[IR] Add hasNPredecessors, hasNPredecessorsOrMore to BasicBlock

Add methods to BasicBlock which make it easier to efficiently check
whether a block has N (or more) predecessors.

This can be more efficient than using pred_size(), which is a linear
time operation.

We might consider adding similar methods for successors. I haven't done
so in this patch because succ_size() is already O(1).

With this patch applied, I measured a 0.065% compile-time reduction in
user time for running `opt -O3` on the sqlite3 amalgamation (30 trials).
The change in mergeStoreIntoSuccessor alone saves 45 million linked list
iterations in a stage2 Release build of llc.

See llvm.org/PR39702 for a harder but more general way of achieving
similar results.

Differential Revision: https://reviews.llvm.org/D54686

llvm-svn: 347256

2 files changed, 5 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 0dcd7371210..d9e0c15dda3 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -682,10 +682,9 @@ void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB,
 
   // DTU updates: Collect all the edges that enter
   // PredBB. These dominator edges will be redirected to DestBB.
-  std::vector <DominatorTree::UpdateType> Updates;
+  SmallVector<DominatorTree::UpdateType, 32> Updates;
 
   if (DTU) {
-    Updates.reserve(1 + (2 * pred_size(PredBB)));
     Updates.push_back({DominatorTree::Delete, PredBB, DestBB});
     for (auto I = pred_begin(PredBB), E = pred_end(PredBB); I != E; ++I) {
       Updates.push_back({DominatorTree::Delete, *I, PredBB});
@@ -989,9 +988,8 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
 
   LLVM_DEBUG(dbgs() << "Killing Trivial BB: \n" << *BB);
 
-  std::vector<DominatorTree::UpdateType> Updates;
+  SmallVector<DominatorTree::UpdateType, 32> Updates;
   if (DTU) {
-    Updates.reserve(1 + (2 * pred_size(BB)));
     Updates.push_back({DominatorTree::Delete, BB, Succ});
     // All predecessors of BB will be moved to Succ.
     for (auto I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index af9d2eaf253..cbf9fc16c3d 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -693,7 +693,7 @@ Value *SimplifyCFGOpt::isValueEqualityComparison(Instruction *TI) {
   if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
     // Do not permit merging of large switch instructions into their
     // predecessors unless there is only one predecessor.
-    if (SI->getNumSuccessors() * pred_size(SI->getParent()) <= 128)
+    if (!SI->getParent()->hasNPredecessorsOrMore(128 / SI->getNumSuccessors()))
       CV = SI->getCondition();
   } else if (BranchInst *BI = dyn_cast<BranchInst>(TI))
     if (BI->isConditional() && BI->getCondition()->hasOneUse())
@@ -2890,7 +2890,7 @@ static Value *ensureValueAvailableInSuccessor(Value *V, BasicBlock *BB,
       if (!AlternativeV)
         break;
 
-      assert(pred_size(Succ) == 2);
+      assert(Succ->hasNPredecessors(2));
       auto PredI = pred_begin(Succ);
       BasicBlock *OtherPredBB = *PredI == BB ? *++PredI : *PredI;
       if (PHI->getIncomingValueForBlock(OtherPredBB) == AlternativeV)
@@ -5774,7 +5774,7 @@ bool SimplifyCFGOpt::SimplifyUncondBranch(BranchInst *BI,
   // backedge, so we can eliminate BB.
   bool NeedCanonicalLoop =
       Options.NeedCanonicalLoop &&
-      (LoopHeaders && pred_size(BB) > 1 &&
+      (LoopHeaders && BB->hasNPredecessorsOrMore(2) &&
        (LoopHeaders->count(BB) || LoopHeaders->count(Succ)));
   BasicBlock::iterator I = BB->getFirstNonPHIOrDbg()->getIterator();
   if (I->isTerminator() && BB != &BB->getParent()->getEntryBlock() &&