Reapply r135074 and r135080 with a fix.

The cache entry referenced by the best split candidate could become
clobbered by an unsuccessful candidate.

The correct fix here is to use reference counts on the cache entries.
Coming up.

llvm-svn: 135113

2 files changed, 39 insertions, 28 deletions

diff --git a/llvm/lib/CodeGen/InterferenceCache.h b/llvm/lib/CodeGen/InterferenceCache.h
index 6c36fa4021f..6434b3a788d 100644
--- a/llvm/lib/CodeGen/InterferenceCache.h
+++ b/llvm/lib/CodeGen/InterferenceCache.h
@@ -127,10 +127,14 @@ public:
     Entry *CacheEntry;
     BlockInterference *Current;
   public:
-    /// Cursor - Create a cursor for the interference allocated to PhysReg and
-    /// all its aliases.
-    Cursor(InterferenceCache &Cache, unsigned PhysReg)
-      : CacheEntry(Cache.get(PhysReg)), Current(0) {}
+    /// Cursor - Create a dangling cursor.
+    Cursor() : CacheEntry(0), Current(0) {}
+
+    /// setPhysReg - Point this cursor to PhysReg's interference.
+    void setPhysReg(InterferenceCache &Cache, unsigned PhysReg) {
+      CacheEntry = Cache.get(PhysReg);
+      Current = 0;
+    }
 
     /// moveTo - Move cursor to basic block MBBNum.
     void moveToBlock(unsigned MBBNum) {
diff --git a/llvm/lib/CodeGen/RegAllocGreedy.cpp b/llvm/lib/CodeGen/RegAllocGreedy.cpp
index d79573c205b..4728a050b17 100644
--- a/llvm/lib/CodeGen/RegAllocGreedy.cpp
+++ b/llvm/lib/CodeGen/RegAllocGreedy.cpp
@@ -160,11 +160,13 @@ class RAGreedy : public MachineFunctionPass,
   /// Global live range splitting candidate info.
   struct GlobalSplitCandidate {
     unsigned PhysReg;
+    InterferenceCache::Cursor Intf;
     BitVector LiveBundles;
     SmallVector<unsigned, 8> ActiveBlocks;
 
-    void reset(unsigned Reg) {
+    void reset(InterferenceCache &Cache, unsigned Reg) {
       PhysReg = Reg;
+      Intf.setPhysReg(Cache, Reg);
       LiveBundles.clear();
       ActiveBlocks.clear();
     }
@@ -206,8 +208,8 @@ private:
   float calcSpillCost();
   bool addSplitConstraints(InterferenceCache::Cursor, float&);
   void addThroughConstraints(InterferenceCache::Cursor, ArrayRef<unsigned>);
-  void growRegion(GlobalSplitCandidate &Cand, InterferenceCache::Cursor);
-  float calcGlobalSplitCost(GlobalSplitCandidate&, InterferenceCache::Cursor);
+  void growRegion(GlobalSplitCandidate &Cand);
+  float calcGlobalSplitCost(GlobalSplitCandidate&);
   void splitAroundRegion(LiveInterval&, GlobalSplitCandidate&,
                          SmallVectorImpl<LiveInterval*>&);
   void calcGapWeights(unsigned, SmallVectorImpl<float>&);
@@ -720,8 +722,7 @@ void RAGreedy::addThroughConstraints(InterferenceCache::Cursor Intf,
   SpillPlacer->addLinks(ArrayRef<unsigned>(TBS, T));
 }
 
-void RAGreedy::growRegion(GlobalSplitCandidate &Cand,
-                          InterferenceCache::Cursor Intf) {
+void RAGreedy::growRegion(GlobalSplitCandidate &Cand) {
   // Keep track of through blocks that have not been added to SpillPlacer.
   BitVector Todo = SA->getThroughBlocks();
   SmallVectorImpl<unsigned> &ActiveBlocks = Cand.ActiveBlocks;
@@ -753,7 +754,7 @@ void RAGreedy::growRegion(GlobalSplitCandidate &Cand,
     // Any new blocks to add?
     if (ActiveBlocks.size() == AddedTo)
       break;
-    addThroughConstraints(Intf,
+    addThroughConstraints(Cand.Intf,
                           ArrayRef<unsigned>(ActiveBlocks).slice(AddedTo));
     AddedTo = ActiveBlocks.size();
 
@@ -794,8 +795,7 @@ float RAGreedy::calcSpillCost() {
 /// pattern in LiveBundles. This cost should be added to the local cost of the
 /// interference pattern in SplitConstraints.
 ///
-float RAGreedy::calcGlobalSplitCost(GlobalSplitCandidate &Cand,
-                                    InterferenceCache::Cursor Intf) {
+float RAGreedy::calcGlobalSplitCost(GlobalSplitCandidate &Cand) {
   float GlobalCost = 0;
   const BitVector &LiveBundles = Cand.LiveBundles;
   ArrayRef<SplitAnalysis::BlockInfo> UseBlocks = SA->getUseBlocks();
@@ -822,8 +822,8 @@ float RAGreedy::calcGlobalSplitCost(GlobalSplitCandidate &Cand,
       continue;
     if (RegIn && RegOut) {
       // We need double spill code if this block has interference.
-      Intf.moveToBlock(Number);
-      if (Intf.hasInterference())
+      Cand.Intf.moveToBlock(Number);
+      if (Cand.Intf.hasInterference())
         GlobalCost += 2*SpillPlacer->getBlockFrequency(Number);
       continue;
     }
@@ -853,7 +853,12 @@ void RAGreedy::splitAroundRegion(LiveInterval &VirtReg,
     dbgs() << ".\n";
   });
 
-  InterferenceCache::Cursor Intf(IntfCache, Cand.PhysReg);
+  InterferenceCache::Cursor &Intf = Cand.Intf;
+
+  // FIXME: We need cache reference counts to guarantee that Intf hasn't been
+  // clobbered.
+  Intf.setPhysReg(IntfCache, Cand.PhysReg);
+
   LiveRangeEdit LREdit(VirtReg, NewVRegs, this);
   SE->reset(LREdit);
 
@@ -1243,17 +1248,18 @@ unsigned RAGreedy::tryRegionSplit(LiveInterval &VirtReg, AllocationOrder &Order,
   DEBUG(dbgs() << "Cost of isolating all blocks = " << BestCost << '\n');
   const unsigned NoCand = ~0u;
   unsigned BestCand = NoCand;
+  unsigned NumCands = 0;
 
   Order.rewind();
-  for (unsigned Cand = 0; unsigned PhysReg = Order.next(); ++Cand) {
-    if (GlobalCand.size() <= Cand)
-      GlobalCand.resize(Cand+1);
-    GlobalCand[Cand].reset(PhysReg);
+  while (unsigned PhysReg = Order.next()) {
+    if (GlobalCand.size() <= NumCands)
+      GlobalCand.resize(NumCands+1);
+    GlobalSplitCandidate &Cand = GlobalCand[NumCands];
+    Cand.reset(IntfCache, PhysReg);
 
-    SpillPlacer->prepare(GlobalCand[Cand].LiveBundles);
+    SpillPlacer->prepare(Cand.LiveBundles);
     float Cost;
-    InterferenceCache::Cursor Intf(IntfCache, PhysReg);
-    if (!addSplitConstraints(Intf, Cost)) {
+    if (!addSplitConstraints(Cand.Intf, Cost)) {
       DEBUG(dbgs() << PrintReg(PhysReg, TRI) << "\tno positive bundles\n");
       continue;
     }
@@ -1268,28 +1274,29 @@ unsigned RAGreedy::tryRegionSplit(LiveInterval &VirtReg, AllocationOrder &Order,
       });
       continue;
     }
-    growRegion(GlobalCand[Cand], Intf);
+    growRegion(Cand);
 
     SpillPlacer->finish();
 
     // No live bundles, defer to splitSingleBlocks().
-    if (!GlobalCand[Cand].LiveBundles.any()) {
+    if (!Cand.LiveBundles.any()) {
       DEBUG(dbgs() << " no bundles.\n");
       continue;
     }
 
-    Cost += calcGlobalSplitCost(GlobalCand[Cand], Intf);
+    Cost += calcGlobalSplitCost(Cand);
     DEBUG({
       dbgs() << ", total = " << Cost << " with bundles";
-      for (int i = GlobalCand[Cand].LiveBundles.find_first(); i>=0;
-           i = GlobalCand[Cand].LiveBundles.find_next(i))
+      for (int i = Cand.LiveBundles.find_first(); i>=0;
+           i = Cand.LiveBundles.find_next(i))
         dbgs() << " EB#" << i;
       dbgs() << ".\n";
     });
     if (Cost < BestCost) {
-      BestCand = Cand;
+      BestCand = NumCands;
       BestCost = Hysteresis * Cost; // Prevent rounding effects.
     }
+    ++NumCands;
   }
 
   if (BestCand == NoCand)