From fa97658b1c71f747cfe0f3e1f1bcbd86d7fa9f75 Mon Sep 17 00:00:00 2001
From: Chandler Carruth <chandlerc@gmail.com>
Date: Mon, 14 Nov 2011 09:46:33 +0000
Subject: [PATCH] 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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144531 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/CodeGen/MachineBlockPlacement.cpp | 28 +++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/lib/CodeGen/MachineBlockPlacement.cpp b/lib/CodeGen/MachineBlockPlacement.cpp
index bd50ac3a4d5..6c8390fee61 100644
--- a/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/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)