReason(I->getParent()) &&
+      Legal->isMaskRequired(I);
   bool LoadAccessWithGapsRequiresEpilogMasking =
       isa<LoadInst>(I) && Group->requiresScalarEpilogue() &&
       !isScalarEpilogueAllowed();
   bool StoreAccessWithGapsRequiresMasking =
       isa<StoreInst>(I) && (Group->getNumMembers() < Group->getFactor());
   if (!PredicatedAccessRequiresMasking &&
       !LoadAccessWithGapsRequiresEpilogMasking &&
       !StoreAccessWithGapsRequiresMasking)
void LoopVectorizationCostModel::collectInstsToScalarize(ElementCount VF) {
   // not profitable to scalarize any instructions, the presence of VF in the
   // map will indicate that we've analyzed it already.
   ScalarCostsTy &ScalarCostsVF = InstsToScalarize[VF];
 
   // Find all the instructions that are scalar with predication in the loop and
   // determine if it would be better to not if-convert the blocks they are in.
   // If so, we also record the instructions to scalarize.
   for (BasicBlock *BB : TheLoop->blocks()) {
-    if (!blockNeedsPredication(BB))
+    if (!blockNeedsPredicationForAnyReason(BB))
       continue;
     for (Instruction &I : *BB)
       if (isScalarWithPredication(&I)) {
         ScalarCostsTy ScalarCosts;
         // Do not apply discount if scalable, because that would lead to
         // invalid scalarization costs.
         // Do not apply discount logic if hacked cost is needed
         // for emulated masked memrefs.

 Optional<VectorizationFactor>
 LoopVectorizationPlanner::plan(ElementCount UserVF, unsigned UserIC) {
   assert(OrigLoop->isInnermost() && "Inner loop expected.");
   FixedScalableVFPair MaxFactors = CM.computeMaxVF(UserVF, UserIC);
   if (!MaxFactors) // Cases that should not to be vectorized nor interleaved.
     return None;
 
   // Invalidate interleave groups if all blocks of loop will be predicated.
-  if (CM.blockNeedsPredication(OrigLoop->getHeader()) &&
+  if (CM.blockNeedsPredicationForAnyReason(OrigLoop->getHeader()) &&
       !useMaskedInterleavedAccesses(*TTI)) {
     LLVM_DEBUG(
         dbgs()
         << "LV: Invalidate all interleaved groups due to fold-tail by masking "
            "which requires masked-interleaved support.\n");
     if (CM.InterleaveInfo.invalidateGroups())
       // Invalidating interleave groups also requires invalidating all decisions
       // based on them, which includes widening decisions and uniform and scalar
VPValue *VPRecipeBuilder::createBlockInMask(BasicBlock *BB, VPlanPtr &Plan) {
   if (BCEntryIt != BlockMaskCache.end())
     return BCEntryIt->second;
 
   // All-one mask is modelled as no-mask following the convention for masked
   // load/store/gather/scatter. Initialize BlockMask to no-mask.
   VPValue *BlockMask = nullptr;
 
   if (OrigLoop->getHeader() == BB) {
-    if (!CM.blockNeedsPredication(BB))
+    if (!CM.blockNeedsPredicationForAnyReason(BB))
       return BlockMaskCache[BB] = BlockMask; // Loop incoming mask is all-one.
 
     // Create the block in mask as the first non-phi instruction in the block.
     VPBuilder::InsertPointGuard Guard(Builder);
     auto NewInsertionPoint = Builder.getInsertBlock()->getFirstNonPhi();
     Builder.setInsertPoint(Builder.getInsertBlock(), NewInsertionPoint);
 
     // Introduce the early-exit compare IV <= BTC to form header block mask.