[LV] Replace UncountableEdge with UncountableEarlyExitingBlock (NFC). (llvm#151311)

Only the uncountable exiting BB is used. Store it instead of a piar of
Exiting BB and Exit BB.

PR: llvm#151311

Author: fhahn

llvm/include/llvm/Transforms/Vectorize/LoopVectorizationLegality.h

@@ -400,19 +400,11 @@ class LoopVectorizationLegality {
 
   /// Returns true if the loop has exactly one uncountable early exit, i.e. an
   /// uncountable exit that isn't the latch block.
-  bool hasUncountableEarlyExit() const {
-    return getUncountableEdge().has_value();
-  }
+  bool hasUncountableEarlyExit() const { return UncountableExitingBB; }
 
   /// Returns the uncountable early exiting block, if there is exactly one.
   BasicBlock *getUncountableEarlyExitingBlock() const {
-    return hasUncountableEarlyExit() ? getUncountableEdge()->first : nullptr;
-  }
-
-  /// Returns the destination of the uncountable early exiting block, if there
-  /// is exactly one.
-  BasicBlock *getUncountableEarlyExitBlock() const {
-    return hasUncountableEarlyExit() ? getUncountableEdge()->second : nullptr;
+    return UncountableExitingBB;
   }
 
   /// Return true if there is store-load forwarding dependencies.
@@ -473,13 +465,6 @@ class LoopVectorizationLegality {
     return CountableExitingBlocks;
   }
 
-  /// Returns the loop edge to an uncountable exit, or std::nullopt if there
-  /// isn't a single such edge.
-  std::optional<std::pair<BasicBlock *, BasicBlock *>>
-  getUncountableEdge() const {
-    return UncountableEdge;
-  }
-
 private:
   /// Return true if the pre-header, exiting and latch blocks of \p Lp and all
   /// its nested loops are considered legal for vectorization. These legal
@@ -659,9 +644,9 @@ class LoopVectorizationLegality {
   /// the exact backedge taken count is not computable.
   SmallVector<BasicBlock *, 4> CountableExitingBlocks;
 
-  /// Keep track of the loop edge to an uncountable exit, comprising a pair
-  /// of (Exiting, Exit) blocks, if there is exactly one early exit.
-  std::optional<std::pair<BasicBlock *, BasicBlock *>> UncountableEdge;
+  /// Keep track of an uncountable exiting block, if there is exactly one early
+  /// exit.
+  BasicBlock *UncountableExitingBB = nullptr;
 };
 
 } // namespace llvm

llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp

@@ -1665,7 +1665,7 @@ bool LoopVectorizationLegality::isVectorizableEarlyExitLoop() {
 
   // Keep a record of all the exiting blocks.
   SmallVector<const SCEVPredicate *, 4> Predicates;
-  std::optional<std::pair<BasicBlock *, BasicBlock *>> SingleUncountableEdge;
+  BasicBlock *SingleUncountableExitingBlock = nullptr;
   for (BasicBlock *BB : ExitingBlocks) {
     const SCEV *EC =
         PSE.getSE()->getPredicatedExitCount(TheLoop, BB, &Predicates);
@@ -1687,15 +1687,15 @@ bool LoopVectorizationLegality::isVectorizableEarlyExitLoop() {
         ExitBlock = Succs[1];
       }
 
-      if (SingleUncountableEdge) {
+      if (SingleUncountableExitingBlock) {
         reportVectorizationFailure(
             "Loop has too many uncountable exits",
             "Cannot vectorize early exit loop with more than one early exit",
             "TooManyUncountableEarlyExits", ORE, TheLoop);
         return false;
       }
 
-      SingleUncountableEdge = {BB, ExitBlock};
+      SingleUncountableExitingBlock = BB;
     } else
       CountableExitingBlocks.push_back(BB);
   }
@@ -1705,15 +1705,15 @@ bool LoopVectorizationLegality::isVectorizableEarlyExitLoop() {
   // PSE.getSymbolicMaxBackedgeTakenCount() below.
   Predicates.clear();
 
-  if (!SingleUncountableEdge) {
+  if (!SingleUncountableExitingBlock) {
     LLVM_DEBUG(dbgs() << "LV: Cound not find any uncountable exits");
     return false;
   }
 
   // The only supported early exit loops so far are ones where the early
   // exiting block is a unique predecessor of the latch block.
   BasicBlock *LatchPredBB = LatchBB->getUniquePredecessor();
-  if (LatchPredBB != SingleUncountableEdge->first) {
+  if (LatchPredBB != SingleUncountableExitingBlock) {
     reportVectorizationFailure("Early exit is not the latch predecessor",
                                "Cannot vectorize early exit loop",
                                "EarlyExitNotLatchPredecessor", ORE, TheLoop);
@@ -1766,7 +1766,7 @@ bool LoopVectorizationLegality::isVectorizableEarlyExitLoop() {
     }
 
   // The vectoriser cannot handle loads that occur after the early exit block.
-  assert(LatchBB->getUniquePredecessor() == SingleUncountableEdge->first &&
+  assert(LatchBB->getUniquePredecessor() == SingleUncountableExitingBlock &&
          "Expected latch predecessor to be the early exiting block");
 
   // TODO: Handle loops that may fault.
@@ -1789,7 +1789,7 @@ bool LoopVectorizationLegality::isVectorizableEarlyExitLoop() {
   LLVM_DEBUG(dbgs() << "LV: Found an early exit loop with symbolic max "
                        "backedge taken count: "
                     << *SymbolicMaxBTC << '\n');
-  UncountableEdge = SingleUncountableEdge;
+  UncountableExitingBB = SingleUncountableExitingBlock;
   return true;
 }
 
@@ -1861,7 +1861,8 @@ bool LoopVectorizationLegality::canVectorize(bool UseVPlanNativePath) {
         return false;
     } else {
       if (!isVectorizableEarlyExitLoop()) {
-        UncountableEdge = std::nullopt;
+        assert(!hasUncountableEarlyExit() &&
+               "Must be false without vectorizable early-exit loop");
         if (DoExtraAnalysis)
           Result = false;
         else