[SimplifyCFG] Avoid increasing too many phi entries when removing empty blocks

llvm/lib/Transforms/Utils/Local.cpp

@@ -112,6 +112,12 @@ static cl::opt<unsigned> PHICSENumPHISmallSize(
         "When the basic block contains not more than this number of PHI nodes, "
         "perform a (faster!) exhaustive search instead of set-driven one."));
 
+static cl::opt<unsigned> MaxPhiEntriesIncreaseAfterRemovingEmptyBlock(
+    "max-phi-entries-increase-after-removing-empty-block", cl::init(1000),
+    cl::Hidden,
+    cl::desc("Stop removing an empty block if removing it will introduce more "
+             "than this number of phi entries in its successor"));
+
 // Max recursion depth for collectBitParts used when detecting bswap and
 // bitreverse idioms.
 static const unsigned BitPartRecursionMaxDepth = 48;
@@ -1047,6 +1053,34 @@ CanRedirectPredsOfEmptyBBToSucc(BasicBlock *BB, BasicBlock *Succ,
   return true;
 }
 
+// Check whether removing BB will make the phis in its Succ have too
+// many incoming entries. This function does not check whether BB is foldable
+// or not.
+static bool introduceTooManyPhiEntries(BasicBlock *BB, BasicBlock *Succ) {
+  // If BB only has one predecessor, then removing it will not introduce more
+  // incoming edges for phis.
+  if (BB->hasNPredecessors(1))
+    return false;
+  unsigned NumPreds = pred_size(BB);
+  unsigned NumChangedPhi = 0;
+  for (auto &Phi : Succ->phis()) {
+    // If the incoming value is a phi and the phi is defined in BB,
+    // then removing BB will not increase the total phi entries of the ir.
+    if (PHINode *IncomingPhi =
+            dyn_cast<PHINode>(Phi.getIncomingValueForBlock(BB)))
+      if (IncomingPhi->getParent() == BB)
+        continue;
+    // Otherwise, we need to add entries to the phi
+    NumChangedPhi++;
+  }
+  // For every phi that needs to be changed, (NumPreds - 1) new entries will be
+  // added. If the total increase in phi entries exceeds
+  // MaxPhiEntriesIncreaseAfterRemovingEmptyBlock, it will be considered as
+  // introducing too many new phi entries.
+  return (NumPreds - 1) * NumChangedPhi >
+         MaxPhiEntriesIncreaseAfterRemovingEmptyBlock;
+}
+
 /// Replace a value flowing from a block to a phi with
 /// potentially multiple instances of that value flowing from the
 /// block's predecessors to the phi.
@@ -1146,7 +1180,7 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
       BBKillable ||
       CanRedirectPredsOfEmptyBBToSucc(BB, Succ, BBPreds, SuccPreds, CommonPred);
 
-  if (!BBKillable && !BBPhisMergeable)
+  if ((!BBKillable && !BBPhisMergeable) || introduceTooManyPhiEntries(BB, Succ))
     return false;
 
   // Check to see if merging these blocks/phis would cause conflicts for any of