Split around single instructions to enable register class inflation.

Normally, we don't create a live range for a single instruction in a
basic block, the spiller does that anyway. However, when splitting a
live range that belongs to a proper register sub-class, inserting these
extra COPY instructions completely remove the constraints from the
remainder interval, and it may be allocated from the larger super-class.

The spiller will mop up these small live ranges if we end up spilling
anyway. It calls them snippets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@136989 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/RegAllocGreedy.cpp

@@ -942,6 +942,12 @@ void RAGreedy::splitAroundRegion(LiveRangeEdit &LREdit,
   DEBUG(dbgs() << "splitAroundRegion with " << NumGlobalIntvs << " globals.\n");
   assert(NumGlobalIntvs && "No global intervals configured");
 
+  // Isolate even single instructions when dealing with a proper sub-class.
+  // That giarantees register class inflation for the stack interval because it
+  // is all copies.
+  unsigned Reg = SA->getParent().reg;
+  bool SingleInstrs = RegClassInfo.isProperSubClass(MRI->getRegClass(Reg));
+
   // First handle all the blocks with uses.
   ArrayRef<SplitAnalysis::BlockInfo> UseBlocks = SA->getUseBlocks();
   for (unsigned i = 0; i != UseBlocks.size(); ++i) {
@@ -971,7 +977,7 @@ void RAGreedy::splitAroundRegion(LiveRangeEdit &LREdit,
     // Create separate intervals for isolated blocks with multiple uses.
     if (!IntvIn && !IntvOut) {
       DEBUG(dbgs() << "BB#" << BI.MBB->getNumber() << " isolated.\n");
-      if (!BI.isOneInstr())
+      if (SA->shouldSplitSingleBlock(BI, SingleInstrs))
         SE->splitSingleBlock(BI);
       continue;
     }

lib/CodeGen/SplitKit.cpp

@@ -1087,6 +1087,24 @@ void SplitEditor::finish(SmallVectorImpl<unsigned> *LRMap) {
 //                            Single Block Splitting
 //===----------------------------------------------------------------------===//
 
+bool SplitAnalysis::shouldSplitSingleBlock(const BlockInfo &BI,
+                                           bool SingleInstrs) const {
+  // Always split for multiple instructions.
+  if (!BI.isOneInstr())
+    return true;
+  // Don't split for single instructions unless explicitly requested.
+  if (!SingleInstrs)
+    return false;
+  // Splitting a live-through range always makes progress.
+  if (BI.LiveIn && BI.LiveOut)
+    return true;
+  // No point in isolating a copy. It has no register class constraints.
+  if (LIS.getInstructionFromIndex(BI.FirstInstr)->isCopyLike())
+    return false;
+  // Finally, don't isolate an end point that was created by earlier splits.
+  return isOriginalEndpoint(BI.FirstInstr);
+}
+
 /// getMultiUseBlocks - if CurLI has more than one use in a basic block, it
 /// may be an advantage to split CurLI for the duration of the block.
 bool SplitAnalysis::getMultiUseBlocks(BlockPtrSet &Blocks) {

lib/CodeGen/SplitKit.h

@@ -185,6 +185,16 @@ public:
 
   typedef SmallPtrSet<const MachineBasicBlock*, 16> BlockPtrSet;
 
+  /// shouldSplitSingleBlock - Returns true if it would help to create a local
+  /// live range for the instructions in BI. There is normally no benefit to
+  /// creating a live range for a single instruction, but it does enable
+  /// register class inflation if the instruction has a restricted register
+  /// class.
+  ///
+  /// @param BI           The block to be isolated.
+  /// @param SingleInstrs True when single instructions should be isolated.
+  bool shouldSplitSingleBlock(const BlockInfo &BI, bool SingleInstrs) const;
+
   /// getMultiUseBlocks - Add basic blocks to Blocks that may benefit from
   /// having CurLI split to a new live interval. Return true if Blocks can be
   /// passed to SplitEditor::splitSingleBlocks.