Add a safe-guard against repeated splitting for some rare cases.

The number of blocks covered by a live range must be strictly decreasing when
splitting, otherwise we can't allow repeated splitting.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130249 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/RegAllocGreedy.cpp

@@ -921,6 +921,7 @@ void RAGreedy::splitAroundRegion(LiveInterval &VirtReg,
   SmallVector<unsigned, 8> IntvMap;
   SE->finish(&IntvMap);
   LRStage.resize(MRI->getNumVirtRegs());
+  unsigned OrigBlocks = SA->getNumThroughBlocks() + SA->getUseBlocks().size();
 
   // Sort out the new intervals created by splitting. We get four kinds:
   // - Remainder intervals should not be split again.
@@ -941,9 +942,20 @@ void RAGreedy::splitAroundRegion(LiveInterval &VirtReg,
       continue;
     }
 
-    // Other intervals are treated as new. This includes the main interval,
-    // local intervals created for blocks with multiple uses, and anything
-    // created by DCE.
+    // Main interval. Allow repeated splitting as long as the number of live
+    // blocks is strictly decreasing.
+    if (IntvMap[i] == MainIntv) {
+      if (SA->countLiveBlocks(LREdit.get(i)) >= OrigBlocks) {
+        DEBUG(dbgs() << "Main interval covers the same " << OrigBlocks
+                     << " blocks as original.\n");
+        // Don't allow repeated splitting as a safe guard against looping.
+        LRStage[Reg] = RS_Global;
+      }
+      continue;
+    }
+
+    // Other intervals are treated as new. This includes local intervals created
+    // for blocks with multiple uses, and anything created by DCE.
   }
 
   if (VerifyEnabled)

lib/CodeGen/SplitKit.cpp

@@ -221,6 +221,29 @@ bool SplitAnalysis::calcLiveBlockInfo() {
   return true;
 }
 
+unsigned SplitAnalysis::countLiveBlocks(const LiveInterval *cli) const {
+  if (cli->empty())
+    return 0;
+  LiveInterval *li = const_cast<LiveInterval*>(cli);
+  LiveInterval::iterator LVI = li->begin();
+  LiveInterval::iterator LVE = li->end();
+  unsigned Count = 0;
+
+  // Loop over basic blocks where li is live.
+  MachineFunction::const_iterator MFI = LIS.getMBBFromIndex(LVI->start);
+  SlotIndex Stop = LIS.getMBBEndIdx(MFI);
+  for (;;) {
+    ++Count;
+    LVI = li->advanceTo(LVI, Stop);
+    if (LVI == LVE)
+      return Count;
+    do {
+      ++MFI;
+      Stop = LIS.getMBBEndIdx(MFI);
+    } while (Stop <= LVI->start);
+  }
+}
+
 bool SplitAnalysis::isOriginalEndpoint(SlotIndex Idx) const {
   unsigned OrigReg = VRM.getOriginal(CurLI->reg);
   const LiveInterval &Orig = LIS.getInterval(OrigReg);

lib/CodeGen/SplitKit.h

@@ -150,6 +150,11 @@ public:
   /// getThroughBlocks - Return the set of through blocks.
   const BitVector &getThroughBlocks() const { return ThroughBlocks; }
 
+  /// countLiveBlocks - Return the number of blocks where li is live.
+  /// This is guaranteed to return the same number as getNumThroughBlocks() +
+  /// getUseBlocks().size() after calling analyze(li).
+  unsigned countLiveBlocks(const LiveInterval *li) const;
+
   typedef SmallPtrSet<const MachineBasicBlock*, 16> BlockPtrSet;
 
   /// getMultiUseBlocks - Add basic blocks to Blocks that may benefit from