Begin chipping away at one of the biggest quadratic-ish behaviors in

this pass. We're leaving already merged blocks on the worklist, and
scanning them again and again only to determine each time through that
indeed they aren't viable. We can instead remove them once we're going
to have to scan the worklist. This is the easy way to implement removing
them. If this remains on the profile (as I somewhat suspect it will), we
can get a lot more clever here, as the worklist's order is essentially
irrelevant. We can use swapping and fold the two loops to reduce
overhead even when there are many blocks on the worklist but only a few
of them are removed.

llvm-svn: 144531

llvm/lib/CodeGen/MachineBlockPlacement.cpp

@@ -377,6 +377,23 @@ MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
   return BestSucc;
 }
 
+namespace {
+/// \brief Predicate struct to detect blocks already placed.
+class IsBlockPlaced {
+  const BlockChain &PlacedChain;
+  const BlockToChainMapType &BlockToChain;
+
+public:
+  IsBlockPlaced(const BlockChain &PlacedChain,
+                const BlockToChainMapType &BlockToChain)
+      : PlacedChain(PlacedChain), BlockToChain(BlockToChain) {}
+
+  bool operator()(MachineBasicBlock *BB) const {
+    return BlockToChain.lookup(BB) == &PlacedChain;
+  }
+};
+}
+
 /// \brief Select the best block from a worklist.
 ///
 /// This looks through the provided worklist as a list of candidate basic
@@ -390,13 +407,20 @@ MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
 MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
     BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
     const BlockFilterSet *BlockFilter) {
+  // Once we need to walk the worklist looking for a candidate, cleanup the
+  // worklist of already placed entries.
+  // FIXME: If this shows up on profiles, it could be folded (at the cost of
+  // some code complexity) into the loop below.
+  WorkList.erase(std::remove_if(WorkList.begin(), WorkList.end(),
+                                IsBlockPlaced(Chain, BlockToChain)),
+                 WorkList.end());
+
   MachineBasicBlock *BestBlock = 0;
   BlockFrequency BestFreq;
   for (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(),
                                                       WBE = WorkList.end();
        WBI != WBE; ++WBI) {
-    if (BlockFilter && !BlockFilter->count(*WBI))
-      continue;
+    assert(!BlockFilter || BlockFilter->count(*WBI));
     BlockChain &SuccChain = *BlockToChain[*WBI];
     if (&SuccChain == &Chain) {
       DEBUG(dbgs() << "    " << getBlockName(*WBI)