[MemCpyOpt] Add some debug output to call slot optimization (NFC)

llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp

@@ -890,8 +890,10 @@ bool MemCpyOptPass::performCallSlotOptzn(Instruction *cpyLoad,
   // trap.  Otherwise the transform is invalid since it might cause a trap
   // to occur earlier than it otherwise would.
   if (!isDereferenceableAndAlignedPointer(cpyDest, Align(1), APInt(64, cpySize),
-                                          DL, C, DT))
+                                          DL, C, DT)) {
+    LLVM_DEBUG(dbgs() << "Call Slot: Dest pointer not dereferenceable\n");
     return false;
+  }
 
   // Make sure that nothing can observe cpyDest being written early. There are
   // a number of cases to consider:
@@ -907,8 +909,10 @@ bool MemCpyOptPass::performCallSlotOptzn(Instruction *cpyLoad,
   //     guaranteed to be executed if C is. As it is a non-atomic access, it
   //     renders accesses from other threads undefined.
   //     TODO: This is currently not checked.
-  if (mayBeVisibleThroughUnwinding(cpyDest, C, cpyStore))
+  if (mayBeVisibleThroughUnwinding(cpyDest, C, cpyStore)) {
+    LLVM_DEBUG(dbgs() << "Call Slot: Dest may be visible through unwinding");
     return false;
+  }
 
   // Check that dest points to memory that is at least as aligned as src.
   Align srcAlign = srcAlloca->getAlign();