[CSSPGO] Fix an AV caused by a block that has only pseudo pseudo instructions.

[wenlei]

So without this fix, it would AV as well if a block only had dbg instruction?

llvm/lib/CodeGen/RegAllocGreedy.cpp

1328

Without slot index for dbg or probe, we don't know whether the MBB is before the split, is it safe to allow insertion of spill there?

[hoy]

llvm/lib/CodeGen/RegAllocGreedy.cpp

1328

I think it's by design. The original code is like below. We were skipping dbg and probe but overlooked that the block could be empty in terms of real instructions.

if (!MBB->empty() &&
      SlotIndex::isEarlierInstr(LIS->getInstructionIndex(MBB->instr_front()),
                                SA->getFirstSplitPoint(Number)))

[wenlei]

lgtm, thanks for the fix.

llvm/lib/CodeGen/RegAllocGreedy.cpp

1328

ok, makes sense. having a probe/dbg is no different from empty block.

llvm/lib/CodeGen/RegAllocGreedy.cpp

if (!Intf.hasInterference()) {
       continue;
     }
 
     assert(B < GroupSize && "Array overflow");
     BCS[B].Number = Number;
 
     // Abort if the spill cannot be inserted at the MBB' start
     MachineBasicBlock *MBB = MF->getBlockNumbered(Number);
-    if (!MBB->empty() &&
-        SlotIndex::isEarlierInstr(
-            LIS->getInstructionIndex(*MBB->getFirstNonDebugInstr()),
+    auto FirstNonDebugInstr = MBB->getFirstNonDebugInstr();
+    if (FirstNonDebugInstr != MBB->end() &&
+        SlotIndex::isEarlierInstr(LIS->getInstructionIndex(*FirstNonDebugInstr),
                                   SA->getFirstSplitPoint(Number)))
       return false;
     // Interference for the live-in value.
     if (Intf.first() <= Indexes->getMBBStartIdx(Number))
       BCS[B].Entry = SpillPlacement::MustSpill;
     else
       BCS[B].Entry = SpillPlacement::PrefSpill;
 
     // Interference for the live-out value.